Authors: Caitlin Bean, Mary J. Russo (revision), Global Invasive Species Team, The Nature Conservancy
- Eucalyptus globulus is an aromatic tree. Commonly attains a height of 150-180 ft. (45.7-54.9 m) and a diameter of 4 to 7 ft. (1.2-2.1 m). It has a straight trunk up to two-thirds of its total height and a well-developed crown.
- Leaves are glossy, dark green, thick and leathery. They average in length from 5.9 to 7.9 in. (15-20 cm). The leaves of the young shoots are ovate, opposite, and horizontal. They are covered with a grey, waxy bloom which is much thicker on the bottom surface of the leaf.
- Solitary white flowers with many stamens, arise from the axils on flattened stalks. They range from 1.6 to 2.2 in. (4-5.5 cm) wide. Sepals and petals are united to form a warty lid which is present on the bud and drops off at flowering. Flowers from December to May.
- The fruit is a hard, woody globose capsule. The fruit is 0.8 to 1 in. (2-2.5 cm) across. The numerous seeds are approximately 0.08 in. x 0.04 in. (2 x 1 mm). Seeds are dark brown with a brownish red chaff.
- Ecological Threat
- Eucalyptus globulus out-competes native vegetation for space, light and nutrients. It is native to Australia.
Scientific Name: Eucalyptus globulus
Common Name: Tasmanian blue gum
The following description of Eucalyptus globulus is primarily adapted from Munz and Keck (1973).
Eucalyptus globulus Labill. is an aromatic tree in the Myrtle Family (Myrtaceae) which commonly attains a height of 150-180 feet and a diameter of 4-7 feet. It has a straight trunk up to two-thirds of its total height and a well-developed crown. The central trunk and tap root are fringed with many lateral stems and roots. The tap root rarely exceeds a length of 10 feet (Hall et al. 1970). The light, yellow-brown bark is deciduous.
The leaves of the older branches are narrowly lanceolate, often curved, alternate and hung vertically. They are glossy, dark green, thick and leathery. They average in length from 1.5-2 dm. The leaves of the young shoots are ovate, opposite, sessile, and horizontal. They are covered with a grey, waxy bloom which is much thicker on the bottom surface of the leaf. Young stems are squared or winged.
The white flowers are solitary in the axils on flattened stalks. They are approximately 4-5.5 cm wide. The sepals and petals are united to form a warty lid which is present on the bud and drops off at anthesis. The flower has many stamens. The ovary is four-loculed with many ovules. Flowers are most abundant from December to May.
The fruit is a hard, woody capsule, broadly top shaped or globose with a wide flat disc. It is loculicidally dehiscent at the top by four valves. The fruit is 2-2.5 cm across. The numerous seeds are approximately 2 x 1 mm (relatively small compared to other woody plant species). Seeds are dark brown with a brownish red chaff (Krugman 1974).
The distinctive features of E. globulus are the juvenile leaves, which are ovate, opposite, sessile, glaucous and occur on squared stems, and the large, solitary fruits (2-2.5 cm), which are warty, glaucous and four-ribbed (Hall et al. 1970).
Eucalyptus globulus will grow on a wide range of substrates, but it is especially common and widespread on soils derived from granite and grano-diorite rocks. It is best developed on moderately fertile loams or heavy, well-drained soil. Blue gum does not occur naturally on poorly- drained soils or on strongly-calcareous or alkaline soils.
Most occurrences of E. globulus are found in areas having an annual rainfall of 60-110 cm, and nowhere does it occur naturally with less than 50 cm annual precipitation. It has an altitudinal range from near sea level to approximately 1100 m. This species is found in a variety of frost situations. Coastal sites are relatively free of frost, while occurrences at higher altitudes may receive over 70 frosts per year.
The natural distribution of E. globulus is confined to Tasmania, Victoria, and New South Wales between latitudes 31 and 43 degrees S. This species is most common in southeastern Tasmania, islands in the Bass Straits, the Ottway Ranges, and Wilson's Promontory in Victoria. In its natural range, E. globulus is most commonly associated with other Eucalyptus species, although within a particular area blue gum is closely tied to a restricted range of habitats. A mosaic pattern is often formed with stands of diverse Eucalyptus species occupying species-specific habitats.
Eucalyptus globulus is one of the most extensively planted Eucalyptus. Its rapid growth and adaptability to a range of site conditions have been responsible for its popularity. It has been particularly successful in countries with a Mediterranean-type climate but has also grown well at high altitudes in the tropics. It has failed only in temperate zones with severe winters, in tropical zones at low altitudes where the temperatures are uniformly high, and in regions with long, hot dry seasons. The fast growing wood has proven unsuitable for sawn timber due to excessive defeat from collapse, splitting and warping. It has been used as satisfactory firewood, fence posts, mining timber, paper pulp, rayon pulp, shelter belts, and as an ornamental landscape species.
By 1853, Captain Robert Waterman had introduced E. globulus to California. He advertised it as a fast growing ornamental. By 1870 blue gum was being planted for commercial purposes. In 1910, Sellers wrote that "The section within the influence of the moderate coast climate is most favorable for its best development. Though its growth is especially thrifty in the northern coast counties and in the coast valleys of Southern California, its timber production appears most rapid in the Santa Clara, Sacramento and other valleys opening about San Francisco Bay.
". . The blue gum groves upon the dry slopes and crests of the Coast Range hills owe their thrift principally to the prevalence of fogs, for ordinarily they will not succeed in dry localities. Owing to fog moisture, however, the valleys opening into San Francisco Bay are rendered the most favorable region for E. globulus planting in California . . Blue gum will grow where the water level is very high and will even endure standing water for a time . . In Southern California deep sandy loam soils have been found to support the best growth while clay-loam and adobe soils support maximum growth both in Bay counties and in the Coast valleys of southern California."
The seeds of the eucalypts are very small in comparison with other forest tree seeds but are produced abundantly. Eucalyptus globulus seeds are among some of the larger seeds in the entire genus (2 x 1 mm). They are formed in a fruit 12 mm in diameter. The mean number of viable seeds produced per 10 grams of total seed is 735 (plus or minus a standard deviation of 380). Most seed is distributed by wind and gravity, but some is moved by such agents as flood, erosion and birds (Jacobs 1955). Usually seed is dropped within 100 feet of the parent tree (Jacobs 1955). The first seeds to leave the globular capsule or hypanthium are infertile. The fertile seeds are located at the bottom of the capsule (Penfold and Willis 1961).
Under optimal conditions (greenhouse environment at 25° C), germination occurs between 5 and 14 days (Boland et al. 1980). In the field, germination should occur no later than 26 days once the appropriate environmental conditions are met. However, seed may remain dormant for several years under dry conditions (Jacobs 1955). Clifford (1953) reported that E. globulus is a species which does not require light for the mature seed to germinate. Under a Eucalyptus canopy, a 1% germination rate is good, given the more usual 0.1% germination success rate (Jacobs 1955).
It is difficult for blue gum seeds to germinate within a dense forest of parent trees. Not only does the presence of deep litter pose a germination problem, but there are also germination-inhibitive chemicals produced in the leaves of mature trees. If seeds do germinate, a thick duff layer may prevent adequate soil moisture conditions for radicle survival, and "it is known that eucalypt litter tends to restrict the development of the roots of seedlings. This is one reason why the genus regenerates best on mineral soil" (Jacobs 1961). Eucalyptus globulus has a higher percent germination rate in open land.
Eucalypts can scatter their seed while the capsules remain on the tree, or if the capsule falls to the ground intact, the seed will spill out in "little heaps." The optimum depth of soil cover for E. globulus seed is 0.5-0.7 cm. However, when germinating in "heaps," E. globulus can emerge from a soil cover of as much as 3.75 cm. Free (1951) explains that when seedlings emerge from seed sown in "heaps," all those that germinate do not emerge. Those that do not emerge may assist in lifting the weight of the soil and breaking the crust. "In nature it is probable that only one plant could reach maturity from any heap" (Free 1951). As a seedling, E. globulus is very sensitive to frost, drought, and fungal and insect attack. Eucalyptus globulus remains very sensitive to frost as long as the immature foliage is retained. Saplings will die at -5° C (Sellers 1910). Eucalyptus globulus can endure a temperature maximum of 45° C if the air is not especially arid (McClatchie 1902).
"Eucalypt flowers are mainly pollinated by insects, but birds and small mammals may also act as pollinating agents. There is no evidence that wind plays anything but a minor role in eucalypt pollination . . The flowers of eucalypts are not highly specialized for insect pollination. The absence of specialization is reflected by the great variety of visiting insects, e.g., Coleoptera, Hymenoptera, Lepidoptera, Hemiptera, and Diptera of which the honey bee is usually regarded as the most important" (Boland et al. 1980). When the cap covering the reproductive organs (the operculum) is shed, the anthers have mature pollen, but the stigma does not become receptive until some days later. This sequence impedes self-pollination of an individual flower.
Eucalyptus globulus is one of the fastest growing eucalypts. A three-year-old tree has been recorded at 14 m high and 22.5 cm in diameter. At six to eight years, it has been recorded at 25-33 m high, and at 30 years, it has been recorded at 50 m high and 1-2 m in diameter (McClatchie 1902). It has been known to seed as early as its fifth year (FAO 1981).
An interesting characteristic of development of E. globulus is the change of its leaves over time. The juvenile leaves, which are produced in the early years of the plant's life, are borne horizontally. They are without petioles, cordate at the base, have a short sharp point at the apex, and are about twice as long as they are broad (Johnson 1926). "They are produced by branches which are square and winged, and they are arranged in pairs at right angles to each other. Both stem and leaves possess a coat of wax which is much thicker on the under than on the upper surface of the leaf. The very young forms of the juvenile leaves are larger, thinner and less waxy" (Johnson 1926). The adult leaves on the other hand are borne spirally on a rounded stem. They are "sickle shaped, very thick and leathery. The petioles have twisted through an angle of 90 degrees, and thus the leaves have no longer a horizontal position but are vertical, presenting their edges to the sun. They are devoid of that heavy coating of wax which is so characteristic of the juvenile leaves" (Johnson 1926). Johnson proposes that the extraordinary differences between the young and mature foliage, specifically the difference in position could be: (1) a protection against insolation and/or (2) protection against transpiration. One of the most characteristic features of the eucalypts is the presence of oil glands in the leaves of all species. Most Eucalyptus biologists advocate the theory that the oils protect the leaf against water loss (Penfold and Willis 1961).
During the first year of life two globular swellings appear on the axils of the cotyledons. These protuberances swell until they finally fuse together. The consequent, rather hemispherical mass is known as a lignotuber (Penfold and Willis 1961). It is an organ of food storage and regeneration. As the lignotuber develops it grows down the stem, enveloping the upper part of the root. As its size increases it tends to bury itself in the soil until only a small portion, or none at all, can be seen. Occasionally the numerous bud strands present in the lignotuber produce leafy shoots.
Under normal conditions, the bud strands remain dormant, but if the tree is cut down, the strands become active and each bud may produce a shoot bearing foliage. Such shoots are commonly known as "sucker growth," or coppice shoots, and a large number are usually formed. Generally, many coppice shoots break off or die, and one shoot will become dominant producing a new main stem (Penfold and Willis 1961). This organ is of great value to the seedlings in preventing mortality from insolation when germination occurs on bare ground (Jacobs 1961).
Along with the lignotuber, the genus Eucalyptus has also evolved a number of other ways to deal with environmental stress. In the axil of every Eucalyptus leaf is a stalked bud which becomes visible immediately as the subtending leaf unfolds. These buds are known as naked buds because they are not covered by protective scales. The naked buds near the tops of growing shoots develop rapidly in conjunction with the shoots themselves and give rise to first order branches. The growth of these buds is inhibited by a hormone (or hormones) produced at the shoot apex, and they will not develop unless the tip of the shoot is destroyed or removed by frost, drought, or insects, for example.
As the branch order increases, the naked buds tend to become smaller, until the third order branches. Then most of the naked buds from the first order atrophy and die (Penfold and Willis 1961). Under the naked buds, and buried in the tissue of the leaf axils, are pads of meristematic tissue which are capable of growing into new shoots. Under normal conditions, these pads are also inhibited by a hormone (or hormones) produced by the leaves and the naked buds of the shoot. Once these organs, called accessory buds, are destroyed or removed, the meristematic tissue grows rapidly forming an accessory shoot (or shoots) within 7-14 days. "The naked buds are the primary cause of the rapid growth rate of the eucalypts in the absence of leaf-eating insects. The accessory shoots are one of the reasons for the hardiness and persistence of this genus in spite of insects and other unfavorable factors. The naked buds may be destroyed by desiccation or frost as well as by insects. The accessory meristematic tissue is resistant to drought and frost unless conditions are severe enough to destroy the parent shoots" (Jacobs 1955).
Another interesting feature of eucalypts is the occurrence of accessory bud-producing tissue in the leaf axil between the stalk of the naked bud and the base of the leaf petiole. When the parent leaf falls this tissue is not occluded by diameter growth of the stem on which it lies. A small shaft of tissue with bud-producing properties grows radially outward from the old leaf axil at a rate corresponding with the diameter growth of the mother stem. It is not uncommon for an accessory pad to develop 2 or 3 shafts of tissue.
These bud-producing tissues are called proventitious bud strands. They are capable of producing leafy shoots (epicormal shoots) but are normally prevented from doing so by the inhibiting substances produced by young and/or upper leaf shoots above them. Should these young shoots or upper leaves be removed, the inhibition is removed and several shoots may develop from the shafts. It has been estimated that a tree 20 m high might have as many as 7000 dormant buds, each one marking the original position of the leaves. This is a remarkable adaptation to complete defoliation by fire, a common phenomenon in the forests of Australia.
In addition to leaf and stem adaptations to environmental stress, Eucalyptus exhibits interesting bark characteristics. As Eucalyptus grows in diameter, the periderm layer is stretched until it finally cracks. A new periderm is then formed further in the phloem, and the tissues outside the periderm dry out and die. This dead outer tissue is called the "rhytidome" (Jacobs 1955). It is deciduous or decorticating bark, and as each layer is renewed, the old bark peels off in long strips. Bark shedding normally occurs in late summer or early fall (Penfold and Willis 1961). It is difficult to define the color and surface texture of decorticating bark because it is characterized by adjoining patches of varying ages. New patches are generally shiny and comparatively bright, while old patches ready to fall are dull and rough (FAO 1981).
The root system of E. globulus consists mostly of strong lateral roots. An abundant supply of moisture is demanded. Since the roots grow quickly toward water, E. globulus should never be planted near wells, cisterns, water pipes, irrigation ditches, sandy or gravelly soils. Large roots have been discovered at a depth of 45 feet below the surface, and surface roots frequently spread over 100 feet away from the trunk (Sellers 1910).
Most mature, undisturbed stands of E. globulus are virtually devoid of herbaceous annual species in the forest understory (Del Moral and Muller 1969). This may be due to the inhibiting effects of Eucalyptus toxins present in the thick accumulation of Eucalyptus leaf litter underneath these stands. This assertion is supported by the fact that annual herbs gradually begin to appear and increase in height and density with increasing distance from the stand, in inverse correlation with the density of Eucalyptus leaf litter. However, there is also a paucity of herbs under mature trees which are well trimmed and cleared from litter periodically, suggesting that "while litter is an important source of toxins in some Eucalyptus species, it is not necessary to the development or maintenance of herb inhibition in the case of E. globulus" (Del Moral and Muller 1969).
Del Moral and Muller (1969) also investigated the transfer of the Eucalyptus herb-inhibiting toxins to the soil by fog drip. The evidence presented in their report indicates that "natural fog drip from E. globulus inhibits the growth of annual grass seedlings in bioassays both on sponges and in soil and suggests that inhibition occurs under natural conditions." The fog drip is known to contain several physiologically active components in significant concentration, including P-consiaryfumic chlorogenic and gentisic acids. The authors conclude that in Eucalyptus globulus "toxin transfer by fog drip alone is capable of severely inhibiting the growth of annual herbs."
The authors emphasized, however, that "toxic fog drip is only one of several mechanisms present in Eucalyptus species capable of producing herb growth inhibition." Other mechanisms mentioned include: (1) the leaching out in quantity of toxic phenolic acids from leaf litter by rain and (2) volatilization of terpenes from leaves and subsequent re-adsorption of the terpenes by soil colloids (soil in this condition is highly inhibitory to germinating seedlings).
Most eucalypts grow in localities where there is marked water shortage for substantial parts of the year. Therefore, they are adapted to seasonal drought stress associated with dry summers. Eucalypts develop an abundance of hard tissue called sclerenchyma which gives them the ability to endure severe wilting without lasting damage (Pryor 1976). They do not economize in the use of water but have wide-ranging root systems and an ability to extract water from the soil at even higher soil moisture tensions than most mesophytic plants. Transpiration rates remain high even when water supply from the soil is dwindling. It is only when severe permanent wilting occurs that there is stomatal closure which inhibits water loss (and, of course, also prevents gas exchange and photosynthesis) and enables the plant to survive a critical water balance situation for some time (Pryor 1976).
The wildlife in a Eucalyptus forest varies depending upon the geographic location of the grove. At Jepson Prairie Preserve, CA, Swainson's hawk and yellow warblers, both of which are "Blue Listed" species of concern, nest in the trees. At Pescadero Creek County Park, south of San Francisco along the coast of California, great blue herons and egrets use the trees to build their rookeries. The following is a report on the wildlife in a Eucalyptus grove in the East Bay. This report is an excerpt from the Preliminary Report on Eucalyptus Control and Management (1983), compiled by the Jepson Prairie Preserve Committee for The Nature Conservancy's California Field Office:
"Contrary to popular belief, many animals, both vertebrates and invertebrates, have adapted to life in the Eucalyptus groves. Moisture from the air condenses on the leaves and the drippage keeps the groves moist and cool even during the dry season. This is a suitable ground habitat for a wide variety of animal life. Amphibians such as arboreal salamander, California slender salamander, ENSATINA, California newt, rough skinned newt, and Pacific tree frog live in the forest, primarily under fallen logs and duff. Amphibians feed on such invertebrates as millipedes, centipedes, sow bugs, COLLENBOLA, spiders and earthworms.
"Several snakes such as the ring-necked snake, rubber boa and sharp tailed snake have adapted to Eucalyptus groves. The ring-necked snake feeds on the California slender salamander, the rubber boa feeds on meadow mice, and the sharp tailed snake feeds strictly on slugs. Other common reptiles include the northern and southern alligator lizards, which live under fallen logs, and the western fence lizard and western skink, which live in the less densely forested groves.
"Over 100 species of birds use the trees either briefly or as a permanent habitat. The heavy-use birds feed on seeds by pecking the mature pods on trees or fallen pods; so they must wait for the pods to disintegrate or be crushed by cars. .Among the birds that feed on seeds in the trees are: the chestnutback chickadee and the Oregon junco. Examples of birds that feed on ground seeds are the song sparrow, the fox sparrow, the brown towhee, and the mourning dove. Birds that take advantage of the nectar from blossoms either by drinking the nectar or by feeding on the insects that are attracted to the nectar include Allen's hummingbird, Bullock's oriole, redwinged blackbird, and blackheaded grosbeak. Birds that use the trees as nest sites include the brown creeper, which makes its nest under peeling shags of bark and feeds on trunk insects and spiders, the robin, the chickadee, the downy woodpecker, and the red shafted flicker. The downy woodpecker and the red shafted flicker peck into the trunk of dead or dying trees to form their nests. When these nests are abandoned, chickadees, Bewick wrens, house wrens and starlings move in. Downy woodpeckers use dead stubs to hammer out a rhythmic pattern to declare their territories.
"The red-tailed hawk prefers tall trees for a nesting site. It therefore favors eucalypts over trees such as oak or bay. Great horned owls use nests that have been abandoned by red-tail hawks or they nest on platforms formed between branches from fallen bark. The brown towhee and the golden crowned sparrow are birds that use piles of debris on the ground for shelter during rains.
"Several mammals have adapted to Eucalyptus. Deer find concealment in dense groves where there are suckers, coyote brush, and poison oak; moles live in the surface layer of the soil, meadow mice, gophers, and fox squirrels are found in the forest.
"A Eucalyptus grove is not a sterile environment. The population density of the animals mentioned can be partially attributed to the presence of eucalypts. With a program of cutting trees and burning debris, some animal residents will disappear because they have restricted home ranges or are sedentary. If an animal's living area (leaf litter, logs, bark) and food supply are destroyed, the animal will either die or attempt to move to another area which is already fully occupied. "The wildlife section draws heavily upon conversations with Professor Robert Stebbins. No errors which may exist should be attributed to the professor."
Eucalyptus globulus can threaten native vegetation in a variety of ways. In coastal California where E. globulus receives enough moisture to propagate from seed, a coastal grove has the potential to spread 10 to 20 feet in diameter a year, eliminating the diversity of native species as it colonizes new ground. This aggressive Tasmanian species releases phytotoxins not only from its litter but also directly from its leaves (see previous Biology-Ecology section under "Allelochemicals").
Phytotoxins exuded through the pores on the leaf surfaces are transported by condensation, fog drip, and rain creating a ring around the base of an individual tree with a relative paucity of herbs. It was also proposed by Dr. Leisner, of the Department of Environmental Horticulture at the University of California, Davis, that another reason for the absence of other plant life beneath the trees might be the strong competition for water exerted by the trees, which outcompete other plants (Brown 1983).
In the inland valleys of California where E. globulus was cultivated as a source of fuel, timber and windbreak, this species does not receive enough moisture to propagate from seed. However, the germination inhibitive chemicals it exudes still restrict the diversity of species directly underneath the crown.
Jepson Prairie Preserve in the Central Valley, California, has had an active eradication program for four years. In the Eucalyptus Control Proposal (Jepson Prairie Preserve Committee 1984), Leitner includes the following as other adverse effects of E. globulus populations:
- In areas of grazing they encourage uneven distribution of livestock and their excreta.
- They maintain disturbed areas (bare ground and weedy patches under trees).
- Their high terpene litter and other phytotoxins not only reduce grassland habitat, but they also adversely affect vernal pool water quality.
- They produce unsafe conditions for prescribed burning due to high fuel loads and high energy content.
With proper management, areas infested with E. globulus can be restored to more desirable vegetation (please see MANAGEMENT PROCEDURES section).
BIOLOGICAL MONITORING NEEDS:
Monitoring is needed to determine the effectiveness of management practices.
BIOLOGICAL MONITORING PROCEDURES:
Detailed observations focused on the vegetational change of the affected area over time will help to determine what method of control would be most efficient for a given site.
BIOLOGICAL MONITORING PROGRAMS:
In April 1985, an extensive vegetation sampling program commenced at The Nature Conservancy's Jepson Prairie Preserve, in California's Central Valley. Jepson Prairie is a grassland with mima mound topography and vernal pools. Approximately 120 acres of this 1600-acre preserve have been or continue to be populated by E. globulus. Transects through the area where E. globulus had been removed and where it still remains were established. A control was set up in an area adjacent to the groves. The transects were approximately 200 m long, with 40 plots per line. Nested plot frequency methods were employed to record density and cover of species found within the half-meter-square quadrats. Permanent plot markers have been placed on the transect. Data will be gathered each spring to determine the effects of management activities.
Contact: Rich Reiner, Stewardship Ecologist The Nature Conservancy California Field Office 785 Market Street, 3rd Floor San Francisco, CA 94103 (415) 777-0487
The following are specific questions that need study to improve conservation or control efforts:
- What can be done to prevent subsequent germination after E. globulus has been removed from an area where it propagates readily?
- What are the state-wide germination needs of E. globulus in California and how do they differ?
- What are the effects of E. globulus allelochemicals on the soil? Are these effects sustained?
- What is the percent increase of native species over time in an area where E. globulus has been removed?
- What pattern of burning and physical removal of regrowth from stumps would be most effective?
This weedy tree species does require active management. Researched methods of control are listed below.
Mechanical methods of control seem to have the least amount of impact on the surrounding area. The following is largely based on personal communication since published information on controlling E. globulus with purely mechanical means is virtually nonexistent. Niel Havlik (1985), Resource Ecologist at East Bay Regional Parks, suggests that initially removing the trees and returning twice a year to remove subsequent regrowth could take over three years for a high percentage of kill. Removal should commence in the spring when biomass production is most vigorous. Regrowth is most vulnerable to cutting when the shoots are six to eight feet high for they are still a major net energy investment for the tree. Havlik (1985) also mentioned that, where the native understory is fairly dense, pulling out the seedlings and saplings up to an inch in diameter has proven to be a successful method of halting a grove's spread.
Although Havlik (1985) found burning to be an ineffective method of control, it is conceivable that a burn program could enhance the effectiveness of manually removing regrowth. The results from the initial efforts of E. globulus removal at Jepson Prairie Preserve seem to indicate that burning did increase mortality. The following is excerpted from the Eucalyptus Control Proposal (Jepson Prairie Preserve Committee 1984) by Leitner, former California Land Steward:
"During the cleanup phase of January-April 1982 and November 1982-August 1983, Eucalyptus slash was stacked over selected stumps and burned. The slash was piled in windrows up to six feet high and six feet wide. Very few mortalities resulted from this burning.
"A second episode of burning happened accidentally when a controlled burn escaped from the Gridley property in the NE 1/4 of Section 13 in early July 1983. The fire eventually burned nearly all the Eucalyptus then under treatment. It top-killed virtually all of the regrowing suckers in the area. As of early November, nearly four months later, about half of the trees showed no regrowth at all, and the other half appeared to be regrowing very slowly. Those trees showing vigorous recovery appeared to have been exposed to the fire very little because of low fuel levels around their bases.
"A third, barely-tested burning technique may be promising under some conditions. Emergent new growth may be treated with a flame gun, which kills the new growth and some cambium at the same time. This technique appears to be most effective when new growth is relatively water stressed, such as in late summer, but has little effect after the onset of fall rains.
"The flame gun was used on re-emerging vegetation following the 1983 wildfire. It apparently was a final blow to already heavily-stressed plants and most of the remaining trees were then killed. A small amount of additional emergent growth was killed by defoliation and shoot removal."
Another alternative in mechanical methods of managing Eucalyptus is to remove the stump with a stump grinder or tractor. The Vermer Company is one manufacturer of stump grinders. The blade cuts in a vertical position and slowly swings back and forth. The whole apparatus can be positioned by a small tractor, aligning it over the stump and anchoring the cutting wheel with its supporting arches. The grinding process takes approximately 30-90 minutes per stump and chews at least two inches down into the ground and root crown.
Another available stump grinder is called the SHAR-20. It is manufactured by Shar Corporation and has a horizontal blade. It rides on a truck chassis, and there is no set-up required. Also there is no soil damage from this stump removal procedure. The SHAR-20 removes the stump faster than a conventional stump grinder and costs approximately $125,000 or $120/hour to rent. The ground level stump can then be covered with dirt or black plastic (or herbicide) to slow and prevent regrowth. A conventional tractor with a blade can ideally knock a four foot stump out of the ground (Jepson Prairie Preserve Committee 1983).
Detailed information on herbicides are available in such publications as the Herbicide Handbook, published by the Weed Science Society of America (Ahrens 1994) or USDA (1984), and will not be comprehensively covered here. The Weed Science Society publication gives specific information on nomenclature, chemical and physical properties of the pure chemical, use recommendations and precautions, physiological and biochemical behavior, behavior in or on soils and toxological properties for several hundred chemicals. In applying herbicides it is recommended that a dye be used in the chemical mixture to mark the treated plants and thus minimize waste.
When considering herbicides as a method of eradicating or controlling E. globulus stands, factors such as species composition, stem size, height growth, location, size of the grove, climate, and time of the year must be kept in mind. These factors will ultimately play a large part in determining the chemical used and method of application. Bachelard et al. (1965) writes, "Although some good results have been obtained [through the use of herbicides] complete control [of Eucalyptus globulus] has rarely been achieved, and the interaction between species, chemicals used, and the time of application make it difficult to make overall recommendations for the control of Eucalyptus vegetation."
This remark is further substantiated by Morze (1971) who wrote that E. globulus should be considered a "resistant" species of Eucalyptus; "resistant species" meaning that even double strength herbicide is not completely effective and that treatment will most likely have to be repeated. Alternating chemicals may prove more effective than using one type alone (Morze 1971).
There are two basic methods of applying herbicides to Eucalyptus stands. The most common (and effective) method of treatment is to bring the chemical in direct contact with the live, woody tissue of the tree. This can be accomplished in a variety of ways. A freshly cut stump acts as a sponge, transferring toxins to its roots, thus spray, liquid or crystal forms of herbicide can be effective. Follow-up treatment will be necessary each time the unaffected buds produce coppice sprouts of a height of six to eight feet. This treatment could take up to three years or until the carbohydrate reserves are depleted.
If felling the tree prior to treatment is not an option, frill-cutting the tree is an excellent alternative. Frilling, the method of placing a ring of downward axe-cuts around the base of the tree and filling the cuts with herbicide, has received very good results. Another common method of treatment is to spray chemicals directly on the foliage. This practice is only employed to treat coppice sprouts or saplings. The method of drilling holes in the trees and filling them with the chemicals or injecting chemicals into the tree has thus far not proven successful in eradicating E. globulus.
As mentioned above, time of application is an important factor when using herbicides. As a rule of thumb, optimum results of herbicide applications are obtained during the early summer months (Morze 1971).
Ammate (ammonium sulfamate), arsenic compounds, systemic herbicides (picloram or Tordon®) and hormone herbicides 2,4-D (2,4- dichlorophen oxyacetic acid) have been commonly used as chemicals for direct application to actively growing cells in E. globulus. Because the chemical is given direct access to the growth tissue, water soluble chemicals can be used. There is little lateral movement of water solutions in a plant stem, therefore the herbicide should be placed around the full circumference of the tree. It is also important that the herbicide be applied immediately after the cut is made to avoid blockage of cells by air pockets and subsequent poor absorption of the herbicide (Forest and Richardson 1965). The stumps should be cut no more than six to ten inches from the ground to insure delivery to the tree's root system.
Ammate is usually used in the crystalline form when treating stumps, but it can also be dissolved and applied in boreholes or sprayed on foliage. As a crystal, ammate takes up water from the air and moves in solution through the cut surface. Capping the stump with black plastic once the ammate crystals have been applied appears to increase mortality (Burke 1985). On relatively susceptible Eucalyptus species, ammate was applied at a rate of 30 percent acid equivalent (a.e.) with a rate of kill between 95 and 100 percent (Bachelard et al. 1965). From this information we can surmise that a rate of 76 percent a.e. would be sufficient on E. globulus.
Debarking or frilling the stumps prior to poisoning was found to be beneficial but not necessary as long as the stump was freshly cut and the surface thoroughly saturated by spraying or by brush application. Spraying should be done in windless conditions and under moderate pressure to avoid spray drift. Dry weather during treatment was found to be very important.
Bachelard et al. (1965) also found Tordon® to be effective at a very low acid equivalent (a.e.) rate. At 0.1 percent a.e. it worked well against more susceptible eucalypt species. Sodium arsenite (8 percent a.i.) was not found to be nearly as effective as the aforementioned chemicals in preventing coppice growth. For all cut-stem treatments it is profitable to add a dye to the herbicide solution so that treated stumps will be readily apparent.
The effectiveness of applying Roundup, a glyphosate, is unclear. Niel Havlik (1985) explained that applying Roundup to a cut E. globulus stump is "fairly effective," although he does not recommend its use. He estimated that if the application did not cover the cambium, or if the a.e. was not high enough, the recovery potential can be up to 70 percent (Havlik 1985).
In Havlik's opinion, the high recovery potential of the Eucalyptus along with the hazards of handling this chemical render this method unworthy of employment. Havlik's opinion seemed to be substantiated after Roundup was chosen for 1981-1982 treatments of E. globulus at The Nature Conservancy's Jepson Prairie Preserve. The mortality resulting from the treatment was low, about 5 percent or less. On the other hand, University of California tests produced 85 percent mortality. Though the former result may have been due to multiple factors, the most probable ones are: (1) too much time elapsed between cutting and herbicide application; (2) wrong form of herbicide used; or (3) inappropriate concentration or quantity of herbicide (Leitner 1984).
As mentioned above, the axe-frill method of herbicidal application involves putting a series of contiguous cuts around the stem in order to provide a small reservoir for chemical solution. It is advisable to make the number of axe cuts not less than one for every two inches of diameter or at three inch intervals around the trunk (Morze 1971). The cut of the axe blade must penetrate into the sapwood. It is important that the cut and herbicide be placed as low as possible to the ground to restrict coppicing. The cut should be frilled at least to the point of run-off. Stems too small for frilling can be cut off leaving a dish shaped or V-shaped stump which will hold the herbicide and ensure penetration.
In one experiment (Mears 1966) picloram was injected into frills no more than six inches apart on a Eucalyptus species. One cubic centimeter of a solution of one part picloram and four parts water were injected into each cut (2 cc is the recommended rate of application). Fifteen percent of the treated area produced coppice sprouts. It was observed that there was a reduced percentage of coppice sprouting on trees that were treated following a rain.
In another experiment (Morze 1971), frill cuts were made 0.9 to 1.2 m from the ground at approximately 5-cm intervals.
Tordon® was proven to be most deadly in these trials. At a strength of 0.5 percent Tordon® 22K (24.9 percent picloram as potassium salt) produced 25-40 percent mortality. Tordon® 101 (10.2 percent picloram and 39.6 percent 2,4-D, both as trisopropanolamine salts) produced 90-100 percent mortality at a strength of 12.5 percent. Summer applications were so much more effective that applications during winter were advised against. Morze (1971) commented that treated trees should die within three months if an herbicide is applied during the growing season. The symptoms of poisoning are said to be arrested by the end of the fourth month and recuperation starts soon afterwards. By then, a second dose should be given to the survivors.
Bachelard et al. (1965) found sodium arsenite (8 percent a.e.) to be a very effective Eucalyptus species deterrent when applied to frills. It was especially effective if applied in late winter or early spring. The best alternative chemical in these experiments was picloram. Picloram was applied as Tordon® 50D which contains 5 percent picloram w/v and 20 percent 2,4-D w/v. This was most effective (on susceptible Eucalyptus species) in a concentration of 1.0 percent a.e. in partial frills or 0.5 percent in complete frills. Other chemicals tested included, 2,4-D, Piquat, and ammate.
Experimentation carried out by the East Bay Regional Park District, CA, in 1973 concluded that axe-frill treatments using 2,4-D, glyphosate (Roundup) were not significantly reduced in effectiveness when applied with 50 percent water. Percent sprout control was still between 90 and 100 percent. Ammate applied at 95 percent concentration yielded 89 percent sprout control.
If a tree has been felled, applying herbicide to the suckers and regrowth for up to three years can produce a high percentage of mortality. However, foliage spraying should be confined to sprouts no more than eight feet tall, the optimum being two to three feet high. Where stems are higher they should be removed and allowed to reshoot to two feet high, at which point foliar treatment can be used (Forest and Richardson 1965).
The overall spraying of coppice can be accomplished either with a hand pumped type of compression sprayer, discharging more or less coarse spray according to nozzle adjustment and pressure, or with a motor-powered knapsack sprayer producing a fine-mist spray. In the first method, spraying is carried out from close range, and a high volume of herbicide mixture is used, some 80 to 120 gallons per acre (909 to 1364 liters per hectare) to wet the stems generously and to saturate the leaves.
In the second method the volume of herbicide used is between 25 and 30 gallons per acre (280 to 337 liters per hectare), the spraying is carried out from a distance of 10 to 12 feet (3 to 4 m), and the treated foliage is made just visibly moist. A single treatment is more effective in the case of the high volume spray, but no treatment is 100 percent effective and replication of treatment should commence 3 to 4 months later. For the second application the strength of the herbicide mixture should be doubled (Morze 1971).
"The most successful sprayings are those in late spring or early summer, when the coppice is some 2 to 3 feet (0.5 to 1.0 m) tall and the young foliage well developed, turgid, and plentiful. Warm, dry weather enhances the absorption and translocation, while a rain shower occurring less than 12 hours after treatment may drastically reduce the poison's effects. Spraying should be done in windless conditions and under moderate pressure to avoid spray drift" (Morze 1971).
When applied at a strength of 0.2 percent a.e., Tordon® 22K yielded a 76 percent kill in a single high volume spray of 100 gallons per acre. Tree density must be taken into consideration along with number of acres. Tordon® 101 yielded 88 percent kill with the same application rate. The mist spray treatments of Tordon® 101 had to be repeated twice to yield 90 to 100 percent mortality.
The data from the experiments by Bachelard et al. (1965) show that picloram applied as Tordon 22K is the most effective chemical for killing sprouts of Eucalyptus that they tested. Complete control of some mildly resistant Eucalyptus species has been achieved with a 0.04 percent acid equivalent. (Bachelard et al. 1965).
Counts of the living trees per plot were made prior to treatment and one year later. Spraying in early fall (rather than late) made a remarkable difference in application efficiency. Ammate at the heaviest rate (100 lbs/acre) gave the most appreciable amount of reduction. Ammate applied with a wetting agent increased its effectiveness (Robertson 1960).
No published methods for the biological control of Eucalyptus globulus were discovered in the course of this research.
An active Eucalyptus control program has been in progress at The Nature Conservancy's Jepson Prairie Preserve since August 1981. At that time contracted woodcutters began removing the trees for firewood. They agreed to cut the trees at ground level, apply herbicide to the freshly cut stumps, and stack the remaining material too small for use as firewood. Unfortunately, the effectiveness of this method was only 5 percent (or less) mortality. It is believed that the herbicide may have been ineffective because it was applied too long after cutting.
Leitner (1986) reports that the current plan is to cut down the remaining trees with chain saws, cutting as close to ground level as possible. Timber will be cut into firewood and the remaining vegetation stacked in windrows. The slash will be burned when it is sufficiently dry. In the S 1/2 SW 1/4 of Section 13, the regrowth will be repeatedly cut until the trees are dead.
Contact: Tom Griggs, Area Manager Cosumnes River Preserve 7100 Desmond Road Galt, CA 95632 (916) 864-2816
The Marin County State Parks have had an active management program against E. globulus for the past five years. Mt. Tamalpais, Angel Island and China Camp state parks have all combatted E. globulus. At Mt. Tamalpais the Resource Ranger, Randy Hogue, is required to organize the removal of 10 percent of the eucalypts per year. The method of treatment he employs is to cut the smaller individuals and paint a 30% solution of Roundup on the live tissue. At China Camp, Ranger Larry Perkins treated cut stumps with ammate crystals 1/4 inch thick and covered them with black plastic. He estimated that there was a 50 percent recovery. He treats sprouting foliage with a 2% solution of Roundup for control.
There is little or no spread of eucalypts at China Camp. Angel Island has had an extensive Eucalyptus removal program. Craig Burke, State Park Ranger for Angel Island, explained that their removal program has included controlled burns, cut-stump treatments with ammate, driving zinc nails into a cut stump and coppice removal. Currently, the park staff is selectively cutting Eucalyptus along the roadside. Apparently the most successful program was one in which seasonal workers were hired to cut wood for sale as firewood in other Marin County state parks. Unfortunately the bureaucracy put an end to that program. They did see some success with the ammate crystals covered by plastic. The zinc nail treatment turned out to be a rumor.
Contact: Dave Boyd, Resource Ecologist Regional Office for State Parks 3033 Cleveland Avenue, Suite 110 Santa Rosa, CA 95403-2183 (707) 576-2185
The current control methods for E. globulus practiced by Niel Havlik (1985), Resource Ecologist for East Bay Regional Parks, is to spray a 2% solution of Roundup on sprout foliage once they achieve a height of 3 to 6 feet. He claims that this method yields a 98 to 99 percent control. It has be applied when there is no wind, otherwise there are hazards with spray drift. In Havlik's opinion this is the most cost- effective method for controlling sprouts.
Contact: Niel Havlik, Resource Ecologist East Bay Regional Parks 11500 Skyline Blvd. Oakland, CA 94619-2443 (415) 531-9300
Eucalyptus globulus has evolved such highly effective mechanisms for coping with the threat of fire that its persistence outcompetes most single attempts to kill it. Whatever method is appropriate for conditions under which it is being treated, repetition of the treatment is practically unavoidable. Under mechanical means of control, removal of the tree and subsequent coppice shoots could take up to six years before carbohydrate reserves are depleted. It is feasible that a pattern of burning and cutting could be more effective. Stump removal, although effective, is costly and impractical on a large scale.
Chemical means of control are of two main types. Application of herbicides can be foliar, in the form of a spray, or directly onto the inner tissue of the trees. Direct application of herbicides to the growing tissue has been proven more effective than foliar spraying. Cut stumps can be treated with crystals of ammate, 2,4-D, and picloram with a high percentage of mortality. Axe frills have been proven to be very effective as reservoirs to hold herbicide as it soaks into the growing tissue; picloram, 2,4-D, and ammate have all been successful to varying degrees.
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Update Of Document: 89-03-22