Authors: Don Pitcher and Mary J. Russo, Global Invasive Species Team, The Nature Conservancy
- Anthoxanthum odoratum is a perennial or winter annual grass that often produces a noticeable seed head by April or May in many of Virginia's hay fields. Anthoxanthum odoratum gives off a distinctive, sweet-smelling odor when mature.
- Leaves are rolled in the sheath and may reach 0.4 in. (10 mm) in width. Upper and lower leaf surfaces, as well as the leaf margins, usually have short hairs.
- A brownish-yellow spike that is approximately 0.75-2.75 in. (2-7 cm) long and 0.5-0.75 in. (1.3-2 cm) wide. Each spike contains many spikelets that are approximately 0.25-0.33 in. (6.5-8.5 mm) long.
- Seedheads develop around April and May and give off a sweet smell.
- Ecological Threat
- This plant is most commonly a weed of pastures, hay fields, roadsides, and ditch banks that is found throughout the southeastern United States and into Kentucky and West Virginia. It is native to Eurasia.
Latin Names: Anthoxanthum odoratum
Common Names: Sweet vernal grass
Anthoxanthum odoratum is a sweet-smelling perennial grass with flat blades and spikelike panicles.
The name Anthoxanthum is derived from the Greek, ANTHOS meaning flower and XANTHOS meaning yellow, alluding to the yellow inflorescence.
Anthoxanthum odoratum can be distinguished from other grass species by its perennial habit and sweet, vanilla scent, especially when dry.
Anthoxanthum odoratum is a sweet-smelling perennial grass of Eurasian origin that has escaped from cultivation in North America. It occurs primarily on poor soils but is not considered a major weed pest. It reproduces by seeds and can be highly competitive with other grasses, particularly during the spring. Sweet vernal grass also shows a remarkable ability to genetically adapt to different environmental conditions. It can be controlled by hand pulling individual plants, hand hoeing, mowing, or by using the herbicide Dalapon. Monitoring needs to be continued after the initial removal of Anthoxanthum odoratum plants because of their potentially long seed dormancy in the soil.
It ranges from northern Florida to southern Canada along the East Coast and west to the Mississippi River flood plain. It also occurs from northern California to Vancouver Island, Canada (Hitchcock 1971, Grant and Antonovics 1978). It was introduced to North America from Europe in the late 1700s as a meadow grass and has since escaped cultivation (Munz and Keck 1973, Grant and Antonovics 1978).
Sweet vernal grass is a native of Eurasia (Hitchcock 1971, Munz and Keck 1973). It occurs on poorer soils in dry fields, lawns, meadows, and waste places (Muenscher 1955).
Growth and Competition
When grown with other common pasture grasses, Anthoxanthum odoratum is highly competitive. In a North Carolina study, it generally dominated mixtures that included Plantago lanceolata, Poa pratensis, Salvia lyrata, and Rumex acetosella (Fowler 1982).
In a study of competition with Lolium perenne, Dactylis glomerata, and Holcus lanatus (Remison and Snaydon 1978), sweet vernal grass proved highly competitive in the early spring due to its rapid growth and early flowering. Later in the summer it declines in aggressiveness. Among these species, sweet vernal grass is the most highly competitive for potassium (Remison and Snaydon, 1978). A Dutch study revealed similar results (Berendse 1983). Even at low densities, competition between Anthoxanthum odoratum and Plantago lanceolata is very strong. Anthoxanthum's competitive ability is highest in early spring.
In competition experiments (Remison 1978) with Holcus lanatus and Dactylis glomerata, Anthoxanthum odoratum had the lowest yield, especially at low densities. It proved most competitive at intermediate densities. Again, however, sweet vernal grass was most competitive early in the growing season, due to its rapid spring growth.
Anthoxanthum odoratum roots are quite shallow, absorbing nutrients mainly from the upper 10 cm of soil (Berendse 1982). It commonly occurs on soils that are low in phosphorus (Roberts 1982). On fertilized soils, sweet vernal grass develops a dense surface root mat (Remison and Snaydon 1978). Although Anthoxanthum tillers profusely, its poor root growth reduces its competitive ability (Remison 1978). The ratio of roots to shoots is therefore quite low at 0.(55) (Remison and Snaydon 1978).
In mixed species plots, intraspecific competition is more important than interspecific competition (Berendse 1983). Like many other grasses, sweet vernal grass contains allelopathic chemicals that suppress the growth of other plant species (Scott 1975). It is "allo-inhibited," growing better with exudate from its own and similar species in permanent grasslands (Newman and Rovira 1975).
The decomposing roots of Anthoxanthum odoratum have an effect opposite that of the allelopathic leachate. Old roots appear to enhance the growth of other grass species, while decreasing the growth of new Anthoxanthum plants. Phosphorus content in the roots averaged 0.155%, a relatively high level. This is probably one of the factors leading to the increased growth of other species on sites with decomposing sweet vernal grass roots (Newberry 1979).
Anthoxanthum odoratum plants average 3.5 leaves per shoot. The individual leaves are unusually short-lived, with a mean survival of only 19 days. Only a few leaves survive for three months. This rapid turnover of leaves is probably a reflection of the plant's rapid growth rate (Sydes 1984).
The biology of Anthoxanthum odoratum populations is complex and depends upon the particular habitat. Different character complexes are favored by different habitat types leading to substantial phenetic differences between ecologically marginal and central populations. Ecologically marginal populations have higher turnover rates than central ones. Individual life expectancy is shorter in marginal populations, while central populations contain a skewed age distribution in favor of older individuals, making them potentially less stable (Grant and Antonovics 1978).
In response to contrasting environments, sweet vernal grass can rapidly differentiate, even without geographic barriers. In one study (Kiang 1982), significant differences in morphology, flowering date, and pollen fertility developed in less than 40 years. Such differences among populations were genetic adaptations to contrasting soil environments. Similar results are reported by Snaydon and Davies (1972, 1976) for populations at distances of less than 30 m apart.
When environmental boundaries are distinct, Anthoxanthum odoratum populations can exhibit sharp differences. Plants growing only 0.1 m apart across a sharp environmental boundary that had existed for less than 120 years showed significant differences in height, yield, flowering date, and morphology (Snaydon and Davies 1976).
Anthoxanthum odoratum reproduces from seeds, flowering early in the spring and forming distinct, identifiable clumps that do not spread vegetatively (Grant and Antonovics 1978). The plants are generally self-incompatible (Antonovics 1972). Plants in North Carolina were reported to produce from 58 to 1,257 seeds per plant (Grant and Antonovics 1978).
Seed Dispersal and Germination
Seeds with awns are able to move further along the soil surface than awnless seeds. Anthoxanthum odoratum seeds have awns that wind and unwind with changes in atmospheric humidity. The movement of seeds by this mechanism is not great, averaging 1.6 cm on dry soil and 1.3 cm on wet soil over a 5-day period, but it may give the seeds a slight advantage in reaching suitable germination sites (Schonfeld and Chancellor 1983).
Harris (1961) found some dormancy in Anthoxanthum odoratum seeds. Most germination began the 4th week after sowing and ceased by the 8th week, but a few seeds germinated more than a year later. In experimental studies in England (Williams 1983a), A. odoratum seeds stored for 48 weeks in dry storage resulted in 86% germination. There is thus indirect evidence of seed dormancy, which may increase a seed's chance of being incorporated into the soil while still viable. For seeds sown on moist soil, however, emergence occurred rapidly (within three months). Those seeds that did not emerge within 10 months were not viable. Emergence was highest for seeds sown at a depth of 1 cm (75% viable) and decreased to 52% for seeds sown at a depth of 5 cm.
In another British study (Williams 1983b), Anthoxanthum odoratum was compared with eight other grassland species. It showed a greater ability to germinate under constant conditions than when conditions were varied. When A. odoratum seeds were stored for 28 weeks, a much higher percentage of the seeds germinated when stored at constant temperatures.
Anthoxanthum odoratum occurs on Nature Conservancy grassland preserves in Oregon from west of the Cascade Mountains to the coast. At present, it is not a significant threat to California preserves, but where present, as at the Northern California Coast Range Preserve, its elimination is the ultimate goal.
With the right combination of control measures, it should be possible to eliminate sweet vernal grass from selected areas. Annual monitoring and removal must continue until soil seedbanks are exhausted.
Control of Anthoxanthum odoratum requires active management once it becomes established in an area. Control through prescribed burning is being attempted on Oregon preserves.
The physical removal of sweet vernal grass by hand pulling or hoeing is an effective method of control for small patches. Control can also be obtained by mowing early in the season and before the seeds have matured. Badly infested fields should be plowed and seeded with a cover crop (Muenscher 1955).
According to Charles Turner of the USDA Biocontrol Lab in Albany, California (pers. comm. 1985), grass species such as Anthoxanthum odoratum present a major problem for biocontrol techniques. The lack of known host-specific insect or disease pest controls means that more experimenting must be done. Such experiments could damage ecologically or agriculturally important grass species. In addition, sweet vernal grass is not considered an undesirable plant everywhere. Keeping insects or pathogens within property boundaries is impossible.
Control by Grazing
Grazing is not known to be a useful control method for Anthoxanthum odoratum.
A variety of herbicides are effective on Anthoxanthum, but they will also kill other, more desirable grasses. Dalapon (Dowpon) provides excellent control of Anthoxanthum odoratum (Fryer and Makepeace 1978). Dr. Jim McHenry (pers. comm. 1985), of the University of California, Davis, recommends it to control sweet vernal grass on Nature Conservancy lands. It will kill both annual and perennial grasses but is not as effective on broadleaf plants. Dalapon should be applied in the spring when there is maximum top growth, prior to the appearance of the seed head. Apply the herbicide at the rate of 10 lbs/100 gal of water along with one quart of surfactant. Surfactants lower the surface tension of the spray and increase the herbicide's effectiveness. Dalapon is cleared for use on right-of-ways and grazing lands (if spot treated). Dalapon has low toxicity to animals and fish and breaks down rapidly and completely in soils (Beste 1983).
Herbicides can be applied uniformly over an area for large infestations or by spot spraying individuals plants. Due to the nonselective nature of Dalapon, spot spraying is recommended for most purposes. Dr. McHenry recommends using a flat-fan nozzle (Spraying Systems Co. #8003 or #8004 nozzle tip) rather than the cone nozzles available on most garden sprayers. Cone sprayers produce greater atomization of the chemicals and increase the chance of unwanted drift. Spraying should be done to dry plants on calm days since dew or rain will dilute the herbicide, reducing its effectiveness. When spraying large areas, a horizontal boom (6-8 feet long) made from aluminum tubing will improve herbicide coverage.
Annual monitoring following prescribed burning continues at Cascase Head and Willow Creek preserves in Oregon.
Monitoring is needed to determine the effectiveness of any sweet vernal grass control measures. Methods will vary depending upon the degree of the problem on a given preserve.
A. odoratum is being monitored on all Oregon preserves where it occurs using nested frequency methods and, in some cases, with density counts (Macdonald pers. comm. 1988). It is not being monitored in California.
For more information on monitoring efforts in Oregon contact: Cathy Macdonald, Land Steward The Nature Conservancy Oregon Field Office 1205 NW 25th Avenue Portland, OR 97210 (503) 228-9561
Management Research Programs:
Research burns were conducted in 1986 and 1987 at Willow Creek Preserve, OR. Additional research burns were done at Cascade Head Preserve, OR, in 1988. It is still too early to evaluate the results of these burns on the control of sweet vernal grass (Macdonald pers. comm. 1988). For further information contact: Cathy Macdonald, Land Steward The Nature Conservancy Oregon Field Office 1205 NW 25th Avenue Portland, OR 97210 (503) 228-9561
Management Research Needs:
Very little research has been done on controlling sweet vernal grass. Much research needs to be done in the areas of biological control, prescribed burning, and mechanical removal techniques.
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