Authors: Don Pitcher, Global Invasive Species Team, The Nature Conservancy
- Xanthium strumarium is an annual that produces a conspicuous prickly 'cocklebur' and ranges from 0.5-6.5 ft. (0.2-2 m) in height. The relatively large, linear to oblong waxy cotyledons helps to distinguish this weed in the early stages of development.
- The first true leaves are opposite, all subsequent leaves are alternate. Leaves are triangular to ovate in outline, have stiff hairs, and are approximately 2-6 in. (5.1-15.2 cm) long. Leaves are irregularly lobed with leaf margins that have relatively inconspicuous teeth.
- Inconspicuous, greenish in color, arising from the area between the leaf petioles and the stems and at the ends of the erect stems.
- An elliptic to egg-shaped two-chambered bur, 0.5-1.5 in. (1.3-3.8 cm) long and covered with hooked prickles. Each bur contains two seeds, one that grows during the first year and one that grows a year later. Two prickles that are longer and wider than the remaining prickles project from the tip of the bur.
- Ecological Threat
- Xanthium strumarium is found throughout the United States and is primarily a weed of agronomic and horticultural crops, nurseries, and occasionally pastures.
Xanthium strumarium is a coarse annual herb. The name Xanthium is derived from the Greek XANTHOS, meaning yellow, from the ancient name of some plant, the fruit of which was used to dye the hair that color.
Many specific epithets have been applied to Xanthium strumarium, including: orientale, canadense, chinense, occidentale, macrocarpum, longirostre, pennsylvanicum, and oviforme. The consensus of taxonomic opinion follows Love and Dansereau's (1959) suggestion that these "species" are actually subspecies or varieties of this highly variable weed. They suggest that X. strumarium consists of seven complexes: strumarium, cavanillesii, oviforme, echinatum, chinense, hybrid, and orientale. There is no evidence of any sterility barriers separating the entities of X. strumarium, but intense inbreeding with occasional outbreeding is responsible for the enormous variation which often results in small, local, but unstable taxa. At least seven varieties or subspecies have been described from California, but today these are generally considered part of the cavanillesii (pennsylvanicum) morphological complex. However, McMillan (1975) considers this a separate species, X. californicum Greene.
Xanthium strumarium is distinguished from spiny clotbur (X. spinosum) by its broader cockleburs, more ovoid leaves on long petioles, and lack of spines.
Xanthium strumarium is a common annual weed spread by water, humans, or other animals. Its origin is still being debated, but cocklebur may be a native California species. It is most abundant on moist open sites but is present on a variety of waste places. Cocklebur is toxic to certain animals. It reproduces from seeds that are viable for up to several years. Biological control measures are currently being investigated and may prove effective in the future. Simple mechanical removal prior to flowering is recommended for control. If pulled following flowering, the plants should be burned. Monitoring should be continued on the sites for several years.
Xanthium strumarium is distributed worldwide (53 degrees north to 33 degrees south latitude) but is most common in the temperate zone. It is a serious weed in Australia, India, South Africa, and the Americas.
There has been considerable controversy regarding the origin of cocklebur. Though first described from Europe, it is probably of American origin. Love and Dansereau (1959) suggest that the cocklebur subspecies most abundant in North America (cavanillesii) originated in Central America. The dates of its introduction to California are not known, but it may be pre-Columbian.
Cocklebur is often associated with open, disturbed areas, particularly flood-prone areas with good soil moisture, but it is found in a wide variety of habitats. It frequents roadsides, railway banks, small streams, and riverbanks, as well as the edges of ponds and freshwater marshes and overgrazed pastures. It does not tolerate shading.
Cocklebur grows on a wide range of soils (sands to heavy clays) and available moisture. On rich soils with abundant moisture and little competition from other plants, it grows tall and luxuriant, forming pure stands. In dry, poor soils, plants may grow to only a few centimeters high, persist through drought, and set seed. The ability to grow under a variety of conditions results in a continuous seed supply, if plants are not controlled.
Cocklebur withstands partial submergence for six to eight weeks by forming adventitious roots from the submerged portion of the stem. These roots float in water and often get infested with oxygen-producing green algae (Dedogonium) which solves the problem of aeration.
Weaver and Lechowicz (1983) describe two types of cocklebur populations. Populations located along shores or water courses tend to be small, ephemeral, and homogeneous with seed dispersal by wind and water. Populations in ruderal (weedy) habitats, agricultural fields, or waste areas tend to be large, dense, and heterogeneous with tall, vigorous plants producing an abundance of seed. Seed dispersal here is primarily the result of human activities. Both types of populations, however, occupy unstable habitats and are continually shifting to newly disturbed areas.
Cocklebur is an extremely competitive weed in corn, cotton, and soybeans fields, particularly in the southeastern and midwestern U.S.. Though not as abundant in California, it is still a serious problem in agricultural areas, as well as in recreation areas and along reservoirs. Some plants appears to have allelopathic properties.
The burs cause an allergic reaction in some people and are toxic to domestic animals (and perhaps to some wildlife). Poisoning threats are greatest in areas where other, more palatable plants have already been consumed. Ingesting an amount of seeds equal to only 0.3 percent of an animal's body weight will cause toxicity. Still, this rarely occurs as the spiny burs are not palatable to animals.
However, the cotyledons are palatable and also have the highest toxicity. Poisoning generally results when these are eaten. This situation occurs most at the edges of ponds, lakes, flood plains, or other bodies of water where shallow flooding followed by recession of the waterline occurs. Under such conditions seeds germinate readily, constantly supplying new generations of potentially poisonous seedlings as the water source dries out. Animals are attracted to such areas because of their need for drinking water. The problem is accentuated because Xanthium seeds do have natural dormancy and germinate over long periods of time. Ingestion of an amount of cotyledons equal to 0.75 to 1.5 percent of the animal's body weight will cause toxicity.
Toxicity decreases rapidly as true leaves are formed. Evidence of poisoning appears in about 12 to 48 hours, the symptoms being nausea, vomiting, lassitude, depression, weakened muscles, and prostration. Severe poisoning may result in convulsions and spasmodic running movements. Ruminants may not vomit. Death may occur within a few hours or days. Fatty substances such as milk, lard, or linseed oil have been recommended as antidotes.
Kaul (1971) includes the following reasons for cocklebur's ability to inhabit such a range of habitats: an effective dispersal mechanism, wide ecological amplitude, heavy output of seeds and high viability and germination under varied environments, high reproductive capacity, large seed size and weight, rapid seedling growth, and a well-developed root system.
Xanthium strumarium is wind-pollinated, self-compatible, and predominately self-pollinated. The staminate heads of X. strumarium are located above the pistillate heads on the main axis and side shoots, an arrangement favoring inbreeding. Moran and Marshal (1978) found the outcrossing rate in natural populations to be 0 to 12%.
In a Quebec experimental garden, individual plants produced from 611 to 1,488 male inflorescences. The 100-150 male florets in each staminate head begin to shed their pollen from a few days before the stigmata are receptive until all female flowers are ripe. The slightest movement of the plant or a gust of wind causes the pollen to rain down over the exposed stigmata of the female flowers immediately below. The pollen of the plant itself is therefore most likely to ensure the fertiliza- tion of its female flowers, and only an accident, a strong wind, or crowded growth, accomplishes cross-fertilization. In Xanthium, inbreeding is thus the rule and outbreeding only an occasional occurrence.
Cocklebur has been widely used as an experimental plant in studies of photoperiod. Love and Dansereau (1959) list 34 articles on Xanthium photoperiod, and many more have been written since that time. According to Salisbury (1969), X. strumarium is a short-day plant and usually does not flower when day length exceeds 14 hours. However, there is evidence of differences in light response among the complexes, as some plants flower with day lengths as long as 16 hours.
At high latitudes, day length is greater than 14 hours during summer, and therefore, X. strumarium does not flower until late summer, once day length is short enough to stimulate flowering. Seeds mature late under these conditions, usually in early autumn. These differences are considered to represent genetic adaptations of the reproductive system to environmental variables as a result of natural selection. The cotyledons do not play a role in flower induction.
SEED PRODUCTION AND DISPERSAL
Open grown X. strumarium plants produce 500 to 5,400 burs per plant. The number of fruits produced is dependent upon the amount of vegetative growth at the time of floral initiation. On crowded plants, production is reduced to 71 to 586 burs per plant. Burs are buoyant and will float for up to 30 days, thus being easily dispersed to beaches and pastures subject to flooding. The burs also become entangled in animal hair or human clothing. The burs are a serious problem in sheep production areas where they become entangled in the wool, reducing its value. X. strumarium burs contain a highly toxic substance, carboxyatractyloside, capable of killing hogs, cattle, goats, horses, sheep, and poultry.
SEED VIABILITY AND GERMINATION
Germination of cocklebur seeds has been extensively researched. More than 80% of cocklebur seeds are viable in most populations. Light is not required for germination, but seedlings seldom emerge from seeds lying on the surface or buried more than 15 cm in the soil (Kaul 1965a).
Seeds of Xanthium strumarium have a high moisture requirement for germination and show little germination in soils at less than 75% of field capacity, but they are able to absorb moisture at high osmotic concentrations. Cocklebur seed viability decreases over time, and seeds do not survive more than a few years. Seedlings are unusually large with foliar-type cotyledons that, through early photosynthetic function, enable the young seedling to become quickly established. Seedlings may be identified in the cotyledon stage by the presence (below ground) of the persistent bur, which usually remains attached to the seedling. The species does not reproduce vegetatively.
Xanthium strumarium plants produce seeds of two types (termed somatic polymorphism). Each bur contains two seeds, with the smaller one often pushed upwards toward the beaked end of the fruit. The lower seed has a shorter dormant period and germinates first. Dormancy in Xanthium involves the presence of a different water-soluble germination inhibitor in each seed type, to which the testa are impermeable. The presence of oxygen causes degradation of these two inhibitors and subsequent rupture of the seed coat, but apparently at very different rates in the two types. Thus at least two batches of seeds are present in each generation to assure germination in the event the immediate environment happens to be unsuitable.
Xanthium strumarium is considered one of the world's worst weeds. Cocklebur seeds are easily spread, due to their ability to float and to 'hitchhike' on humans and animals. The plants can quickly become dominant in an area because of their prolific seed production and high germination and survival rates.
Control of cocklebur requires active management once it becomes established in an area.
According to Weaver and Lechowicz (1983), young plants of Xanthium strumarium regenerate readily from the lower nodes if trampled, clipped, or otherwise injured. Fruit on older plants or shoots which have been cut or damaged will continue to ripen provided fertilization has occurred prior to the injury. Burs may persist on dead plants for up to 12 months. Since plants can regrow, mowing is not an effective control measure for cocklebur.
Physical removal of the plants by hand pulling or hoeing them is effective if done prior to flowering. If left until after seed development, plants should be carefully removed so as not to dislodge the burs, piled, and burned.
Burning is an effective means of destroying cocklebur seeds, but prescribed fire has seldom been used for this purpose.
Much work has been done on the insect pests of Xanthium strumarium . Hilgendorf and Goeden (1982, 1983) provide good reviews of sap and foliage feeding (phytophagous) insects associated with X. strumarium. They list 60 different species that attack it in different parts of the world. Although many of these also attack cultivated plants, eight feed only on plants in the Heliantheae Tribe (ragweeds and cocklebur).
The insect fauna of Xanthium species in the central U.S. is richer in species and trophically more specialized than in California. In California, these insects probably switched from ragweed when cocklebur reached the state. Nine insect species feed on cocklebur as immatures.
Several species of insects have been introduced to Australia to control Xanthium strumarium, but results have generally been disappointing. The most promising control species there appears to be Nupserha antennata Brun., a beetle native to India and Pakistan. Insect species associated with X. strumarium have also been studied in Pakistan (Baloch et al. 1968) and India.
Hilgendorf and Goeden (1983) suggest that Oedopa sp. nr. capito (Diptera) is probably the only insect species worthy of study as a potential biocontrol agent for Xanthium strumarium. Oedopa is restricted to the genus Xanthium, feeding on its roots. Baloch and Ghani (1969) suggest that a combination of insect species, with different feeding habits, would improve the chance of suppressing Xanthium populations.
Weaver and Lechowicz (1983) list 14 species of fungi that infect Xanthium in the U.S. and Canada. The rust Puccinia xanthii Schw., which occurs throughout the U.S., southern Canada, parts of Europe, and India, is an obligate parasite on species of Xanthium and Ambrosia. It attacks all aerial parts of the plant except the flowers. Infected plants mature more rapidly than healthy plants and show decreased transpiration, dry weight, bur production, and percent germination. The spores overwinter on dead plant parts. Fungal and bacterial pathogens have had some success in controlling X. strumarium in India. Kalidas (1981) induced rapid wilt in X. strumarium by using phytopathogenic toxins from seven different fungal and bacterial agents. Plant death was evident within 6 to 8 hours with each toxin. Sharma (1981) also describes a powdery mildew that infects cocklebur in India.
Nematodes reported from X. strumarium are Aphelenchoides ritzema-bosi Schmidt and meloidogyne hapla. Cuscuta pentagona (dodder) is a higher plant parasite that has been found on cocklebur. Orobanche ramosa L. (broom rape) is another parasitic plant found on a variety of cultivated and weedy plants, including Xanthium.
CONTROL BY GRAZING
Because of its toxicity and unpalatability, grazing is not a viable control method for Xanthium strumarium.
Cocklebur is susceptible to a wide variety of soil- and foliar- applied herbicides commonly used for the control of broad-leaved weeds , but certain Xanthium complexes are more susceptible than others. Dr. Jim McHenry , of the University of California, Davis, recommends the following herbicides for cocklebur control in California's preserves:
(1) 2,4-D amine, a phenoxy-type herbicide used for broadleaf weed control, should be applied to plants at the 3- to 5-leaf stage of growth. Application should be at the rate of 1 to 1.5 lbs/100 gallons of water, with one quart of surfactant/100 gallons. (Surfactants lower surface tension of the spray and increase the herbicide's effectiveness.) 2,4-D does not affect grasses.
(2) Dicamba (Banvel) is a broad spectrum herbicide used against perennial broadleaf weeds. It may persist in the soil for up to eight weeks. The suggested mixture is 0.5-0.75 lb/100 gallons water, with one quart of surfactant/100 gallons, and an application rate of 0.5-1.5 pints/acre (not to exceed 2 gal/acre in growing season). Dicamba is more selective than 2,4-D.
(3) Bromoxynil (Buctril, Brominal) is a contact herbicide which affects only the plants or portions of a plant actually contacted by the chemical. Therefore, adequate distribution of the chemical over the foliage is essential. Bromoxynil should not be used on grazed lands but is effective in controlling a wide variety of broadleaf weeds, including Xanthium strumarium. The suggested rate of application is 0.56-1.12 kg/ha.
(4) Selective weed oils. There are several petroleum oils used for weed control. The herbicidal use of oils depends on their chemical and physical properties. Most contact oils evaporate slowly and owe their plant toxicity to their high content of aromatic compounds. Spraying oil on cocklebur will be effective only if entire plants are coated.
Herbicides can be applied uniformly over an area (for large infestations) or by spot spraying individual plants. 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, as cone sprayers produce greater atomization of the chemicals and increase the chance of drift into unwanted areas. Spraying should be done on calm days when plant surfaces are dry.
Herbicide Resistance: Populations of this plant exist in the United States that are resistant to ALS inhibitors (B/2) and Organoarsenicals (Z/17).
Management Research Needs
The origin of Xanthium strumarium needs to be determined. Additional research is needed on the possible toxic effects of X. strumarium on wildlife, on biological control measures, and on the effects of prescribed fires.
Uchytil, Ronald J. 1992. Xanthium strumarium, Fire Effects Information System, U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory 
University of California, Jepson Flora Project 
Flora of North America, www.eFloras.org 
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