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An Assessment of Exotic Plant Species of Rocky Mountain National Park

Euphorbia esula L. Tithmalus esula
Leafy spurge (Euphorbiaceae)


Current level of impact
Known locations in RMNP: Beaver Meadows, McGraw Ranch.
Assessment: Several widespread and dense populations in RMNP. When added together, all populations would cover and estimated area less than 5 hectares. Currently not found in areas with no known disturbance. However, leafy spurge has potential to invade and modify native plant communities.

Distribution
Origin: Naturalized from Europe, brought to the U.S. as a seed impurity in 1827.
Geographic distribution: Widely scattered throughout U.S. Throughout most of the northern half of the U.S., north into Canada from Nova Scotia to Alberta. In Colorado, found in cultivated soil from 5000 to 6500'.
Ecological distribution: Weedy species of open habitats. Waste areas, pastures, roadsides, cultivated fields. Infests irrigation ditches, roadsides, fields, woodlands, shelter beds, disturbed sites, rangelands, and especially sub-irrigated meadows.
Soils: Tolerant of a wide range of soils, and may occur in rich damp soils such as the bank of rivers or on dry nutrient poor soils common to the western U.S. Grows most rapidly on coarse-textured soils.

Reproduction
Perennial herb, reproduces by extensive slender rootstocks and seeds. Flowers May to September. Flowers are insect pollinated. On heavy soils, about 60% of new shoots are produced from seed, while on lighter soils plants mainly spread by vegetative propagation.
Seed production: 129,714 seeds/plant.
Seed longevity: Seeds may remain viable in soil for at least 5-8 years. In addition, both the crown buds and root buds can remain viable in soil for a number of years.
Seed dispersal: Seeds are contained in a dry capsule which explodes when dry, projecting seeds up to 15'. Seeds can float, and may also be spread by animals such as birds.
Germination: Seeds often have a rather high germination rate, ranging from 60-80%. The majority of seeds germinate within the first two years, usually in May or June.

Competition
Generally spreads most rapidly in semi-arid situations and infestations are common on hillsides, dry prairies, or rangelands. Once established, leafy spurge is capable of crowding out desirable native species. A good competitor for resources because of its early spring emergence. May also exhibit allelopathy toward other species.
Level of impact: A serious weed because of its spreading nature and persistence. For example, between 1973 and 1983, leafy spurge more than doubled its acreage infested in North Dakota. This rapid rate of spread can be attributed to leafy spurge's high seed production and rapid vegetative growth.

Control
An extensive roots system containing large nutrient reserves makes leafy spurge extremely difficult to control.
Cultural: Grasses can compete with leafy spurge; however, over-grazing will limit the ability of grasses to compete. In some cases, fertilization and irrigation may also help to improve grass growth and competition. However, these conditions may also favor other weedy species. The most effective control methods will likely combine a number of management techniques to reduce root reserves and stress the plants.
Mechanical: Because leafy spurge can reproduce rapidly from roots, cultivation or shallow removal can actually cause a net increase in shoots and number of stems.
Chemical: Herbicides can be used to kill the upper portions of the plant, but resource reserves in the roots will allow new shoots to regenerate. Timing of application is critical when using herbicides to control leafy spurge. Research at North Dakota State indicates that picloram (Tordon 22K), 2,4-D, and dicamba (Banvel) are most effective when applied in spring when the true flowers emerge. Picloram is most effective for leafy spurge control, and has yielded control as high as 80% for two years after a single application. Picloram may also be mixed with 2,4-D to provide control. When a mixture of 0.25 to 0.375 pound ai/A picloram mixed with 1 to 1.5 pounds ai/A of 2,4-D is applied for three to five consecutive years, shoot may be controlled 80-90%. Roundup (glyphosate) applied in the fall has relatively good control if followed in spring with pretreatment of Roundup or 2,4-D.
Biological control: Grazing can be used to help deplete root reserves. Using sheep or goats to graze leafy spurge, followed by fall herbicide applications may be one integrated method of control. Several insects are currently being investigated for biological control and may be especially useful in areas where herbicide use is difficult or risky. The spurge hawkmoth (Hyles euphorbiae L.) lays eggs on the plants and the larvae emerge and feed on the plant. Two beetles (Apthona nigriscutis and A. flava) also show some promise as biological control agents.

Notes
Leafy spurge has been reported to cause severe irritation to mouth and digestive tract in cattle which may result in death.

References

Beck, K.G. 1994. Leafy spurge: Biology and Management. Colorado State University 
    Cooperative Extension no. 3.107. 2 pp.

Best, K.F., G.G. Bowes, A.G. Thomas, and M.G. Maw. 1980. The biology of Canadian 
    weeds. 39. Euphorbia esula L. Canadian Journal of Plant Science 60:651 -663.

Biesboer, G.G. 1987. Element Stewardship Abstract for leafy spurge (Euphorbia 
    esula). The Nature Conservancy, Minneapolis. 17 pp.

Cole, M.A.R. 1991. Vegetation management guidelines: Leafy spurge. Natural Areas 
    Journal 11:171 -172.

Lym, R.G. and C.G. Messersmith. 1985. Leafy spurge control with herbicides in 
    North Dakota: 20 year summary. Journal of Range Management 38:386-391.

Lym, R.G. and C.G. Messersmith. 1990. Cost-effective long-term leafy spurge 
    (Euphorbia esula) control with herbicides. Weed Technology 4:635-641.

Stubbendieck, J., C.H. Butterfield, and T.R. Flessner. 1992. Euphorbia esula L. 
    pp. 98-106. In An Assessment of Exotic Plants of the Midwest Region. Final 
    Report. Department of Agronomy, University of Nebraska, Lincoln.

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