Leafy Spurge, Euphorbia esula

by Sherry Lajeunesse, Roger Sheley, Celestine Duncan, and Rodney Lym

Leafy spurge (Euphorbia esula) infests close to 3 million acres of land in 29 states. This invasive perennial weed overruns and destroys grazing lands for cattle and horses, degrades wildlife habitat and wildlife-associated recreation, decreases rangeland plant diversity, threatens native plants, and reduces land values. The legal responsibility for managing this noxious weed is an additional liability. Intensive, long-term, integrated management is necessary to reduce leafy spurge infestations.

Identification

Leafy spurge is a long-lived, deep-rooted perennial that reproduces vegetatively and by seeds. Stems are hairless and pale green or blue-green. They grow 16 to 32 inches in dense patches. The narrow, hairless leaves are alternate on the stem. Stems and leaves contain a milky latex. The small flowers are green and inconspicuous, but are surrounded by a pair of yellow-green, heart-shaped leaves that are often mistaken for flowers.

Origin, History, and Distribution

In its native Eurasia, leafy spurge is widespread and adapted to many sites and habitat types ranging from riparian to dry hillsides. Infestations are usually small and widely scattered, probably due, in part, to naturally occurring biological factors and intensive grazing by sheep and goats, conditions under which leafy spurge evolved.

The weed was brought to northeastern North America as an ornamental in 1829. It soon spread from yards and gardens, and by the 1900s infestations had reached the west coast of North America. The weed is increasingly widespread throughout the United States and Canada.

Potential Invasion

A highly competitive plant, leafy spurge displaces native vegetation, forming pure stands in many cases. As of 1997, it infested more than 2.7 million acres, mainly in the northern Great Plains of the United States and the prairie provinces of Canada. Loss of plant diversity and loss of wildlife forage and habitat are additional environmental impacts of leafy spurge.

Economic losses in North Dakota alone exceed $14.4 million annually due to reduced forage production and use, and control costs. Because cattle will not graze in areas with a 10% to 20% leafy spurge cover, the weed reduces carrying capacity of infested rangeland to near zero. The value of heavily infested land can also be reduced because of legal responsibilities, expense, and difficulty associated with management. However, leafy spurge has been shown to provide good forage for sheep and goats.

Management

High genetic variability, even within the same plant, enables leafy spurge to adapt quickly to localized conditions, sometimes making control inconsistent. Early and late-season growth, when many other plants are dormant, and deep root systems allow the weed to avoid direct competition. Leafy spurge is particularly successful in areas with few desirable species that can consistently compete for surface moisture, especially if those species that can consistently compete for surface moisture, especially if those species and avoidance of leafy spurge also enhance success of the weed. As population and human activity increase, soil disturbances and transport of weed seeds into uninfested areas provide more opportunities for successful establishment.

Five methods are used to mange leafy spurge: prevention, plant competition, physical control, biological control, and chemical control. The only effective management programs developed as of the 1990s incorporate several or all of these methods. Leafy spurge control must be considered a long-term management program.

Prevention

All new infestations of leafy spurge originate from seeds or vegetative buds on root pieces which can be brought into uninfested areas by activities such as outdoor recreation, agriculture, and construction. Seeds can be transported in mud on motorbikes, construction and farming equipment, tires, vehicles, and on the feet of livestock. Check or clean equipment when moving from infested to uninfested areas. Landscaping soil and fill dirt can also contain seeds and root pieces; avoid importing such materials from weedy areas.

Livestock feed and crop seeds can contain leafy spurge seeds. Certified weed-free crop seeds and feed, including feed for pack animals, are available. Before moving livestock from infested areas, hold cattle 6 days and sheep 11 days in corrals or small pastures, to give seeds time to pass through the digestive tract. Monitor and treat the holding areas regularly for seedling establishment. Prevent the formation of seeds, which can be carried into uninfested areas, by using herbicides or sheep grazing.

Grazing

Plant communities can be manipulated to favor particular plant groups over others by implementing a careful grazing program and by reseeding. It make take 3 to 10 years, or even longer, to shift a plant community from leafy spurge to grasses. The use of grazing animals must be considered a long-term suppression program.

Sheep can be used to graze leafy spurge often enough and at sufficient stocking rates to weaken the plants over time. Grazing rotations must be timed to minimize stress to grasses when they are growing stems and flowering. Generally, sheep graze forbs more heavily in the spring and summer, when plants are young and succulent, and will select grasses more heavily in fall and winter. An intensive grazing system that includes a minimum of two grazing periods, each followed by a rest period to allow grass regrowth, is recommended, rather than season-long grazing by sheep. Begin grazing leafy spurge in early spring when plants are 2 to 6 inches tall. Schedule rotations to prevent leafy spurge from producing seeds. When "the yellow is gone" (the yellow-green bracts), the sheep should be moved to other patches. A goal of grazing management should be prevention of seed production. However, sheep that graze leafy spurge plants with mature seeds should be corralled for at least 11 days to pass seeds from their digestive tracts before being moved to uninfested areas.

Sheep have been used in conjunction with insect biological control agents in the genus Aphthoma (the flea beetles) with positive results when sheep are grazed once in the spring and again in the fall. The schedule avoids disturbing adult beetles when they are laying eggs and feeding.

Goats also will consume leafy spurge (along with other woody species) and are particularly useful where terrain is too rough for sheep, or where burrs and other plant species problematic for sheep are plentiful. Goats can be used initially to clear the site of problematic plants, to be followed by a sheep-grazing program. Although goats are sometimes better suited to a particular area. They require more management than sheep, and markets may be more limited.

Fall grazing by goats followed by herbicide application (picloram at one quart+2,4-D at one quart product per acre) in mid-September has provided good control in northern climates. Allow 3 to 4 inches (7.5 to 10 cm) regrowth after grazing before herbicide application (Lym et al, 1994). When using goats, monitor desirable species regularly, especially woody species such as young trees and desirable shrubs, to prevent overgrazing.

Biological Control

Insects, pathogens, and animals that consume or otherwise use leafy spurge are considered biological controls. With careful management, biological controls can be integrated with other management methods. Biological control agents will not eradicate a weed, but in some cases will reduce weed populations over a period of years.

Insects and pathogens that attack leafy spurge in its area of origin, Eurasia, are carefully studied and screened for their appropriateness for importation into the United States. As of 1996, 13 species of insect biological control agents of leafy spurge had been approved by the U.S. Department of Agriculture for introduction into the United States. As of 1996, 13 species of insect biological control agents of leafy spurge had been approved by the U.S. Department of Agriculture for introduction into the United States (Rees et al. 1996). The most effective and well-established agents appear to be six species of root-and foliage-feeding beetles in the genus Aphthona, and a stem-and root-boring beetle, Oberea erythrocephala. All of the biological control species stress the plant to some extent. Most species are widely available through the USDA Animal and Plant Health Inspection Service (APHIS), or from local insectaries.

Plant diseases can also be used to manage weeds. Several promising bacteria and fungi that attack leafy spurge roots, crowns, and seedlings have been identified and are being investigated. The integration of pathogens with other control methods is being investigated.

Current information about the availability and effectiveness of biological control agents is available from the state Departments of Agriculture, the U.S. Department of Agriculture, the Cooperative Extension Service, and regional colleges and universities.

Chemical Control

Annual applications of herbicides are one component of an effective management strategy. When leafy spurge patches are small and isolated, it is possible to eradicate them with herbicides. Treat an extra 10 to 15 feet around leafy spurge patches to control spreading roots and seedlings. Neither biological control nor grazing should be used to manage small patches because these methods work slowly, will not prevent seed production and will not eradicate the weed. Eradicating small patches of leafy spurge should be the highest priority of a management program.

Picloram is the most effective herbicide for controlling leafy spurge. For small, isolated patches where annual retreatment is not feasible, picloram at one gallon of product per acre will give 90% or more leafy spurge control the first year after treatment. Control will decline gradually to approximately 70% three years after treatment and more rapidly thereafter. This application rate should not be used on infestations larger than half an acre.

Research at North Dakota State university has shown that cost-effective treatment for leafy spurge control is a tank mix of picloram at one point per acre + 2,4-D at one quart per acre (based on a concentrate with four pounds active ingredient per gallon), applied during true flowering and repeated annually. This treatment has provided 85% leafy spurge control after four annual applications, and was the most cost-effective herbicide treatment for leafy spurge, Fall application of this treatment does not effectively control leafy spurge. Picloram at one quart per acre +2,4-D at one quart per acre provides greater control, does not require retreatment as frequently, but is more expensive. This treatment can be applied either during true flowering or in the fall. An economical management option is to apply picloram at one quart per acre + 2,4-D at one quart per acre the first year, followed by picloram at one point per acre + 2,4-D at one quart per acre for three consecutive years.