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Euphorbia esula: a challenge for non-herbicidal weed control in north America

G.R. Leather and J.V. Lovett

USDA-ARS, Frederick MD USADepartment of Agronomy and Soil Science, University of New England, Armidale NSW 2351

Summary. While present in Australia, leafy spurge, Euphorbia esula, has not attained major weed status. However, in the central plains of the USA and Canada it has proliferated to infest millions of hectares of crop land, range land, recreational and conservation areas. Leafy spurge is a deep-rooted perennial that reproduces by seeds and vegetative regrowth from crowns and roots. Attempts to control this weed with herbicides, grazing animals, insects, and plant pathogens have met with limited success. New research focusing on the use of allelopathic plants and their allelochemicals as natural herbicides for the biological control of leafy spurge is in progress. The challenge is to target the perennating organs of leafy spurge as the site of action for the phytotoxic compound(s).


Leafy spurge was first collected by William Oakes in 1827 at Newbury, Massachusetts, USA. This native European plant was not known to be anywhere else in the USA. Gray's 1848 Manual of Botany suggested that leafy spurge was likely to become a troublesome weed and, in 1875, it was found in New York. By 1913, leafy spurge had spread over the northeastern states and west to Michigan (3). Dunn (9) reported in 1979 that it was found in 26 states from coast to coast and that infestations were out of control in some States. It now appears that infestations in the US are a resuli of multiple introductions. The leafy spurge found along the east coast of North America appears to be of European origin and, although it has spread from the original site of introduction, it has had little impact as a weed on crop land. Recently, however, there have been reports of increasing infestations associated with lucerne and cattle farming in the east.

The central Great Plains infestation of leafy spurge is traced to the Ukraine area of Russia (9, 34) and probably resulted from more recent introductions than the eastern counterparts. Most likely this weed arrived: (i) in ship ballast, (ii) in seed stock of immigrants, (iii) in cereal seed introductions, or (iv) in brome grass seed introductions (33). In North Dakota, it was first discovered growing along a street in the city of Fargo in 1909 (10). Since that time it has become `the weed that has taken possession of farms and driven their owners to the poorhouse' (2). Estimates of the annual economic impact of this weed now exceeds $300 million in the US. One of the many European plants introduced into Australia, leafy spurge is rated as being of economic importance (14) but, unlike other species of Euphorbia, is not regarded as a weed of great significance (1). Given the American experience with this plant and the rapid spread of many other weed species introduced, accidentally or by design, into Australia, the difference in weed status is of considerable agronomic interest.

Weed status

Leafy spurge is a dicotyledonous deep-rooted perennial that reproduces by seeds and by vegetative regrowth from crowns and roots. The stems are erect, usually 40-90 cm high, but can grow to 3 m in fertile soil. The flower is a compound structure consisting of a whorl of four to five petal-like members with a conspicuous yellow-green color. Ripe fruits dehisce explosively and the seeds are propelled up to 7 m from the stalk. Thus, from a single plant, an infestation could expand to over 10 m in a single season. Each stalk produces 10 to 50 fruits with one, two, or three seeds each giving the potential for 150 seeds per stalk (30). The roots are woody and of two types: 'long' roots arise from the primary root of the seedling, grow longitudinally and contain numerous shoot buds; 'short' roots arise from the long roots, have no cambial activity, do not produce buds, but are important for water and nutrient uptake (27,28). Shoots arising from root buds quickly form a crown with numerous buds near the soil surface. Seedlings have shoot buds in the transition zone as early as the second week after germination (27,28).

Leafy spurge begins growing in early spring and competes vigorously with forage and range crops. The most important aspect of the leafy spurge plant as a weed is the root system and associated vegetative buds. Roots penetrate the soil to a depth of 3 m and root buds have been found at 2 m with the capacity for stems to emerge from 1 m deep in the soil (6,7,8). However, an estimated 56% of the leafy spurge root mass is located in the uppermost 15 cm of the soil. Removal of the stem by cutting, burning, grazing or herbicides results in numerous new shoots from the crown and root buds.

Leafy spurge contains toxic chemical substances that cause skin inflammation to grazing cattle and scours when ingested, leading to weakness and possible death (18,29). Latex is present throughout the plant and a copious amount of this white, sticky substance is exuded when the plant is wounded. There is evidence that some of the chemicals in leafy spurge are phytotoxic and, when released, inhibit the growth of other plant species, that is, the plant has allelopathic properties (29,31). Manners (20), suggested that kaemferol 3-glucuronide and 1-he xacosanol were important allelochemicals which contributed to the spread of this weed.

Another important aspect of this weed in North America is that it may have hybridized from Euphorbia esula X E. virgata (11). The genetic diversity of leafy spurge may have contributed to the failure of biological control by insects and disease biological control organisms. Another deep-rooted perennial, skeleton weed, Chondrillajuncea , is similar to leafy spurge in the nature of its spread in the USA and Australia (23). However, the importation and establishment of the pathogen, Puccinia chondrillina, is having a significant impact on reducing the density of this weed in range lands (32). Thus, although occurring in several forms, skeleton weed has remained sufficiently genetically stable to be subject to biological control efforts.


It is possible that the spread or non-spread of leafy spurge is related to the agricultural practices of a region. In the USA, areas that are grazed by sheep are devoid (or nearly so) of leafy spurge. However, on large expanses of cattle range where overgrazing is common, leafy spurge proliferates. Observations by scientists made in Europe and Asia, where leafy spurge is native, confirm that insects and plant pathogens do not kill this weed but, along with agronomic practices, the populations of leafy spurge are held static.

The prevalence of sheep grazing in much of Australia may be a major contributing factor to the relative lack of success of leafy spurge in this country. However, given the impact of overgrazing by cattle on spread of the weed in North America, this is not an entirely satisfactory explanation for the status of the plant in Australia.

The trend to limited or conservation tillage has favoured the proliferation of perennial weeds (24), especially early in the growing season (22). In addition to leafy spurge and skeleton weed, perennial thistle, Cirsium arvense, Carolina horse nettle, Solanum carolinense, and dandelion, Taraxacum officinale are broadleaf weeds that have increased populations in limited tillage agriculture. The economic impact of perennial weeds may be long-term in that the influence on yields increases with the age of infestation (13). Whether these influences are a result of the accumulation of allelochemicals in the soil is not yet confirmed (12).


Leafy spurge is a weed unchecked in the upper midwest of the USA and into Canada. Banks have foreclosed on farms that have little resale value because they are completely infested with leafy spurge. The cost of chemical control exceeds income from many of the infested land areas. Messersmith (unpublished data) has shown that leafy spurge can be reduced to manageable levels with a 10-year chemical and crop rotation program. A number of insects has been introduced but site and variety specificity has limited successful control. Plant pathogens, both endemic and imported, are understudy for use against teal)/ spurge. However, climatic conditions including lack of long periods of dew for germination, have slowed progress. Messersmith (21) suggested that a successful biological control program would use several organisms and require 20 years to establish. Sheep and goats readily graze leafy spurge and at present provide the best means of control. These `biocontrol agents' are not likely to be accepted by farmers and cattle ranchers.

During the past 20 years, there has been an increase in research on the use of allelopathy in agriculture and it has been suggested that allelochemicals have the capacity to contribute to agriculture as components of weed management systems (19). For example, several investigators have addressed weed control using crop and non-crop plants as sources of allelochemicals (12,25,26). Progress has been made on the use of cover crops and their residues to reduce weed populations, and on the identification of other crop plants as having allelopathic potential (12,25). A few allelochemicals have served as structural templates for herbicides, and the development of allelochemicals as 'natural herbicides' is under investigation.

Several chemical compounds from plants have been identified as potential inhibitors of leafy spurge seed germination and its subsequent growth. Hydroquinone, isolated from small everlasting, Antennaria microphylla, is phytotoxic to leafy spurge seedlings (15,30) and the mechanism of its action has been studied (15). Artemisinin, from annual wormwood, Artemisia annua, is an anti-malarial compound that has plant growth regulating properties (4). Both hydroquinone and artemisinin are from plants considered to be weeds of minor importance in the US. Sorgoleone is a phytotoxin exuded in nearly pure form from the roots of Sorghum bicolor. In bioassay, sorgoleone inhibits leafy spurge seedling growth at very low concentrations (F. Einhellig, pers. comm., 1991).

Recently, we have tested a non-protein amino acid, 5-hydroxynorleucene from sunn hemp, Crotalaria juncea, seed on germination and growth of leafy spurge (16). In sand culture, two sunn hemp seeds inhibited the growth of leafy spurge by 60-80%. Important in these tests was the observation that regrowth from root and crown buds was also inhibited (17). A partially purified seed extract containing 5-hydroxynorleucene inhibited growth of leafy spurge at a treatment of 25 ppm, establishing that leafy spurge was more sensitive to this toxin than several other dicot weeds tested (5). Interestingly, we found that as purification of 5 -hydroxynorleucene proceeded, phytotoxic activity based on concentration did not increase (Leather, unpublished data). We suspect that other natural phytotoxins may be working in concert to yield the observed activity, as noted in other observations of allelopathy (12).

Current research on natural phytotoxins at the US Department of Agriculture laboratory in Frederick, MD, is directed toward identification of new compounds that may be active on leafy spurge, and determining the translocation of allelochemicals to root and crown buds in this weed. Proposed research to further meet the challenge will be to determine if the aforementioned phytotoxins might work as natural herbicides in concert with each other or with synthetic herbicides to reduce the vegetative reproduction of leafy spurge and other perennial weeds.

Combining a number of elements into integrated systems for managing pest organisms is well established in the field of entomology and not unknown in weed management, for example, approaches to skeleton weed in Australia (19). Past experience of attempts to manage leafy spurge by agronomic and other means should not be overlooked. While, relatively, unsuccessful when used alone, they might usefully be combined with natural and synthetic herbicides to reduce and maintain leafy spurge populations at acceptable levels.


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