Euphorbia esula, commonly known as leafy spurge or wolf’s milk, is a herbaceous perennial native to temperate areas of Europe and Asia, where it can be found in dry meadows, stream banks and sandy areas. At maturity, leafy spurge is typically 20-90 cm (to 3 ft.) tall.
Leafy spurge can be recognized by its glabrous, oblong to lanceolate leaves, Euphorbia-type inflorescences, and dense, clump-forming habit. The alternately-arranged leaves on the main stems can reach 8 cm (to ~3 in.) in length and 1cm (~0.5 in.) in width, and the leaves nearest the base of the plant have shortened petioles. Euphorbia esula blooms continuously from mid-spring to early autumn. The inflorescences are umbels, with each cluster of insect-pollinated flowers containing a single pistillate flower and 12-15 staminate flowers. The thusly-incomplete flowers lack both sepals and petals. Pairs of heart-shaped bracts, each 12-16 mm (0.5 in.) in length, cup individual flowers in the inflorescence.
Euphorbia esula was introduced to North America in the early 1800s, likely by means of contaminated crop seed. On this continent, this invasive species can now be found growing in grasslands, shrublands, riparian areas, dry roadsides, disturbed areas including burn sites, floodplains, prairies, stream banks, and big sagebrush communities. It most commonly occurs in the western and central parts of the United States and Canada, and less frequently on the coasts. In Canada, leafy spurge has been ranked sixth out of eighty-one invasive species most detrimental to natural habitats (see: Catling and Mitrow. 2005. A prioritized list of the invasive alien plants of natural habitats in Canada. (PDF) Canadian Botanical Association Bulletin). It is listed as a noxious weed or invasive species in 21 states, and even quarantined in Washington and Oregon. Reasons for the plant’s success as an invader include its hardiness over many habitat types, extensive root systems, seed dispersal mechanisms, resistance to fire and herbicide, and toxicity to wildlife.
The major means of spread is by root system, which both spreads widely (radiating outward 0.2-3m (to 10 ft.) per year) and penetrates deeply (up to 9m (30 ft.) below ground (PDF)). Live pieces of root can regenerate to form whole plants when conditions are favourable. This deep root system is also the key to leafy spurge’s fire tolerance, as the above-ground part of the plant alone is burned while the root system remains intact. Leafy spurge then easily grows back from the roots, thriving in the absence of post-fire competitors.
Several hundred tiny brown seeds are produced in three-chambered fruits from mid-spring to the first frost. Under warm and dry conditions, the mature capsules explode, sending seeds up to 4.5 meters (15 ft.) away from the parent plant. Seeds are also dispersed by water and by certain animals, including sheep, goats, and mourning doves.
Leafy spurge seedlings also emerge earlier in the year than the seedlings of most native herbaceous plants. Growing quickly, they outcompete and shade out seedlings of the native flora. The tenacious seedlings become capable of vegetative regeneration within one week of emergence, and are supported by roots that have been reported to reach up to 61cm (2 ft.) deep in less than two weeks following the appearance of cotyledons.
All parts of the plant produce a milky sap containing ingenol–an irritant to humans, horses, cattle, and wildlife. This sap can cause blistering and dermatitis externally, while ingestion has been known to cause inflammation of the mouth and throat as well as abdominal pain. The leaves may have allelopathic properties, as studies show that areas with large amounts of Euphorbia esula leaf litter have reduced plant biodiversity in comparison to other areas.
The above properties make leafy spurge particularly noxious and difficult to control. Chemical control has proven to be both difficult and expensive as the leaves are coated with a protective wax that prevents aqueous chemicals from easily entering them, while the roots possess a mechanism that prevents large uptake of unwanted soil chemicals. Herbicides can also negatively impact native plants surrounding leafy spurge populations. On a small scale, sheep and goats have been used to clear patches of the plant. With some success, six types of Eurasian insects have been introduced to Canada and the United States as a means of biological control of leafy spurge. In general, it is recommended that leafy spurge be placed under an integrated control program involving biocontrol, chemical control, and re-establishment prevention.