Spurge Family

Many species of spurges have become noxious weeds in agriculture, especially in pastures, because these plants can be toxic to cattle if eaten in large quantities. One example of an economically important weed is the leafy spurge or wolf's-milk (Euphorbia esula). This species was originally native to temperate regions of Europe and Asia, but became an invasive weed when it was introduced to North America. The introduction of this important weed probably occurred numerous times as a seed that was present in the ballast that ships often carried to give the vessels stability when sailing from Europe to North America. This ballast was commonly soil obtained locally in European harbors, and then discarded at American ports upon arrival.

Leafy spurge now has a wide distribution in North America, but it is especially abundant in the Midwestern prairies. This species occurs in a diverse range of open habitats, including agricultural fields and pastures, and grazed and natural prairies. Leafy spurge is a herbaceous, perennial plant that grows an extensive root system that can penetrate as deep as 30 ft (9 m) into the soil. The leafy spurge also produces large numbers of seeds, which are effectively dispersed by various means, including animals.

Leafy spurge is a severe problem because it can poison livestock if they eat too much of this plant. The only exception is sheep, which can tolerate the latex of leafy spurge, especially early in the growing season. The latex of leafy spurge is also toxic to humans, causing dermatitis upon contact, and severe damage to the eyes and mucous membranes if contact is made there. Leafy spurge is invasive in some natural communities and in semi-natural habitats such as grazed prairie, where this species can become so abundant that it displaces native species.

Infestations of leafy spurge have proven to be very difficult to control. Herbicides will achieve some measure of success locally, but this sort of treatment has to be repeated, often for many years. Recent investigations have focussed on the discovery of methods of biological control, using herbivorous insects or diseases native to the natural Eurasian range of leafy spurge, which keep this plant in check in its natural habitats. So far, these methods have not proven to be successful.

Various other species of spurges have also become agricultural weeds in North America, although none as troublesome as the leafy spurge. Some additional, weedy spurges include the spotted spurge (Euphorbia maculata) and cypress or graveyard spurge (E. cyparissa), both of which likely became pests after escaping from gardens in which they had been cultivated.