pronounced: yoo-FOR-bee-uh het-er-oh-FILL-uh
Euphorbia was named for Euphorbus, a physician in ancient Greece; heterophylla is from the Greek 'ετερος (heteros), different and φυλλον (phyllon), a leaf.
There is much confusion between this plant and Euphorbia cyathophora, the painted spurge. Many Websites and books on plants refer to the painted spurge as Euphorbia heterophylla, and even GRIN gives Euphorbia heterophylla as a synonym of Euphorbia cyanophora. Kew Gardens, however, in its recent review of botanical names and synonyms, has come down firmly on the side of those who regard them as separate species. The plants are obviously not the same – the confusion must be in the nomenclature.
Milkweed is an erect annual herb to about 1.5 m high. The stems are hollow, usually with scattered hairs. The leaves are ovate to rhombic, 0.5 to 5 cm wide, 2–12 cm long, hairless above, hairless or with a few appressed hairs below, paler toward the base, the margins entire or lightly toothed. The leaves are opposite at the lowest 1 or 2 nodes, and in the fertile region, the others alternate; the leaf stalk is 0.5 – 4 cm long. The uppermost leaves are never pink or red at the base.
The flowers are male or female in terminal clusters, each flower-head (a cyathium) has a solitary terminal female flower surrounded by male flowers enclosed in a cup-shaped involucre with a solitary conspicuous gland.
The fruit is a capsule 3 – 4 mm long, 5 – 6 mm wide, hairless, and 3-lobed. The seed has 3 longitudinal ridges. The seeds are released explosively from the ripe pods – a single plant can produce up to 4,500 seeds – and are spread by birds and contaminated machinery.
Toxic latex oozes from damaged stems and leaves. This is a widespread weed in the tropics, in crops including sugarcane, pineapples, cotton and row crops. Plants grow rapidly and often shade out the seedlings of the crop. The plant is reckoned to be toxic to stock.
The native range of the plant is from the southern USA south to Argentina, and in the West Indies.
