- Name also: Radium Weed, Cancer Weed, Milkweed, Wartweed
- Family: Spurge Family – Euphorbiaceae
- Growing form: Annual herb.
- Height: 10–20 cm (4–8 in.). Stem often branched from base, glabrous. Containing latex.
- Flower: The small male and female flowers which lack perianth are borne in groups in the centre of a bowl-like involucre (formed by fused bracts), the whole resembling a single green flower. Stamens numerous. Carpels 3, fused. Subtending bracts ovate, similar to stem leaves. Inflorescence a 3-branched, doubly 2-sided compound umbel. Nectariferous glands in cyathia sickle-shaped, light-coloured, with long points.
- Leaves: Alternate, short-stalked. Blade elliptic–widely obovate, with entire margin, dark green.
- Fruit: 2 mm (0.08 in.) long, 3-valved capsule. Each valve narrowly winged.
- Habitat: Gardens, flowerbeds, vegetable gardens, heaps of earth, wasteland, loading areas.
- Flowering time: June–September.
The large Spurge family is very diverse: species can be herbs or prickly bushes, succulents and tall trees. Most of the species are native to warm countries and many resemble cacti. Spurges and cacti can immediately be differentiated however by pricking the plant’s leaves or stem with a needle: if while latex flows out it is a spurge. Latex is a highly poisonous cocktail and has even given some poisons, euphorbine and euphorbone, their names.
Somewhat small and delicate, petty spurge doesn’t look much like a cactus, but its latex is just as much reason to avoid it. Its unpleasant taste and toxicity mean that even hungry diners leave it alone, but they don’t save the plant from competition with other plants. This demanding species only thrives in rich, calciferous habitats, which are usually colonized in Finland by larger plants. In practice petty spurge grows in Finland only in places that have been disturbed by humans, e.g allotment flower beds and alongside plants that require nutrition. Weeders could decide to leave this relatively harmless and rare weed to grow in peace.
Petty spurge’s habitats can also be populated by sun spurge (E. helioscopia), whose inflorescence is five-branched and whose blade is serrated at the tip.