Argemone mexicana aka Mexican Poppy

Argemone mexicana, or Mexican prickly poppy, belongs to the poppy family. Its native range runs from central Mexico to Honduras. It now grows wild in many other parts of the world. It favours places with a marked dry season. Roadsides, fields, and waste ground provide suitable sites. Gardeners grow it as an annual, though some plants live longer. Most reach about 2 feet [0.6 metres] tall. RHS gives a garden height range of 1.6–3.3 feet [0.5–1 metre]. Spread ranges from 0.3–1.6 feet [0.1–0.5 metres].

The upright stems branch and may bear scattered prickles. A cut stem releases yellow, milky sap. The blue-green leaves have pale veins and deep lobes. Sharp spines line their edges and tip the lobes. The leaves grow in an alternate pattern along the stems. Upper leaves clasp the stem rather than having leaf stalks. A pale wax coats their surface and can rub off. The leaves feel smooth above, despite their sharp edges.

Single flowers open at the ends of the branches. Each cup-shaped bloom has four to six petals and many stamens. Most flowers are yellow, though cream and white forms occur. Flowering spans spring and summer, with timing varying by climate. Bees, flies, and beetles visit the blooms. The plant can also pollinate itself. Green, spiny capsules follow the flowers and split when dry. One capsule can hold about 400 seeds. Those seeds may lie dormant in soil for years.

Grow this poppy in full sun with good soil drainage. It tolerates poor soil and drought, but not shade. RHS lists UK hardiness H3; PFAF gives USDA zones 7–10. It suits gravel gardens, rock gardens, and informal beds. The flowers and prickly seed heads provide garden interest. Grow new plants from seed; pruning is not needed. Disease problems are few, but whiteflies, thrips, and aphids may attack. It reseeds freely and can invade crops or overgrazed pastures.

All parts contain toxic alkaloids, including the seeds. Eating it can cause vomiting, diarrhoea, sight problems, fainting, or coma. It also poisons grazing animals, including through contaminated hay. Reports of edible leaves do not establish safe food use. Traditional medicine has used the plant for pain and skin complaints. These uses do not remove its poison risk. Seed oil has served as lamp fuel and soap material. People have also used it to treat wood against termites.