Ambrosia trifida aka Giant Ragweed

Ambrosia trifida, or giant ragweed, grows for one year. It is native to North America. It belongs to the daisy family, Asteraceae. People also call it great ragweed, horsebane and wild hemp. It is not native to Britain or Ireland. Its known range spans Canada, the United States and northern Mexico. It grows in regions with a temperate climate. It grows on disturbed alluvial ground, where water has left soil. There, it can form large stands with few other plants.

This upright summer plant reaches about 6.6 feet [2 metres]. PFAF says it grows at a moderate rate. It completes its life cycle within one year. Its small green flowers offer little garden show. It blooms from August to October. Each plant has male flowers and female flowers in separate blooms. Wind moves pollen from flower to flower. This pollen is a major cause of hay fever in North America.

PFAF suggests a sunny site with soil that drains well. The plant also grows in light shade and prefers moist ground. It grows in sand, loam and clay soils. Its stated pH range runs from mildly acid to mildly alkaline. PFAF lists USDA zones 1–11, though the plant lives for one year. The source suggests sowing seed where it will grow in April. PFAF notes that it bases its growing advice on limited facts.

Its spread and pollen make it less suited to garden seats or paths. PFAF warns that it is invasive and notes its use in garden beds. Some Indigenous peoples of North America grew it for food and medicine. Seeds found at ancient sites were much larger than today's wild seeds. This suggests growers chose plants that had larger seeds. The supplied facts do not show any part is safe to eat. The seed-oil details refer to a different ragweed. They do not prove this species has an edible use.

Past folk remedies used the leaves for insect bites and skin problems. Other records note leaf and root teas for several ailments. These past uses do not prove that the treatments are safe. Firms also gather its pollen to make allergy drugs. Crushed heads yield a red dye. The sap can stain skin red. Touching or eating the plant can cause allergic reactions in some people. Weigh its allergy risk alongside these records of its uses.