Ambrosia artemisiifolia aka Annual Ragweed

Ambrosia artemisiifolia, or common ragweed, is a summer annual. It belongs to the daisy family, Asteraceae. Its native range covers much of North America. It has also become established in many other world regions. It is not native to Britain or Ireland. The species name refers to its wormwood-like leaves. Despite its fine foliage, this plant is a noxious weed.

Ragweed grows fast and forms upright, branching clumps. Most plants reach 1–3 feet [0.3–0.9 metres] tall. Some reach 6 feet [1.8 metres] in height. The spread can reach 3–4 feet [0.9–1.2 metres]. Its straight stems have grooves and pink to red tones. Hairs cover the stems and the undersides of young leaves. The green leaves are simple but deeply cut, or pinnatifid. They give the plant a fine, fern-like look. Leaves may grow opposite each other or alternate along stems. Their bases are broader than their tips.

The small flowers lack a showy display. Each plant bears separate male and female flowers. Male flowers form terminal racemes, sometimes with short side spikes. Female flowers sit in clusters at leaf axils below them. Flowers start green, then turn yellow-green and brown with age. Flowering spans summer and fall. Male flowers shed large amounts of pollen from late summer onward. Wind carries this pollen between plants. The brown fruit is a woody achene with four to seven pointed projections. These give it a small, crown-like shape. Seeds can remain viable for up to five years.

Ragweed grows in full sun and can tolerate partial shade. It favors dry sites and soil that drains well. It tolerates drought, poor ground, sand, clay, and shallow rocky soil. Its seeds provide food for songbirds and game birds. Their high oil content makes them useful wildlife food. Yet its weed growth and health effects limit its landscape value. Leaf teas have a history of use in herbal medicine. That use does not establish that the leaves are safe food.

Airborne pollen causes hay fever and other allergic reactions. Contact with the plant can also cause dermatitis. Hoeing and hand pulling are reported methods of control. Some strains in the United States resist glyphosate. The ragweed leaf beetle has helped control it in Europe. This beetle is also being introduced in other countries.