Family Asteraceae > Subfamily Asteroideae > Tribe Heliantheae > Subtribe Ambrosiinae.

Plants of the World Online accepts 46 species of which Australia has 4 that are naturalised.
They are annual or perennial herbs with some shrubs.
From a few cms to over 4 m high they may have a woody base and often have rhizomes (underground stems).
There are hairs and glands on most parts.

Leaves are alternate or opposite and sometimes both are on the same plant.
They can be entire, lobed or once or twice deeply divided (pinnate and bi-pinnate).
The edge can be smooth or toothed and there may be hairs and glands.

Male and female flowers are in separate tight clusters or heads with both on the same plant.
Male inflorescences are a terminal raceme or spike, with flowers with or without a pedicel along a midrib.
Female heads of solitary or clustered flowers are in the upper leaf axils below the males.
Males are on a peduncle (inflorescence stalk), females are not.

Male heads have flowers with white or purplish petals, 5 stamens and an aborted ovary.
Female heads have flowers with no petals just an ovary with a style and long stigma lobes.
Fruit are ovoid achenes that may be smooth or have blunt or sharp spines or wings.
Achenes are dry, indehiscent fruit with one seed from an ovary with a single locule.

Species found in Australia are:
Lacy ragweed Ambrosia tenuifolia.
Perennial ragweed Ambrosia psilostachya.
Annual or common ragweed Ambrosia artemisiifolia.
Burr ragweed Ambrosia confertiflora.