Appearance

Ambrosia tomentosa' is a long lived perennial herbaceous plant in the sunflower family (Asteraceae) growing up to 3 ft (0.91 m) in height. A. tomentosa develops extensive horizontal roots. It is native to the Great Plains region.

Foliage

A. tomentosa leaves are alternate, up to 5 in. (12.7 cm) long. They are finely dissected and deeply lobed with coarsely toothed margins. The upper leaf surface is greenish-gray and may have rough hairs. The lower leaf surface is covered with white short, dense hairs.

Flowers

The yellow flowers of A. tomentosa are very small and inconspicuous. The male flowers are held in umbels at the top of the stem while the spiny female flowers are found in the leaf axils. The spiny female flowers help to set this species apart from other Ambrosia species.

Fruit

A. tomentosa fruits are light brown burs with up to 10 short spines. They each contain 2 seeds. A. tomentosa also reproduces vegetatively by sending shoots up from the horizontal root system.

Ecological Threat

A. tomentosa is considered a serious cropland weed. It spreads both by seed and creeping roots, keeping primarily on, but not limited to, dry or poorly drained sites. A. tomentosa can be found growing in cultivated fields, pastures, prairies, and waste areas.

Selected Images

Maps

Invasive Listing Sources

• Pacific Northwest Exotic Pest Plant Council, 1998
• Wyoming Noxious Weeds

References