Roots of tarweed go deeper than most of the winter annual grasses, reducing competition with them for soil nutrients and moisture. Penetration rates in sand of over 1.5 inches per day have been observed. From late spring until early summer the shoots elongate and branch out with bract-like leaves on woody stems that stand 1 to 2 feet tall. In August and September tarweed produces composite heads that have 3 to 5 ray flowers and 3 to 12 disk flowers. The ray flower is incomplete, having only a carpel, but the disk flower also has anthers that produce abundant pollen, an important food source for honeybees.
Achene dispersal is caused by rain, wind, and wildlife and continues into the winter.
The ray achenes are quite different in appearance, hardness, and the vigor of resulting seedlings, but there appears to be no morphological difference between the plants they produce. The ray achene is 3 mm long, ovate-shaped, and extremely hard. In laboratory tests no germination was achieved without scarification. The factors causing ray achenes to germinate in the field are unknown. Tarweed produces at least 5 times as many fertile ray achenes as fertile disk achenes.