Because tarweed germinates in the fall and grows in close association with dense stands of winter annuals, there is probably also some competition for light and nutrients during the growing season, but the degree of competition is unknown. The shallow-rooted, short stature, early maturing alien annual grasses use less light and water than the late successional perennial grasses or taller annual grasses. This results in a surplus of moisture that tarweed is able to utilize. Because tarweed relies on stored soil moisture for summer growth, it is most competitive on deep fine textured soils. Tarweed is distributed widely over the range but is more common in swales, and tarweed often dominates the better forage producing sites.
Annual grasses are dominate in some years, and annual forbs or annual legumes in other years.
The amount of competition between tarweed and these winter annuals is less in grass dominate years and greater in forb-dominate years. The nitrogen-fixing ability of annual legumes tends to increase soil fertility, which increases forage production and water use and therefore reduces tarweed densities. Instead of being a highly competitive invader like some alien annual grasses, tarweed seems to have been able to invade the annual grassland by taking advantage of underused resources of moisture, nutrients and light. References Perrier, G.K., W.A. Williams and J.W. Menke. 1982. Tarweed an unloved annual-type range plant.