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 statured, 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 dominant 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 dominant years and greater in forb-dominant 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. These techniques can greatly reduce a population of tarweed, leaving very few plants to flower and set seed. However, the timing of these activities is critical.