Yellow starthistle root growth was measured under field conditions using a minirhizotron camera system. Roots grew 1.0 to 1.3 cm d−1 and quickly reached the bottom of the 100-cm tube. When plants were grown under 80 and 92% shading, roots grew 45 and 64% slower, respectively, than when plants were grown unshaded. Using a neutron probe, we evaluated the effect of yellow starthistle density on soil moisture depletion to 180 cm in the soil profile. At the end of the growing season, we also measured aboveground biomass and seedhead production. Total plant dry weight and seedhead number at the lowest density (0.6 plants m−2) were 43 and 97% of the maximum values, respectively, suggesting that yellow starthistle is capable of reaching near-maximum yield at low densities. Moisture depletion was density dependent, and plants in low-density plots used more soil moisture from deep in the profile than from shallow soil (30 cm) early in the season. In contrast, yellow starthistle densities > 90 plants m−2 rapidly depleted moisture from all depths in the soil profile by preflowering growth stages. High yellow starthistle density expands the moisture depletion zone and leads to increased shallow moisture depletion. In high-density plots, soil moisture did not recharge, compared with bare-ground plots, after subnormal winter and spring precipitation. These results illustrate the importance of reducing yellow starthistle densities in grassland restoration efforts, where shallow soil moisture is critical to the establishment of seeded perennial grasses or annual forbs and where moderate to deep soil moisture is essential for the establishment and survival of transplanted shrubs and trees.
Nomenclature: Yellow starthistle, Centaurea solstitialis L. CENSO.