Information on weed responses to soil fertility levels is needed to aid development of fertilizer management strategies as components of integrated weed management programs. A controlled environment study was conducted to determine shoot and root growth response of 22 agricultural weeds to fertilizer phosphorus (P) applied at 5, 10, 20, 40, or 60 mg kg⁻¹ soil. An unfertilized control was included. Wheat and canola were included as control species. Shoot and root growth of all weeds increased with added P, but the magnitude of the response varied greatly among species. Many weeds exhibited similar or greater responses in shoot and root biomass to increasing amounts of soil P compared with wheat or canola. With increasing amounts of P, 17 weed species increased shoot biomass more than wheat, and 19 weed species increased shoot biomass more than canola. However, only 10 weed species exhibited greater increases in root biomass than canola, and no weed species increased root biomass more than wheat with added P. Canola was among species taking up the greatest percentage of available P at all P doses. However, percentage P uptake by wheat relative to other species varied with P dose. Only four weed species extracted more P than wheat at low P levels, but 17 weed species extracted more P at high soil P levels. These findings have significant implications as to how soil fertility may influence crop–weed competition.

Nomenclature: Canola, Brassica napus L. ‘Excel’; spring wheat, Triticum aestivum L. ‘Katepwa’.