Weed germination is inhibited under dry conditions. Thus early season drought may actually reduce weed infestations. Drought that occurs after weed emergence toughens or hardens plants. Weed response to severe drought stress includes leaf cuticle thickening, reduced vegetative growth, and rapid flowering. Drought-stressed weeds are more difficult to eliminate with postemergence weed control efforts.
Weeds compete with crop plants for moisture, nutrients, and light.
Many weeds are highly efficient at using available soil water. For example, cocklebur can extract moisture four to five feet around each plant and crabgrass two to three feet around each plant. Both species are capable of drawing moisture from up to four feet deep in the soil. When rainfall is limited, effects of weed competition on crop yield may be even greater than during years of adequate moisture. The combined effects of drought and weed competition limit yield potential considerably.