With many North American no-tillers dealing with several years of serious drought, it’s critical to understand how weeds respond to low moisture conditions much better than most no-tilled crops.

While numerous studies have evaluated weed responses to drought, a global analysis at the University of Nebraska evaluated 1,196 paired observations from 86 scientific articles published over 30 years. In this analysis of studies from Asia, Australia, Europe and North America, graduate student Mandeep Singh looked at the water stress impact on weed germination, growth characteristics and seed production.

What Was Learned

Below-ground weed growth as measured by root biomass decreased by 39% at moderate water stress levels (30-60% of field capacity) and by 69% with severe water stress (less than 30% of field capacity).

With above-ground weed characteristics, plant height was reduced 24% with moderate water stress and by 37% with severe water stress. Leaf area decreased by 43% with moderate water stress and by 44% under severe water stress conditions. Weed shoot biomass decreased 39% with moderate water stress and by 61% under severe water stress conditions.

Weeds under water stress prioritize root growth to increase rooting depth, allowing them to extract water from deeper levels while providing more water to roots over longer periods of time. The analysis also showed grass weeds grew slightly better than broadleaf weeds under most drought conditions.

Even when water stressed, weeds continue to produce seeds. For example, the number of seeds produced per weed plant decreased by 50% under moderate water stress and by 88% under severe water stress.

More Drought, More Competitive Concerns

Don’t ignore the fact that weeds will continue to germinate, grow, survive and produce seeds even under drought conditions. Since drought more likely in the years ahead, try to develop more diverse strategies to deal effectively with managing weeds in a water-limited environment.

Scouting for weeds is extremely important under drought conditions. Since weeds under drought stress can respond less to herbicides, your best bet is to wait for rain and apply herbicides to healthy and actively growing weeds.