Greener Roadsides

Effects of Drought on Weed Management

Steven Dewey
Utah State University

Star thistle one of the invasive increasers during dry conditions.

Leafy spurge also takes advantage of drought.

Have you ever noticed that weeds seem more abundant or more difficult to control under dry conditions? Well, its not just your imagination. Many weeds have developed ingenious ways to survive and even thrive under conditions that would kill many other plants. When drought conditions persist, many desirable plants are weakened or even killed, and weeds fill in the spaces to take over. Why are some weeds able to cope so well with drought? Some have unusually long roots that permit them to reach water stored deep in the soil where some desirable plants can t reach (i.e. leafy spurge, field bindweed, and Russian knapweed). Some weeds keep their extensive root system in the top few inches of soil and absorb water very quickly, essentially giving them first rights to any precipitation (i.e. downy brome, kochia, and yellow starthistle). When rainfall is light and infrequent, there is often none left over for plants whose roots majority of roots are deeper. Other weeds can become dormant during of extreme heat and drought - a little bit like hibernating - allowing them to avoid the harsh effects (i.e. hoary cress and quackgrass). Still other weeds are able to germinate, grow, and produce seeds very quickly, taking advantage of scarce precipitation and completing their life cycle before other plants (i.e. Russian thistle, shephers purse, and wild mustard). Whatever the reason, weeds usually win in the battle for limited resources like water.

Drought can also affect the performance of many herbicides. Soil-active herbicides normally need precipitation to move into the soil where they can be taken up by weed roots. If the soil is dry and no precipitation occurs, herbicides may remain on the soil surface, ineffective and vulnerable to loss One of the recommendations to minimize the effects of drought on soil-applied herbicides is to mechanically incorporate the materials shallowly as soon as possible after application.

Foliage-active herbicides also are frequently affected by drought. Dry soil and air conditions make it especially difficult for herbicides to get into plants and move about because they adversely affect the processes known as absorption and translocation. Foliar absorption refers to the passage of herbicide molecules from the outer surface of a leaf, through the protective covering called the cuticle, and into the living cells inside the leaf. Cuticles are layers of waxes and resins that cover the surface of all leaves and stems of most plants to protect them against excessive evaporative water loss. Cuticles typically become much thicker and more difficult for herbicides to penetrate when drought conditions exist. Absorption of some herbicides can be improved under drought conditions by the addition of specific adjuvants listed on their labels.

Translocation is the movement of herbicides through vascular tissues to various places within a plant. When weeds are stressed by drought, herbicide translocation is often affected, moving more slowly and perhaps not as far as if applied under good moisture conditions. Because control of perennial weeds from a foliar-applied herbicide depends much on translocation to distant roots or rhizomes, its no surprise that control can be reduced.

As a general rule, herbicides work best if applied when environmental conditions are favorable for plant growth. Whenever possible, avoid applying herbicides when weeds are under stress. Always read and follow all label instructions regarding environmental conditions that can affect the performance of the herbicides you use.

NOTE: Per the author, invasive plants that often increase or spread during drought include leafy spurge, squarrose knapweed, diffuse knapweed, Scotch thistle, hoary cress, jointed goatgrass, yellow starthistle and kochia. Plan accordingly.