Prolonged periods of dry conditions affect weed growth, herbicide performance, and complicate weed management strategies.

Competition for water between crops and weeds increases as soil moisture becomes limited, so weed control is even more important when water is scarce.

Fewer weeds are usually observed during dry conditions and they tend to be less vigorous. However, some weeds like Russian thistle and field bindweed develop extensive root systems early and take advantage of limited surface soil moisture, making them more competitive and difficult to control. Other weeds, like hairy fleabane and horseweed, can adapt to both wet and dry conditions.

While weed vigor and size may be reduced under dry conditions, they can still produce a lot of viable seed. When soil moisture is adequate, a properly timed shallow cultivation can be use to control weeds. However, during dry conditions, tillage can result in a loss of soil moisture, potentially limiting crop growth, particularly in newer plantings.

Since herbicides play a vital role in grape production, it’s important to understand how pre- and postemergent herbicide performance changes under droughty conditions.

Preemergent herbicides require moisture for activation and movement into the soil where weed seeds germinate. Sunlight degrades preemergent herbicides from the soil surface if it not activated by water and moved into the soil soon after application. This is not a concern in annual crops where irrigation water or mechanical methods are used for crop stand development and herbicide activation.

However, in perennial crops like grapes, growers rely upon rainfall for herbicide incorporation and activation. Product labels specify how long the herbicide can remain on the soil surface after treatment before rainfall must occur. Some loss of the herbicide by sunlight degradation can be expected once the herbicide is applied to the soil surface, so apply them as close to rainfall as possible. Sprinkler, basin-flood, or furrow irrigation can also be used to activate the herbicide in lieu of rainfall.

Performance of postemergent herbicides also changes during dry conditions. Systemic herbicides are especially impacted under droughty conditions. Examples include 2,4-D, glyphosate, and sethoxydim. Water-stressed weeds have thicker waxy leaves (cuticle), reducing herbicide absorption into the plant.

Plant growth processes are also altered, reducing movement of herbicides within the target weeds to sites of activity. If movement of herbicides with the carbohydrates or water stream in the target weeds is reduced, control will be reduced. Since contact herbicides do not require movement within weeds, they are less likely to be affected by drought conditions. However, drought-stressed weeds can sometimes have spindly shoots and/or leaves, making them harder to kill. Examples of herbicides commonly affected include carfentrazone, paraquat, and oxyfluorfen.

Using a higher label rate and appropriate spray adjuvant can help compensate for drought-stressed conditions. Using an Nbased additive (like ammonium sulfate) and/or a crop oil adjuvant may also help increase absorption into the leaf surfaces of stressed weeds. Apply the spray mixture in enough water to adequately cover the weeds and increase the chance of absorption and/or contact activity. Making applications toward the end of the day as the temperature begins to decrease often aids efficacy.

Achieving effective weed control can be difficult during conditions of drought. Weed growth will be altered, making them less susceptible to control. In grapes, apply preemergent treatments as close to rainfall as possible, otherwise consider irrigation as a source of herbicide activation.

Postemergent herbicide performance, especially systemic-type herbicides, is often reduced under dry conditions. Apply these herbicides when weeds are young and succulent to improve control. In some cases, it may be better to wait to make postemergent treatments after the weeds receive moisture and are less stressed.

Also, using N-based additives and/or certain adjuvants can help aid in control. If at all possible, spray later in the evening when the temperature begins to decrease.