[From the August 2014 issue of the Green Bulletin, a newsletter for landscape and structural professionals]

While one of the best methods to reduce weeds is to not water them, there are some that survive even in drought conditions (Fig. 1). As we continue to be impacted by the drought in California, we need to consider our weed management strategies:

• Which weeds will survive?
• How do drought conditions affect control?

Which weeds will survive?

Many weeds, once established, need very little water to survive. Weeds with extensive, deep root systems are able to “mine” water or otherwise have a

physiological mechanism to survive under low soil moisture conditions. Many of these plants are perennials, but annuals, especially those that can quickly develop deep roots, can also be found in dry areas. For example, yellow starthistle (Centaurea solstitialis) (Fig. 2), is a winter annual that develops a taproot to 6 feet deep or more allowing it to access deep moisture in summer. Some weeds, such as yellow nutsedge (Cyperus esculentus) and broadleaf plantain (Plantago major) will establish best in wet areas, but after maturing can tolerate very dry conditions.

Other weeds that survive well in drought conditions include the mallows (Malva spp.), knotweed (Polygonum aviculare L.), horseweed and hairy fleabane (both Conyza spp). Perennial weedy grasses that can survive in low soil moisture include dallisgrass (Paspalum dilatatum) (Fig. 3) and bermudagrass (Cynodon dactylon).

How do drought conditions affect control?

Not only does low soil moisture affect weed establishment, weed control with herbicides is also affected. Soil moisture factors that affect herbicide activity include degradation rate (residual) and herbicide uptake. For a herbicide to be active it must be absorbed by the plant then translocated to the site of action in high enough quantities to be toxic.

The effect of drought on preemergence herbicides. Most preemergence herbicides need rain or irrigation within 3 weeks of application to be effective, but should optimally receive this “activating” moisture within a day or two of application. Herbicides that remain on a dry soil surface may degrade or volatize off or can attach to soil particles and blow away. Rain or irrigation moves the herbicide down into the soil where germinating seeds will take it up. Because the preemergence herbicide must be at proper depth to work effectively, the amount of water applied must be enough to move the herbicide to that depth. Even though water use may be restricted, the herbicide application should be timed such that enough water is allocated to the area for good weed control.

It's important to remember that preemergence herbicides generally will not control weeds that have already or are close to emerging so any weeds germinating between the time of herbicide application and delayed irrigation will not be controlled. Also, water stress may cause some weeds to germinate later that normal. In that case, a previously applied herbicide may not still be present in a high enough concentration to be effective. Overall, drought conditions can shift weed control to postemergence herbicides due to preemergence herbicide failure and late weed emergence.

The effect of drought on postemergence herbicides. As noted above, a herbicide must be absorbed into the weed and moved to the site of activity to be effective. Therefore, anything that inhibits these steps will reduce a postemergence herbicide's activity. Plant stress under drought conditions may result in a thickened cuticle, the waxy covering of a plant's leaf. The thicker the cuticle, the harder it is for the herbicide to be absorbed into the leaf. Additionally, plant leaves tend to get dustier in dry weather conditions, which can inhibit herbicide uptake. For example, glyphosate is very strongly adsorbed to organic soil particles. If the weeds are dusty, not only is it physically difficult for glyphosate to reach the leaf surface, it may also binds with the dust and the overall herbicidal activity is reduced.

Furthermore, plants that are actively growing will move the herbicide more effectively than those that are stressed. Under drought conditions, postemergence herbicides may not provide the kind of control you expect.

Overcoming the effect of drought on postemergence herbicide activity. Where plants are stressed but a postemergence herbicide is still the management choice, it may be worthwhile to actually water the site a few days prior to the herbicide application to ensure that weeds are growing and can take up the herbicide. Overhead irrigation will also help rinse dust off the plants.

Surfactants can be used if there is no contradiction on the herbicide label for their use. Surfactants are products that enhance the ability of an herbicide to enter into a leaf or to stay in an aqueous solution thereby improving herbicide uptake. Surfactants can reduce the surface tension of liquids, causing greater droplet contact with the leaf surface and aid in movement into stomata, can slow evaporation of herbicide droplets, and in some case assist in passage of hydrophilic herbicides through hydrophobic wax layers of the cuticle.