Weed management is highly impacted by high temperatures combined with low and variable precipitation. Hot weather and drought affect the germination, growth, hardiness and competitive ability of both desirable plants and weeds. Many weeds, once established, need very little water to survive.
Perennial weed species such as spotted knapweed, field bindweed and Canada thistle have extensive, deep root systems and are able to “mine” water. Perennial weeds and other plants also have the ability to establish a dormant state for some time during water and heat stress and can then resume growth during more favorable conditions.
Annual weed species such as cheatgrass and pigweeds that germinate during late fall or early spring have a competitive advantage for water and other resources over native and desirable species that emerge later.
Dry conditions can also affect the proportion of the weed species present – some weeds do better than others under various environmental conditions, and multiple flushes of weeds may occur as some rainfall is received throughout the season.
Not only does low soil moisture affect weed establishment and growth, weed control with herbicides is also affected. Soil moisture factors that affect herbicide activity include residual (longevity) rates and herbicide uptake. For an herbicide to be active it must be absorbed by the plant and then translocated in high enough quantities to be toxic.
Most preemergence herbicides (applied before weeds emerge from the soil) need rain or irrigation within three weeks of application to be effective but should optimally receive moisture within a day or two. Rain or irrigation moves the herbicide down into the soil where germinating seeds will take it up. Herbicides that remain on a dry soil surface may degrade (break down), volatize (evaporate) or attach to soil particles and blow away, as some herbicide molecules tend to bind more tightly with soil particles as soil moisture decreases. Weed seeds may germinate, grow roots and emerge below an herbicide layer on the soil surface. Because the preemergence herbicide must be at proper depth to work effectively, the amount of moisture following application must be enough to move the herbicide to that depth. Water stress may also cause some weeds to germinate later than normal. In that case, a previously applied herbicide may not still be present in a high enough concentration to be effective.
A post-emergent herbicide (applied to weeds that have already emerged through the soil) has to come in contact with plant leaves and remain on leaves long enough to be absorbed into the plant to be effective. Plant stress under drought conditions may result in a thickened cuticle, or waxy covering of a plant’s leaf to conserve water within the plant, and this in turn makes it harder for an herbicide to be absorbed into the leaf. Plants that are not actively growing or are dormant due to water stress will also absorb and move the herbicide less effectively. Herbicide symptom development may be very slow and possibly incomplete if the herbicide is metabolized (digested) too slowly or if it does not move to the root system and other parts of the plant.
Dust can also inhibit post-emergent herbicide uptake. If the weeds are dusty, not only is it physically difficult for an herbicide to reach the leaf surface, it may also bind with the dust and the overall herbicidal activity is reduced.
When plants are stressed but a post-emergence herbicide is still the management choice, it may be worthwhile, if possible, 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. Water output while applying herbicides can also be increased by reducing driving speed and herbicide amount proportionally. For example, cutting driving speed by half, as well as the amount of herbicide in a spray tank, would result in treatment of half as many acres per tank at the recommended herbicide rate, while doubling the amount of water applied.
Applying herbicide rates at the upper end of the labeled rate range can also improve control of drought stressed weeds. Increasing the application rate increases the herbicide concentration in each individual spray droplet, allowing more herbicide to penetrate the cuticle and enter the leaf. While accurate spray equipment calibration is always important, it may be even more so when spraying drought stressed plants. With maximum herbicide rates along with adjuvants or additives being used and potentially more water, it is vital to avoid over- or under-application.
Adding a surfactant can increase the effectiveness of all herbicide applications and in particular during drought conditions. Surfactants are products that allow liquefied mixtures to blend, adhere and absorb better. They help to break the barrier between unlike chemicals and surfaces, initiate greater droplet contact with the leaf surface, slow evaporation of herbicide droplets allowing the herbicide to stay on the leaf surface long enough to be absorbed and can assist passage of herbicides through wax layers of the cuticle of weeds. The herbicide label will specify the proper type of surfactant to use, and if the product already contains added surfactants.
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