Fall is a good time to get a head start on weed control by applying preemergence herbicides. Herbicides are chemicals that kill plants. Preemergence herbicides target plant seeds, providing a chemical barrier. As the seeds begin to sprout, they encounter this chemical barrier, and cell division in the young root system is prevented. This results in seedling death before the plants ever emerge from the soil.
Preemergence herbicides, applied in October or November, are especially helpful in reducing winter annual weeds. Winter annuals, such as cheatgrass, medusahead and annual mustards, sprout in the fall, lie dormant through the winter and then begin growing again in early spring. These persistent weeds begin growing earlier in spring than other desirable plants, outcompeting them for nutrients and water.
Summer annual weed seeds, such as puncturevine, pigweed, tumbleweed (Russian thistle) and kochia, sprout in early spring and are actively growing in the summer. Applying a preemergence herbicide in mid-February to mid-May will help control these weeds.
Many people report poor weed control, even after applying preemergence herbicides, and they wonder what went wrong. There are several possible reasons why the preemergence herbicide application failed.
Do not apply preemergence herbicides after the targeted weeds have sprouted. Applying a preemergence herbicide in the early spring to control winter annuals is ineffective because the winter annuals are already actively growing. Once weed seeds have sprouted, even if they are winter annual seedlings lying dormant in the soil, preemergence herbicides offer no weed control.
Do not forget to water preemergence herbicides into the soil to activate the chemical barrier. Most require ¼- to ½-inch of water. Planning your preemergence herbicide application to coincide with a major rain event is the best way to ensure effectiveness. But predicting meaningful rainfall in our area is difficult at best, so irrigation may be needed.
In addition, new studies suggest that preemergence applications that are watered by several smaller precipitation events are not as effective. As the soil is wetted by these short events, and then dries out in between, the preemergence herbicide is concentrated in the very top layer of the soil. Weed seeds can sprout below this thin layer and emerge from the soil unscathed.
Do not disturb the soil after watering in preemergence herbicides. They stick to soil particles and will stay in place unless the soil is moved by wind or water…or humans. Once the preemergence herbicides are applied and wetted, disturbing the soil could displace the chemical and result in poor weed control.
Preemergence herbicides are degraded by sunlight. If irrigation or precipitation is delayed, they should be raked into the soil so they remain viable until activated by water. Once activated by water, preemergence herbicides last three to 12 months or longer. Read the product label to determine the persistence of the product. The label will also specify the areas and situations in which these products may be used. Remember to read, understand and follow label directions.
Melody Hefner is the Urban IPM and Pesticide Safety Program coordinator for University of Nevada Cooperative Extension. Have a gardening question? Ask a Master Gardener at firstname.lastname@example.org.