By Melody Hefner, Urban IPM and Pesticide Safety Program Coordinator
As much as I like to garden, by fall I’m grateful for the freezing nights and the reduced garden chores. During a session of garden clean-up, I remembered that it was time to start thinking about a fall preeemergence herbicide application to reduce the weeds I would otherwise have to pull next spring.
Herbicides are chemicals that kill plants. Postemergence herbicides kill plants that have sprouted and are actively growing. 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 inhibited. This results in seedling death before the plants ever “emerge” from the soil.
Preemergence herbicides are applied to the soil and generally stick to the soil particles. They will stay in place unless the soil is moved by wind or water. They generally must be watered in, either by irrigation or precipitation, to be activated in the soil. Planning your preemergence herbicide application to coincide with a major rain event is the best way to go, but predicting “meaningful” rainfall in our area is difficult at best. Preemergence herbicides are degraded by sunlight, so if irrigation or precipitation is delayed, they can be raked into the soil so they remain viable until activated by water. Once activated by water, they 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.
It is important to put some planning into your preemergence application. Preemergence herbicides are not discriminating and will inhibit all seedling growth. In areas you are planning to establish plants from seed, such as a lawn or garden bed, you need to be very careful what product you choose. You need to be sure that the product will remain active long enough to prevent weed seeds from sprouting, but not so long that it will interfere with your planned seeding. Remember to read, understand and follow label directions.
Preemergence herbicides are especially helpful in reducing annual weeds. Winter annuals, such as cheatgrass, medusahead and annual mustards, actually sprout in the fall, lie dormant through the winter and then begin growing again in the very early spring. Because they begin growing much earlier than other plants in the spring, they are able to out-compete the spring-sprouting seeds for nutrients and water.
Unfortunately, with our crazy weather this year, these winter annual weeds have already started to sprout in some areas. If they have sprouted, a preemergence herbicide will not control them. Look closely at your application site and see if you have seedlings of the winter annuals showing up. If so, pulling, hoeing or tilling will aid in control of those young plants. A preemergence herbicide application after removing the existing weed seedlings will help reduce continued sprouting of those weeds this fall. Summer annual weed seeds, such as puncturevine, pigweed, tumbleweed (Russian thistle) and kochia, sprout in the early spring. Depending on the persistence of the preeemergence herbicide you choose, a second round of preemergence herbicide may need to be applied in mid-March to mid-May to help control these summer annuals weeds.
Melody Hefner is the Urban IPM and Pesticide Safety Program Coordinator for the University of Nevada Cooperative Extension. Have a gardening question? Contact a master gardener at firstname.lastname@example.org.