After several years of drought, we welcome all the precipitation so far this year, but wet winters can lead to weedy springs. Weed seeds can remain viable in the soil for years, waiting for the right moisture conditions in the soil to sprout. If you’ve had trouble in the past with weeds, it may be time to start planning an application of preemergence herbicide to reduce the weed pulling chores later this spring.
Herbicides are chemicals that kill plants. Post-emergence 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 stick to the soil particles. They will stay in place unless the soil is moved by wind or water. They must be watered in, either by irrigation or precipitation, to be activated in the soil. It is not recommended to apply them when the ground is frozen as they will not be activated by frozen 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. You want to apply when there will be enough precipitation to activate them, but not so much precipitation that they are washed by rain water to unintended areas or, worse, to the storm drains and into our rivers.
Preemergence herbicides are degraded by sunlight, so 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, they last three to 12 months or longer. Read the product label to determine the persistence of the product and the specific areas and situations where these products may be used.
It is important to put some planning into your preemergence application. Preemergence herbicides are not selective and will inhibit all seedling growth. In areas where 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. Applying a preemergence herbicide in the early spring will not control winter annuals as they are already actively growing. Apply a preemergence herbicide in October or November to help control winter annuals. Summer annual weed seeds, such as puncturevine, pigweed, tumbleweed (Russian thistle) and kochia, sprout in the early spring. Applying a preemergence herbicide in mid-February to mid-May will help control summer annuals weeds.
Melody Hefner is the Urban IPM and Pesticide Safety Education Program Assistant for the University of Nevada Cooperative Extension. Have a gardening question? Ask a Master Gardener at firstname.lastname@example.org.