We probably will have a bumper crop of weeds this year after all the rain and snow. You can get a head start and stop the weeds before they emerge from the soil. Herbicides applied to actively growing leaves are referred to as postemergent herbicides. Preemergent herbicides, on the other hand, are applied to kill seedlings as they germinate. Preemergents are used to control annual grass and broadleaf weeds.
It’s a common misperception that preemergent herbicides kill seeds directly. Instead, when sprouting seeds encounter the herbicide, cell division in the young root system is inhibited, resulting in death of the young seedling. These products generally do not control established vegetation, so it’s important to remove existing weeds from the site prior to applying a preemergent herbicide. The active period for most products varies from three to 12 months or more. In dry soils, preemergent herbicides break down slowly and will likely remain active for the specified period listed on the label. The label will also specify the sites and situations in which these products may be used.
Some preemergent herbicides are specifically designed for industrial sites such as roadways, railroad yards and unplanted or non-crop areas. Plant damage or death of established desirable vegetation may result if these particular products are used in home landscapes. Other products are intended for application around specific types of plants and cannot be applied around plants not listed on the label. Most preemergents must not be applied around food crops. Check the label before applying preemergent herbicides to areas that you plan to plant with seed to determine how long you must wait to avoid damaging desirable seedlings. Also, dirt clods, plant residues and trash must be removed from the site before applying preemergent herbicides.
No single herbicide kills every type of weed, so match the product to the specific problem weed. These products are applied to the soil, often as dry granules and sometimes as a liquid spray, and are then watered into the top inch or so of soil. Generally, at least one-half inch of water is necessary to move the product into the soil, so the average rainstorm in western Nevada (0.26 inches) is not enough, but this year is not average. You might be able to time an application right before a rain.
To find out when to apply a preemergent, whether there are organic alternatives, or what the risks with these products might be, read my publication on preemergent herbicides: http://www.unce.unr.edu/publications/files/ho/2011/fs1167.pdf.
JoAnne Skelly is Associate Professor & Extension Educator, Emerita at University of Nevada Cooperative Extension. She can be reached at email@example.com.