Donaldson, S. and Skelly, J. 2011, Using Preemergence Herbicides for Weed Control in the Home Landscape, Extension, University of Nevada, Reno, Fact Sheet 11-67

Weeds are constant (and unwanted) companions in our gardens and landscapes. Some years, when we get a bit of extra moisture and the temperatures are just right, we get a bumper crop in our gardens as well as in vacant lots and disturbed soil areas along roadsides and other bare areas. Where do these weeds magically come from? Their seeds have been stored in the soil for years, waiting for the right conditions to germinate and grow. If your weed problem is major, you may consider the use of a product called a preemergence herbicide to control these unwanted germinating seeds.  

What are preemergence herbicides, and how are they used?

Herbicides are chemicals that kill plants. No single herbicide will kill every type of weed, unfortunately, so it’s essential to match the product to the specific weed problem. Some herbicides are applied to actively growing plant foliage and are referred to as postemergence herbicides.

Preemergence herbicides, on the other hand, are applied before weed seeds germinate. They are used to control annual grass and broadleaf weeds. 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. Gardeners will need to use a hose or sprinkler to water in the product thoroughly.  

How do preemergence herbicides work?

It’s a common misperception that preemergence 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 the preemergence herbicide.

The active period for most products varies from three to 12 months or more. Read the product label to determine the persistence. In dry soils, preemergence herbicides break down more slowly and will likely remain active for the specified period.

The label will also specify the sites and situations in which these products may be used. Some preemergence herbicides are specifically designed for industrial sites such as roadways, railroad yards and unplanted or non-crop areas. Plant damage or death of established vegetation may result if these products are used inappropriately. Other products are intended for application around specific types of plants and cannot be applied around plants not listed on the label. Check the label before applying preemergence 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, weed residues, prunings and trash must be removed from the site before applying preemergence herbicides.

It’s important to keep in mind that preemergence herbicides, for the most part, are not very discriminating, and affect many seedlings equally, whether weeds or desired plants. In areas where you are planning to establish plants from seed, such as a lawn or garden bed, avoid the use of these products unless you can leave the area fallow for a sufficient time period that the herbicide will have degraded. Study product labels to determine effectiveness on the weed species of interest, and potential damage to desirable species.

Also consider the sites in which you’re applying the product. Some preemergence herbicides cannot be used with some food crops, for instance, and others may damage certain types of adjacent vegetation. The label will advise you on these issues.

Tips for using preemergence herbicides effectively:

  •  Apply the product before weed seeds germinate. 
  • Remove any dead vegetation, trash or other debris from the site before applying the product. 
  • Read and follow label directions carefully and completely. For most products, you must water it into the soil. If you don’t, it won’t work! 
  • Make one application in the fall to control winter annuals, and another in spring to control summer annuals. Apply when fall rains start in order to control next year’s crop of winter annuals.
  • Avoid disturbing the soil in the application area to avoid holes in the chemical barrier. Traffic from dogs, vehicles or other sources across the treated area can cause the application to fail.
  • Don’t use them in areas where you plan to grow plants from seed. Preemergence herbicides affect many germinating seeds, not just those of weeds.

When should you apply a preemergence herbicide?

Since preemergence herbicides must be applied prior to weed seed germination, correct timing of application is essential. Some of our most prevalent weeds, including cheatgrass and mustards, sprout in the fall in response to onset of seasonal precipitation. These weeds are called winter annuals, and they get a head start on other plants by sprouting in the fall, lying dormant through the winter, and then growing very early in the spring. Because they have already established their root system in the fall, a spring preemergence herbicide application will not control them. Instead, a fall application is needed. Wait for the onset of fall rains and incorporate the herbicide according to label directions in October or November. If there are no fall rains, apply at this time and water the herbicide in thoroughly according to label directions.

Depending on how long the product is active, a second application in mid-March to mid-May, or when forsythia have bloomed and the petals are falling, is needed to control lawn weeds such as dandelions, or summer annual weeds such as puncturevine and tumbleweeds.  

Is there a natural product I can use?

Is the product approved for organic production? Iowa State University (ISU) has studied the use of corn gluten meal as a preemergence herbicide and fertilizer. The product stops roots from forming in germinating plants. It is essential that a short dry period be maintained after seeds have germinated in order to kill the seedlings. It is not appropriate for use in seeded garden crops, but can be used with transplants or mature plants. ISU suggests the product will be effective for five to six weeks after application. In addition to killing germinating weeds, the corn gluten meal slowly releases nitrogen into the soil as it decomposes, feeding existing plants. 

For a complete list of common preemergence herbicides and their properties, use the link below to access the full article. 

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Extension Director's Office | On the campus of University of Nevada, Reno