If you do not plan to do any seeding of lawn grasses before fall, certain weeds, such as crabgrass, annual bluegrass, purslane, prostrate pigweed, knotweed, and others, can be treated with a pre-emergence herbicide to keep the seeds from germinating. This will prevent weeds from establishing this spring. Use pre-emergent herbicides only on established lawns, because they will affect most germinating seeds, including desirable ones. Although pre-emergence herbicides are used primarily to control annual grasses, they also help control annual broadleaf weeds, minimizing the need for control of those weeds later in the season.
When using pre-emergence herbicides, it is important to know if the weeds you want to control are annuals, biennials, or perennials- which type of weed it is determines when the seeds of that weed germinate. If the pre-emergence herbicide is applied at the wrong time, it may not remain effective until the seeds of the problem weed sprout. Annuals live for one year. There are summer annuals and winter annuals. Summer annuals germinate in the spring and winter annuals germinate in the fall. Pre-emergence herbicides for summer annuals must be applied in the spring, which is when they germinate. Examples of summer annuals include crabgrass, knotweed, and purslane. (Incidentally, it comes as a surprise to many people that we have very little crabgrass in Montana. However, we have a lot of quackgrass, and the two are commonly mistaken for each other and are similar in appearance. What works for one, won't work for the other; crabgrass killer will not work on quackgrass and vice-versa.)
The second type of annual is the winter annual. Winter annuals germinate in the fall, and if you apply pre-emergence herbicides for winter annuals in spring, the herbicide will probably be broken down by soil microbes before the seeds germinate, and the application will be wasted. Examples of winter annuals include annual bluegrass and common chickweed. Apply pre-emergence herbicides for these weeds later in the summer.
Biennials live for two years. They germinate in one season, pass the winter as a rosette of leaves close to the ground, produce flowers and seeds the second year, then die. Dandelions are usually biennials, although many live for several years. Pre-emergence herbicides are not usually applied strictly for dandelions because seed are produced and germinate throughout the growing season.
Perennials live for two years or more and are more difficult to control because they reproduce both from seed and from vegetative buds on the roots. Pre-emergence herbicides have little affect on shoots arising from these vegetative buds, and no effect on vegetative buds of the perennials that have deep root systems. Pre-emergence herbicides are usually applied in spring for germinating perennial seeds. Examples of deep-rooted perennials are field bindweed (the morning glory weed') and Canada thistle.
If you are not sure of the identification of a problem weed, take a sample to your county Extension agent or county weed office for identification. He or she can also make appropriate herbicide recommendations for your particular situation.
The period of control these herbicides provide will be determined by the product used, the amount applied, soil temperature and moisture, and the soil texture. The period of effectiveness ranges from two or three weeks to many months. Most of these herbicides are broken down in the soil by soil microbes. Problems with leaching or runoff are usually minimal when the products are applied properly, unless soil is sandy, gravelly, or if the water table is close to the surface. By reading the herbicide label carefully, you can select the herbicide that will meet you needs and be appropriate for the environment. Your county Extension agent or county weed control officer can help you if you need additional assistance.
References:
Danneberger, T.K. 1993. Turfgrass Ecology and Management. G.I.E. Inc., Cleveland, OH. 201 pp.
Written by Sherry Lajeunesse, Extension Urban Pest Management Specialist. 1997