Skip to main content

To the average homeowner, they hear the word pre-emergent and they have no idea what it is. The main idea of a pre-emergent is a preventative of Annual weeds. In Idaho, some common annual weeds are Crabgrass, Barnyard Grass, Puncture Vine, Downy Brome, Prostrate Knotweed, Common Purslane, Prostrate Spurge, Kochia, Witchgrass, Common Lambsquarters, Shepardspurse, Prickly Lettuce, Purslane Speedwell, Common Mallow, Black Medic, Common Chickweed, Redroot Pigweed to name a few (

Annual Weeds

The definition of an annual weed is basically a plant that completes its life cycle in a single years’ time frame. Some annuals are summer annuals and others are winter annuals. In a lawn, Crabgrass, Puncture Vine, Common Mallow, Prostrate Spurge, Common Purslane, Downy Brome, Prickly Lettuce, Black Medic and Kochia are the main target weeds for a pre-emergent application. However, even if your lawn receives a pre-emergent application, this does not mean your lawn is guaranteed to be weed proof.

Since weeds germinate at different times your lawn could still get weeds. A pre-emergent application, once watered to the soil level, will provide a barrier for annual weeds for about two to three months. During this time the annual weed seeds will be prevented from being able to germinate and grow. Perennial weeds, like dandelions, are not always affected by a pre-emergent application; especially if you already have them in your lawn at the beginning of the spring season. The pre-emergent does, however, prevent any seeds they drop during that time from germinating.


Some pre-emergents are designed to control both before and just after germination, such as Dimension, which is a restricted-use product. A restricted use product is not a product that a homeowner can go buy from a local dealer. There are two kinds of use products that the EPA allows for the treatment of properties; first, they are general use products that can be bought at any home and garden store. These products have a lower dose of active ingredients in them so homeowners can apply them to their properties. Secondly, you have restricted use products.

These are ones that can only be applied by a person or company who has a professional applicator license and the proper endorsement. Restricted use products have a higher dose of active ingredients and are designed to gain control faster on any given pest/weed issue on a property. The applicator must understand and follow the label of these products. Care and training are taken to ensure that they apply the product at the right rate and in compliance with EPA guidelines.

Most pre-emergents need to be applied before germination occurs, which is usually when soil temperatures reach forty-five degrees. Normally in Idaho, this is about mid-March to early April. It must be noted that pre-emergents do not guarantee that you will not get weeds in your lawn. This is because a pre-emergent is photosensitive, which means that light can and does cause it to break down over time. Your lawn will eventually get weeds and need further treatments as the season progresses. We like to think of a pre-emergent as one necessary weapon in the arsenal that you need in your battle against weeds.


March is the best time to apply your pre-emergent application if you are doing it yourself. If you have a professional company caring for your lawn, they will have it as part of your lawn care program, as long as you have requested it. If they didn’t offer it to you during their initial sales visit, it would be a good idea to request it so these weeds do not cause issues in your lawn and become an eyesore. In my years as a Professional Applicator, I have seen a great benefit to having a pre-emergent application, so please take advantage of this service and enjoy a beautiful weed free lawn.