Ask a Master Gardener: I used a pre-emergent. Why do I still have weeds?

(Posted March 25, 2021)

I used a pre-emergent. Why do I still have weeds?

Pre-emergent herbicides (one example is Preen) can be a real help in controlling weeds in flower beds and around landscape plantings.  But, they are only effective when used properly. Pre-emergents work by creating a chemical barrier in the soil. A weed seed may sprout, but the barrier prevents roots from developing, so the tiny sprout cannot grow or mature. Root growth is not prevented in existing weeds or plants, so there is no harm done to those already growing in place. The barrier develops when the herbicide granules are watered into the soil. Depending on rainfall and other things, a proper application will last about three months. The first application is generally recommended for mid- to late-March, followed by subsequent applications in June and in August or September, depending on the weather.

Prior to using the pre-emergent, it is important to clean out the area being treated.  The herbicide granules must contact the soil, so garden debris from the past season needs to be removed. Existing weeds will not be killed, so those should also be eliminated beforehand as pulling them later will disturb the chemical barrier you are trying to establish.

The chemical barrier also will be disrupted and lose effectiveness if there is any digging, raking or hoeing in the area. If possible, get new plants in before application.  Remember also that the herbicide works on all seeds, so do not use it in areas where you plan to start plants from seed. Perennials that reseed also will be negatively affected.  Existing perennials that are still dormant will be fine. Once down, the granules need to be watered into the soil gently by hand or rainfall. Make sure they are not washed away.

There are different concerns for using pre-emergents for weed control when growing vegetables. Timing becomes even more important and differs from product to product. Compare product labels to determine the best product for your use. Products also vary widely in their application. Some caution against use with specific ornamentals.

Whichever product you choose for any application, remember that the most important first step is to read and follow the label directions. Most companies also offer contact information, by phone or e-mail, if you have more questions.

This column is written by the Madison County Master Gardeners. Watch for details about their new Ask A Master Gardener Help Line, coming soon.

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