When Should You Apply a Preemergence Herbicide?

Unlike last year, soil temperatures have remained rather cool for this time of year. Therefore, an application toward the end of April or first part of May should be early enough for most of Iowa. Another way to tell if soil temperatures are near the range for crabgrass germination is by looking for other plants to give us a clue. Crabgrass seed germination usually coincides with the time when forsythia blossoms start to fall. If you do not know where a forsythia bush is growing, perhaps you can find a redbud tree or a lilac bush. A preemergence product needs to be applied by the time redbud trees reach full bloom or when lilacs are beginning to bloom. Thus, observing forsythia, redbud, or lilac is an excellent means of determining early or late warming of the soil temperature. This also helps us to correctly time our preemergence application. Making an application too early in the spring may result in poor control because not enough herbicide is available. On the other hand, applying a preemergence herbicide after crabgrass has emerged will definitely result in poor control because a preemergence herbicide needs to be applied prior (pre) to crabgrass emergence. For more information, refer to: FG-401, Home Lawn Care: Crabgrass Control.

This article originally appeared in the April 14, 1993 issue, p. 49.

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