Crabgrass Prevention

by Lisa Johnson, Horticulture Educator for Dane County

Forsythia is one of the harbingers of spring. The bright yellow blooms indicate that spring has finally sprung! Forsythia’s bloom coincides roughly with soil temperatures at 55F at 1/2 inch depth — the conditions crabgrass seeds need to germinate. So, when forsythia blooms, it is time to apply pre-emergent crabgrass products.

The key to a pre-emergent is to apply and water in before crabgrass seeds germinate, but not too far before, because many products available to homeowners have a limited window of efficacy, usually about 90 days.

Products available to professional lawn care companies have a longer period of efficacy, so you often see companies applying these products earlier than when the forsythia blooms.

Pre-emergents don’t actually kill the seed; they create a chemical barrier that doesn’t allow seeds to germinate. If the soil is disturbed and the barrier disrupted, germination may still occur.

One problem with pre-emergents is that they will also prevent desirable lawn grass seed from germinating, if it is sown while the herbicide is still active. So, be careful when and where you apply the pre-emergent and where you seed later.

If you have dead patches of lawn in spring and want to reseed there, avoid using a pre-emergent in that area. The best time to sow lawn grass seed is actually late August or early September, but if you need to seed in spring, just make sure to keep the seed watered, if it is a dry spring (the same, if it is a dry fall). Wait until the seed has germinated and is about 2 inches tall before fertilizing. The lawn, in general, should not be fertilized until Memorial Day.

by Lisa Johnson, Horticulture Educator for Dane County

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