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Preventing Crabgrass

Key reasons for success with crabgrass control

By: Sandy Feather 2016
Penn State Extension

Q. I used a crabgrass pre-emergent last spring, but wound up with a bumper crop of crabgrass by the end of the season. I have used it in past years with great success. Can you explain what happened?

A. There are a number of reasons why crabgrass pre-emergent herbicides fail. It is important to have it in place prior to crabgrass seed germination. Most of the products available to homeowners do not control growing plants, but kill the roots as they emerge from the seed. It is also important not to apply them too early, because they do break down over time and exposure to the elements.

Speaking of the elements, it is very important to apply crabgrass pre-emergent herbicides prior to rain or plan to water it in after application. Most of these products will break down on exposure to sunlight; watering them in washes them down to the soil surface, where the grass shades them.

However, excessive rainfall or irrigation can wash the product out of the upper inch or so of soil, where the crabgrass seed is ripe for germination. We did have about six weeks of frequent rainfall that may have affected the pre-emergent you applied.


Excessive rain can lengthen the germination period for crabgrass seed, and it can speed the chemical and microbial degradation of the herbicide. Also, a thatch layer over a half-inch can interfere with the performance of pre-emergent herbicides. Thatch is the accumulation of sloughed off stems, rhizomes and roots that is the natural by-product of grass as it grows. You can find it between the soil surface and the crowns of the grass plants.


Any activity that disturbs the herbicide barrier digging, core aeration, dethatching will also decrease the effectiveness of pre-emergent herbicides. Environmental conditions, especially cool soil and drought, can delay the germination of crabgrass seed beyond the effective residual of the herbicide. Once the rain quit around mid-July, we did have an extended period of hot, dry weather.

aeration soil cores
Core aeration helps when done once or twice a year

Finally, the best defense against all kinds of weeds is a dense, thick lawn. Proper fertilization, irrigation and mowing can have a huge impact on weed populations.

Allowing your lawn to grow at least 3 inches tall can help shade out germinating weed seeds, and it also encourages the grass to have a deeper, more extensive root system. Managing insect and disease problems helps keep the lawn from thinning out, which keeps the weeds from moving in.


Spot-seeding your lawn

Mulching leaves when mowing

September is lawn month


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