CRABGRASS PRE-EMERGENTS

Crabgrass pre-emergent is a component of almost all lawn care programs. The goal of using a crabgrass pre-emergent is to minimize the number of crabgrass plants that will germinate in your lawn.  It’s important to realize that no crabgrass pre-emergent will eliminate crabgrass; however 90 - 95% control is not out of the range of the crabgrass pre-emergent products available today. This does mean, though, that even if you achieve 95% control of crabgrass on a 5000 square foot lawn you could have 250 square feet of crabgrass.

One aspect of crabgrass pre-emergents that many people don’t know about is that crabgrass pre-emergents will also help minimize more than just crabgrass plants. To understand what to expect from crabgrass pre-emergent you need to know what type of weeds it can control.

Annual weeds (like crabgrass) reproduce from seed each year and are therefore controlled by pre-emergent herbicides. Biennial weeds reproduce from seed, but require two seasons to complete their life cycle. Therefore, pre-emergent herbicides will control germinating seeds of biennials, but will not control a vegetative top that established the previous year. As with the other weed types, germinating seeds of perennial weeds are also controlled by pre-emergent herbicides. However, pre-emergent herbicides will not control perennial weeds that have already germinated and are established.

Most crabgrass pre-emergents do not affect growth from vegetative structures such as rhizomes, stolons, fleshy roots, tubers, and bulbs. Pre-emergent herbicides kill plants by direct contact with living root tissue or through root absorption. If absorbed, the pre-emergent disrupts internal plant growth processes which results in seedling death.

Crabgrass pre-emergents today have about an 8 - 9 week effective residual in the soil, which allows for a pretty wide window of application timing in the spring. Given that most crabgrass germination in southeast Michigan occurs in mid-to late May, a crabgrass pre-emergent application anytime in April to mid-May should be pretty effective.

Can you apply crabgrass pre-emergent after crabgrass has germinated? The short answer is - yes. Just because crabgrass, in general, might start germinating in mid-May doesn’t mean all crabgrass plants are going to start germinating on the same day. In fact some crabgrass plants may not germinate until June, or even July. Additionally, there are few crabgrass pre-emergents that are even effective on controlling young, newly germinated crabgrass plants.