In the Garden
Amy Jordan, Special to The Gazette
Originally published 12:59 p.m., March 27, 2008
Updated 12:59 p.m., March 27, 2008
Crabgrass can be a problem and frustration to many homeowners when it invades lawns.
It thrives in the heat of the summer and can be difficult to control.
Crabgrass is an annual weed, so it is killed each winter and germinates again in the spring. It typically germinates in this part of the state around May 1.
It is a grassy weed, so it is not easy to selectively control with herbicide once it is growing among turfgrass plants. This is because the herbicide formulations don’t adequately differentiate between the two types of grasses. There are some products available that are labeled for control of established crabgrass in lawns, but total control is doubtful and they may cause damage or discoloration of the desirable grass.
Prevention of crabgrass is generally the best option. There are products available for this purpose that will be discussed, but first we will address cultural practices that prevent weed invasion.
Crabgrass is rarely a problem in lawns that are dense and mowed at the proper height. This is because crabgrass seed requires contact with the soil and sunlight to grow. Lawns that are dense and mowed appropriately do not have exposed soil, preventing invasion by weeds. Therefore, the most important way to have a lawn with minimal crabgrass (or other weeds for that matter) is to have a healthy lawn and mowed at the upper end of the range for your type of grass. For tall fescue that would be about 3 inches.
New lawns that are planted in the spring also may have crabgrass competition. One way to avoid this is to plant cool-season grass like tall fescue in the fall rather that spring. If planting in the spring is necessary, mow at the appropriate height and work on getting a healthy lawn with proper watering and fall fertilization.
There are products available referred to as “crabgrass preventers” that can be put down in the spring to reduce crabgrass invasion in the lawn. If a lawn had problems with crabgrass last year or there are some thin or bare spots in the yard, these might be a good option. Crabgrass preventer is another name for a pre-emergence herbicide. It doesn’t actually “prevent” the seed from germinating but rather the germinating seed takes up the herbicide and is killed. It must be put down before the seed germinates, and it has little effect on existing crabgrass plants. Once the products have been applied, microorganisms and natural processes start to break down the herbicide, so they don’t last forever and proper timing is important.
April 15th is a good target date to have crabgrass preventers applied. That gives the product time to evenly disperse in the soil by the time the seed germinates. However, because weather varies year to year, a better indicator of proper timing is the blooming of ornamental plants. The Eastern Redbud is a good indicator plant for crabgrass preventers. When the redbud is in full bloom, apply the product. Most products last about 60 days, so may have to be applied twice (about 8 weeks apart) for adequate control. Exceptions to this are products containing Dimension (dithiopyr) and Barricade (prodiamine). Both of these products have a long residual effect and will provide season-long control. Dimension can also be effective on crabgrass that has germinated when applied at the proper rate, as long as the plants are still small.