Of all the weeds, crabgrass is the most notorious – and for a weed, that is a bad thing. In fact, 60 percent of respondents to a recent survey on TV indicated they had a problem with crabgrass last year.
Let’s face it. Crabgrass is downright ugly, and we don’t want it growing in our lawns. But getting rid of it after it has established is more difficult than preventing it from germinating in the first place.
Like most grassy weeds, crabgrass is an annual grass – it lives for one growing season. However, while it is alive, crabgrass will produce thousands of seeds that lie dormant until spring, just waiting to start over with a new crop. Research has shown that less than 50 percent of the crabgrass seed produced the previous year will germinate the following spring. When you consider that one single crabgrass plant can produce more than a thousand seeds, well that’s a lot of crabgrass.
Having A Thick Lawn Helps
Seed germination depends mainly on soil temperature, and this is true of crabgrass seed as well. Researchers studying crabgrass at the University of Maryland and Cornell University have determined that turf density (the size of the gap between individual grass plants) plays an important role in the fluctuation of soil temperature. In other words, the thicker the lawn, the longer it takes to warm up, whereas a thin lawn will warm up much quicker.
The research conducted by these two universities also suggests that under conditions where the turf density is average (between thick and thin) the germination and emergence of crabgrass can occur for 10 to 12 weeks depending on the season.
They suggest that if you apply a pre-emergent crabgrass control too early in the spring the effectiveness of that material may dissipate and allow crabgrass to breakthrough areas of the lawn. On the other hand, if the pre-emergent is applied too late to a thin lawn, the seeds will have already germinated rendering the pre-emergent useless.
What does this all mean? Simply that the timing of when the pre-emergent is applied is critical to the control of crabgrass in the lawn.
Applying At The Right Time
As a rule of thumb, lawn experts suggest that a crabgrass pre-emergent should be applied in early spring during the time when forsythia flowers start to wither and before lilac begins to bloom.
If you don’t have forsythia or lilac as your “guide,” then pay attention to local temperatures: When the temperature is consistently in the 60’s, that is a good time to apply a pre-emergent to control crabgrass. Air temperatures above 60° F for 4-5 consecutive days is high enough to increase soil temperature to approximately 55° F, which is warm enough for crabgrass to germinate. Adjust this schedule depending on the thickness of your lawn (thick = a little later; thin = slightly earlier), and you should have no problem controlling crabgrass in your lawn.
In addition, there are several lawn management practices that you should follow to prevent crabgrass from appearing in your lawn:
- Mow at the highest recommended cutting height for your grass type and there will be fewer crabgrass plants in your lawn. This promotes a thicker lawn where it is difficult for weed seeds to germinate.
- Also avoid light, frequent watering of the lawn. This promotes a thin lawn, while providing the ideal watering conditions for crabgrass.
- Follow a regular fertilization schedule to help the grass thicken up and fill in bare spots naturally. Again, this helps promote a thick lawn.
- Be sure the pre-emergent product is applied evenly to the entire lawn. Crabgrass can easily germinate in areas where the pre-emergent is not applied.
After Spring Is Over
Crabgrass won’t be fully noticeable in the lawn until early summer, not the spring.
Often, homeowners incorrectly identify tall fescue as crabgrass. Similar to crabgrass, tall fescue (image at left) is coarse and grows in clumps and when mixed in with more common lawn grasses, like Kentucky bluegrass or fine fescues, it can look unsightly and even ugly. Hence, homeowners confuse tall fescue for crabgrass.
If it is visible in the spring, most likely it is tall fescue and not crabgrass. In fact, a recent study conducted by Scotts revealed that in the spring, a majority of the weeds that consumers believed to be crabgrass were a perennial grass such as coarse tall fescue. This same study done in late summer found that 40 percent of all suspected crabgrass infestations were other types of grassy weed as well. Since tall fescue is a perennial grass, a pre-emergent won’t have any effect on it. (note: If the presence of tall fescue is unwanted in the lawn, then simply spray it with a non-selective herbicide and then reseed the area. Remember to read and follow label directions carefully.)
Once crabgrass germinates and establishes in the lawn, it can be very difficult to eliminate. It will survive through heat, drought, disease and low fertility while other grasses thin out.
Controls are available for established crabgrass, but the results aren’t immediate. Using controls, it can take up to several weeks for the plant to die and disappear. You can also dig up each plant individually, but that can be time-consuming, not to mention it will leave large chunks of bare soil in your yard.
The homeowner who follows a regular fertilization schedule, mows at the proper height and applies a pre-emergent to control crabgrass at the correct time during the spring will have very little trouble with crabgrass in the lawn.