Crabgrass is an annual grassy weed that is bright green in color. It has a much thicker blade than your typical northern turf grass and it grows very low to the ground, usually only a couple of inches tall. It typically starts growing in late spring and dies out in September. Crabgrass is most common in lawns that are mowed too short and/or under irrigated. It can be very difficult to control once a stand has been established. The best way to control crabgrass is to prevent it from growing in your lawn in the first place.
The Crabgrass Lifecycle
This season’s crop of crabgrass comes from the seeds of crabgrass from previous seasons. Crabgrass likes hot weather and typically does not take hold in your lawn until mid-May to early June depending on the weather conditions of that specific year. A hot spring will yield earlier germinations and a cool spring will yield later germinations. During the warm summer months of June, July and August crabgrass does exceptionally well and can spread throughout your lawn with ease. At this point crabgrass is well established with an exceptional root system. Once crabgrass reaches this point it is very difficult to control. As temperatures begin to drop through September and into October, you will notice the crabgrass dying off. The first frost of the fall will cause it to turn purple and die off.
How Crabgrass Pre-Emergent Control Works
Crabgrass pre-emergent control works by putting down a 1 inch thick barrier at the surface of the soil that prevents crabgrass seeds from germinating. Applications are most beneficial if done in the spring before any crabgrass has germinated. If crabgrass seeds have already germinated the application can still be very beneficial. It will not control already established crabgrass in your lawn but it will prevent any further crabgrass from germinating.
What Causes Crabgrass?
Crabgrass is the result of a poor performing lawn, not the cause of a poor performing lawn. There are numerous factors that can cause excessive crabgrass in your lawn.
Factors such as:
Mowing height is too short
The main cause of crabgrass in lawns is a short mowing height. As depicted in the figure to the right, you can see how a short mowing height allows sunlight to reach the surface of your soil. This not only dries out your soil (which puts your grass at a disadvantage), it also raises the soil temperature which causes crabgrass seeds to germinate. Excessive sunlight exposure causes crabgrass pre-emergent products to break down prematurely.
Lack of water
The second leading cause of crabgrass in lawns is a lack of water. Water is essential to keeping your turf grass healthy. When grass receives insufficient water, it gets stressed and starts to go dormant; so with no competition, crabgrass is free to take hold in your lawn. When grass goes dormant, the blades get thinner; this allows more sunlight to reach the soil and further dry it out and heat it up. Also when your turf grass dries out, the lack of water raises the soil temperature, forcing crabgrass seeds to germinate in your lawn. The extra exposure to sunlight and heat breaks down your pre-emergent barrier.
Excessively hot temperatures
One factor that is out of your control is the temperature. Seasons with high temperatures, especially for extended periods of time cause lawns to be extremely susceptible to crabgrass invasion. The heat causes lawns to dry out. When grass dries out it becomes drought stressed and starts to go dormant. It is no longer competition for crabgrass, allowing crabgrass to thrive in your lawn. Once again the soil temperatures are raised promoting crabgrass seeds to germinate as explained above.
Thin or bare spots
Mother Nature does not like bare spots. When an area of soil is exposed, nature does its best to fill that spot with anything that grows almost immediately. In Michigan, once warmer weather arrives these spots will fill with crabgrass.
One of the more common crabgrass causes in recent years is improper/excessive weed whipping and edging. Many homeowners and commercial cutting companies have made a habit of running weed whips around the edge of your lawn along curbs, streets, sidewalks, driveways, patios, tree rings, landscaping, pools, flower beds and whatever you may have in your lawn. Most often the grass is whipped angling down toward the edge, often times less than an inch tall, sometimes even down to the dirt. This causes the same problems as mowing too short except more extreme.
Curb lines, driveway edges, street edges
Curb lines, driveway edges, street edges and other edge lines in turf are under increased crabgrass pressure. In sunlight, paved surfaces heat up much faster than the soil around them. This causes the soil within 8-12 inches of paved surfaces to heat up and dry out much quicker than soil in the middle of your lawn. Also edge lines are often subject to physical damage like car tires, pet damage, kid traffic and numerous other factors.
Physical damage to turf surface
Sometimes unavoidable but still a leading cause of crabgrass in lawns is physical damage to the turf surface. Your crabgrass pre-emergent provides a one inch thick barrier that prevents crabgrass. Anytime this barrier is broken from processes like, digging hole, edging, driving vehicles on lawn, construction, and any other factor that can disturb the top one inch of soil.