The Q&A Archives: Crabgrass

Question: Within the past 5 years, I've had two lawns installed from sod: Marathon II, and St. Augustine. The first died after a year or so, possibly because of shade conditions. The second expired because I was instructed to water too much, and the roots rotted. Now, crabgrass is thriving. Should I just let it take over?

Answer: Although I'm not a fan of crabgrass lawns, you may have run out of options if you've tried to grow both Marathon and St. Augustine and still end up with crabgrass.

Crabgrass is a warm-season, annual grass, which grows best in the heat of mid-summer. It overwinters as seed. It comes up about mid-May or later. It is killed by fall frosts. Crabgrass is not shade tolerant, and grows best in full, hot sun. Several lawn care practices can be used to help control crabgrass.

First, a healthy, relatively dense lawn is your best prevention against invasion of undesirable weeds. A healthy, thick turf gives vigorous competitive turf that allows little or no weed invasion. Properly fertilizing and watering your lawn will help insure an adequate supply of plant nutrients and water for vigorous grass growth.
Second, adjusting the mower height upward will provide more shade at the soil surface. Without enough light, crabgrass seeds, along with some other weed seeds, will not germinate as readily, if at all. Most home lawns can be mowed at 2 1/2 to 3 inches.

Third, a "pre-emergence" weed killer can be used. This destroys crabgrass seeds as they germinate before they emerge from the soil. Depending on where you live, apply pre-emergents from late April to late May. For home use, granular forms, easily applied with fertilizer spreaders, are available.

Usually one application put down at the proper time gives control. If an application is made very early in the season, a second application may be necessary during mid- to late June. Always follow pre-emergent herbicide applications with one-quarter to one-half inch of water. This helps move the herbicide to the soil surface where it provides the control. Also, watering it in may reduce your exposure to the product. Post-emergent herbicides for crabgrass are usually not recommended. They're most effective when plants are very small.

Crabgrass is likely to be more of a problem weed in warmer areas adjacent to curbs, driveways and sidewalks. It may only be necessary to apply a pre-emergent herbicide in those areas.

Crabgrass is often confused with perennial-type grasses. Perennial grasses, which grow each year from roots, are not controlled with pre-emergent crabgrass materials. A good thick turf, mowed at three inches during the warmer summer months, will effectively control most crabgrass for many homeowners.

You can try attacking the crabgrass now, before the weather gets hot and it begins to thrive, and then overseeding the area with grass seed. With proper maintenance, the desired grass can eventually overtake the crabgrass. Best wishes with your lawn~

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