First we need to determine what the "weed" in question really is. If the "weedy grass" is greening up right now, then it is NOT crabgrass! If the "weedy grass" is regrowing and comes back every year, then it is one of the perennial grassy weeds such as nimblewill, tall fescue or quackgrass. If it is not crabgrass, traditional crabgrass herbicides will not control it as it is a perennial grassy weed.
Crabgrass is considered a summer annual weed. This means that is has a life of less than one year. Summer annuals germinate in the spring, grow through the summer and die with the first hard frost. These plants produce a tremendous about of seed in the mid-to late summer when the day length starts to shorten. It is likely that if you had crabgrass in your lawn, it will be there this year too. A common saying for crabgrass is, "One year's seeding equals seven years weeding."
Usually crabgrass appears near sidewalks and driveways where the turfgrass has been killed due to car or foot traffic. It also thrives in full sunlight and high temperatures and can easily out compete common cool-season grasses under these conditions. But crabgrass is a good indicator that there are other problems with the growing conditions in your lawn. These conditions must be corrected to achieve long-term crabgrass control. This is usually accomplished by changing your cultural and mechanical management practices.
The primary and most effective weed control tactic in a lawn is proper mowing. It has been estimated that regular, proper mowing can eliminate 80 percent of weedy species. Without enough light, crabgrass seeds, along with some other weed seeds, will not germinate as readily, if at all. Mowing height can have a big impact; lawns mowed higher (over two inches) tend to have less problem with annual grasses such as crabgrass. Close-mowed lawns tend to open up, allowing weeds like crabgrass to invade. Most home lawns can be mowed at two-and-a-half to three inches.
Other cultural practices include proper fertilization and watering of the lawn. Through proper fertility, you can increase turfgrass vigor and reduce weed competition. This does not mean over fertilizing! Two important factors to consider when fertilizing lawns are: how much and when to apply. For grasses grown in full sun you can apply up to but not more than three applications of nitrogen per year.
Each application consists of applying one pound of nitrogen per 1,000 square feet. Most often we can have great lawns using just two applications per year. These applications would be applied in early/mid-May and then in early September. If you are applying three applications, you would make those applications in early/mid-May, early September and late October/early November.
Light, frequent watering favors crabgrass. Oftentimes crabgrass invades areas seeded in late spring because of bare soil, frequent watering, and onset of hot weather, ideal for its growth. In general, about one to one and a half inches of water per week is needed to maintain green color and active growth. Water your lawn as infrequently as possible but water thoroughly so moisture gets down to the depth of the roots.
Exceptions to this general rule would be for newly seeded lawns where the surface needs to stay moist, newly sodded lawns that have not yet rooted into the soil of the site, or when summer patch disease is a problem. Otherwise, avoid frequent waterings, which promote shallower root systems and weeds (i.e. crabgrass). Given a choice, water early in the day when lawns are normally wet from dew. Avoid midday due to evaporation, and at night due to potential increased chances of some diseases. Also remember that cool season grasses naturally slow down in growth and may go dormant in hot weather.
If you are going to use a herbicide to control crabgrass, the most effective method is to use a preemergent herbicide. A preemergent herbicide controls the seed emergence and must be put down before germination occurs. So application timing is crucial. Crabgrass germination typically begins when soil temperatures reach 62 degrees Fahrenheit at a depth of one to two inches. The general rule of thumb for preemergent herbicide applications is to wait until the lilacs are coming into bloom in your neighborhood. We want to apply the chemical as the soil temperatures reach 50 degrees Fahrenheit for three consecutive days. this will allow the preemergent herbicide to form a barrier before the crabgrass seedlings emerge. If the chemicals are applied too early in the spring it has the potential to break down before the end of the germination window. Applications made late in the season aren't effective as germination has already occurred. Always read and follow the label for timing and rates.
April is also the time used to establish a new lawn by seed. Do not use preemergent herbicide control if you also plan on reseeding or overseeding the lawn this spring. Preemergent annual grass weed killers will also damage germinating desirable grass seed. There is one exception and this is the herbicide siduron (Tupersan). It is the only preemergent that can be used at the same time as seeding. Oftentimes siduron is combined with starter fertilizer. Be sure to read and follow all label directions.
If crabgrass plants are appearing in lawns in mid to late summer, remember that they are annual plants and die as temperatures drop in fall. Postemergence crabgrass herbicides need to be applied when crabgrass plants are very small; typically crabgrass is noticed too late for these to be effective. The suggested strategy to avoid crabgrass next season would be to improve the lawn through cultural practices and consider a preemergent herbicide in spring.
The best defense against weeds, insects and disease is maintain a healthy, dense grass by proper fertilizing, water, mowing and cultivation. A thick turf canopy can effectively shade the soil and reduce the number of seedlings that are able to establish.