University of Vermont Extension 
Department of Plant and Soil Science

Spring News Article

Dr. Leonard Perry, Extension Professor
University of Vermont
 Crabgrass is one of the most common weeds found in lawns, poor soils, and open areas.  If using a herbicide for control, proper timing is critical.  Use of the correct cultural practices may avoid the need for any herbicides.
Crabgrass is appropriately named, as it often grows low to the ground, spreading outwards resembling a crab.  In unmown areas it may get taller.  Being “annual” means that it grows each year from germinating seeds, completes its life cycle by the end of the season producing more seeds, then dies.  One reason it is a problem is from its prolific seed production, as much as 150,000 seeds per plant per year.  Only half of these may germinate the following year, the other half remaining viable and able to germinate in later years.  While weak lawns may end up largely crabgrass, healthy lawns can tolerate a few of these or other weeds.  
It follows that a good way to control crabgrass is to prevent either seed formation, or seed germination.  Seeds begin germinating in the spring when soil temperatures reach at least 55 to 60 degrees (F) for anywhere from 3 to 10 consecutive days.  Different studies have shown different numbers of days needed, depending on how warm the soil gets.  Keep in mind open, bare ground will warm sooner in spring than one covered with dense turfgrass, shielding the soil from the warm sun.
Commonly used for control of crabgrass seed germination are preemergent herbicides.  These often are found combined with lawn fertilizers, so you can apply both at once.  Make sure to read and follow all label directions and precautions when applying herbicides.

Timing of such weed chemicals is crucial, as they act on the germinating seeds, so must be applied before seeds begin germinating. Applied too early, and the chemicals wont last the whole season.  Keep in mind crabgrass seeds can germinate until soil temperatures reach the 90’s.  In the Northeast, germination can occur over three months, or much of the summer.  Applied too late, and the chemicals wont be effective as seeds already will have germinated. 

Although soil temperatures are the sure guide to timing, the common guide for northern climates is by the time forsythia blooms begin dropping.  If a warm spring, this may be mid to late April.  If a cool spring, this may be early May.  If you err, it is best to be just a little early than too late.  Some of the preemergent herbicides are timed for best application about eight to ten days before the seeds germinate.  If such herbicides are combined with lawn fertilizers, the latter are effective when grass begins growing, often in mid April in northern areas.

Beware that if you are creating a new lawn from seeds, or reseeding bare spots, such herbicides may kill desirable lawn seeds just as they do those of crabgrass.  If you apply early, and it is a warm season or there are many crabgrass seeds in the soil from previous years, you may need to reapply a preemergent herbicide in six to eight weeks (mid to late June). 

If creating new lawns, the main source of crabgrass is from the soil.  Seeding of new turfgrasses is best done in late summer.  This allows plants to establish in fall and better compete with weeds such as crabgrass emerging in spring.  Use the right selection of turfgrasses, and the proper fertilizer.  Your local full service garden supply store should be able to provide recommendations on both. 
For established lawns, proper culture to maintain them dense and healthy is the key to crabgrass prevention.  If you haven’t done a soil test, do so, and correct the pH or soil acidity if needed.  Apply proper fertilizer, also according to the soil test.  This way you don’t apply nutrients the soil may not need, such as phosphorus, which may end up in waterways instead.

Provide good irrigation during the season if possible.  If it doesn’t rain, grass can use an inch of water a week, watered deeply and less often.  Sprinkling the surface more often results in roots near the surface, more subject to stress and prone to weeds taking over.

Perhaps THE most important key to a healthy lawn, better able to compete with crabgrass and other weeds, is high mowing of two and a half to three inches.  Mow often, never removing more than a third of the grass at any one mowing.
If, in spite of your best efforts, you still get weeds such as crabgrass, consider hand pulling if only a few.  For large areas, consider other herbicides if only scattered weeds.  If weed infestation is high, as over 40 percent, or there are many bare spots, you may consider a complete lawn renovation.   

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