University of Vermont Extension
Department of Plant and Soil Science
Dr. Leonard Perry, Extension
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
producing more seeds, then dies. One
reason it is a problem is from its prolific seed production, as much as
seeds per plant per year. Only half of
these may germinate the following year, the other half remaining viable
able to germinate in later years. While
weak lawns may end up largely crabgrass, healthy lawns can tolerate a
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
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
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
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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