Annual grassy weeds such as crabgrass are best controlled with
pre-emergence herbicides that are applied before crabgrass seed
germinates in the spring. These products work by preventing the weed
seeds from germinating, but have no effect on established plants. It
is critical to apply them at the proper time for optimum control.
Crabgrass seed germinates when temperatures in the upper inch or so
of soil reach 55 to 58 degrees F for four or five consecutive days.
This usually occurs in mid-April in our area. Another rule of thumb
is to have your crabgrass pre-emergence herbicide in place when
forsythia blooms. Crabgrass continues to germinate in our area until
mid – late July, until we get into very hot, dry weather.
Pre-emergence products include Dimension (dithiopyr), Halts (pendimethalin),
Barricade (prodiamine) and Team (benefin and trifluralin). They are
often sold in combination with fertilizer, such as Step 1 in the
Scotts Turf Builder program. Organic gardeners can use corn gluten.
It does not provide the level of control that these other products
provide at first, but if you use it for a few consecutive years, you
should see some improvement.
Seedheads of crabgrass
Pre-emergence herbicides can fail to perform as expected for a
number of reasons, including improper timing of application,
improper rate, wrong spreader settings or any practices such as core
aeration after pre-emergence application that would disrupt the
protective barrier created by the herbicide. Also, pre-emergence
herbicides should be watered in after application to activate their
seed-killing properties and protect them from breaking down on
exposure to sunlight.
There are a few products that can be used to control crabgrass after
it germinates, but they are most effective on very small crabgrass
plants - those with two or three leaves. The mature crabgrass plants
that have grown all summer would laugh at them! These include Ortho
Weed-B-Gon Crabgrass Killer for Lawns (calcium acid methanearsonate)
and Weed Hoe (monosodium methanearsonate).
Crabgrass germinates when Forsythia blossoms drop-off
Finally, 2010 was a great year for crabgrass. Since it is truly a
warm season grass, the hot weather favored crabgrass over our
perennial, cool season lawn grasses. Hot, dry summer weather makes
cool season grasses go dormant unless you water regularly. Even
then, they are not growing strongly because they are so heat
Cultural controls for crabgrass
In addition to herbicides,
cultural practices that encourage a thick, healthy lawn are critical
to winning the war on weeds of all kinds. Start with a soil test to
see what needs to be done to adjust the soil pH and fertility to
meet the needs of your lawn. Penn State soil test kits are available
from your local Penn State Extension office.
If you have lawn areas that are very different - for
example, if your front lawn is exposed to full sun while mature
trees shade your back yard heavily - order two kits and have those
areas tested separately. You should also take separate tests for
flowerbeds or vegetable gardens. The lab’s recommendations are based
on what you tell the lab you are growing in a given area. The kits
come with instructions for taking a good sample. The cost of the kit
includes the testing. Your only other cost is for the postage to
send the sample up to Penn State's Agricultural Analytical
Laboratory in University Park, PA.
Raise the cutting height!
Other cultural practices that help your lawn outcompete the weeds
include raising the mowing height to 2.5 - 3 inches. In addition to
shading out germinating weed seeds, leaving your lawn higher helps
it maintain a deeper, more extensive root system. This also makes it
more drought resistant. If you water your lawn during hot, dry
weather, it is important to water deeply and infrequently. Apply one
to one-and-a-half inches of water once a week, rather than watering
a little bit every day. Deep watering also encourages a deep root
system. Shallow, frequent watering actually favors crabgrass because
it weakens the lawn, encourages a shallow root system and creates
openings for more crabgrass seed to germinate through the growing
Growing bigger Apples