Crabgrass
By: Sandy Feather
©2008
Penn State
Cooperative Extension
Q. My lawn got
overrun with crabgrass this summer. I put down a a pre-emergent weed
control back in April, but it didn't seem to do much good. Do you
have any suggestions of what I should do now with my extensive
crabgrass problem?
A. Crabgrass is
a summer annual weed that grows from seed every spring. It would be
a waste of time, energy and money to spray crabgrass this late in
the season. It will die when we have our first frost in the fall.
The best thing you can do now is to mow regularly to remove the
seedheads before they mature as much as possible. Bag and dispose of
your clippings, rather than composting them. Like most annual weeds,
crabgrass is prolific seed producer. Also, anything you can do to
encourage a thick, healthy lawn will go a long way to crowding out
the crabgrass. A combination of cultural and chemical controls will
give the best result.
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 improvement. 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.
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. In 2008, the weather was a very important
factor in the failure of crabgrass control products. The rainy
weather through June and early July, and the cool weather so far in
August have extended the germination period for crabgrass seed right
into August. Although the effective life of pre-emergence herbicides
varies from one product to another, the frequent rain likely reduced
that time for all of them.
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).
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 Cooperative
Extension office. In Allegheny County, kits are available for $12,
with additional kits for separate soil tests costing $9 each. You
can send a check, payable to Penn State Cooperative Extension (PSCE)
to Penn State Cooperative Extension, 400 North Lexington Street,
Pittsburgh, PA 15208. Please write Attn. Soil Test Kit in the lower
left corner. 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.
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. This also encourages a deep root system.
Shallow, frequent watering actually works to crabgrass' advantage
because it weakens the lawn and creates openings for more crabgrass
seed to germinate through the growing season.
Q. I read your column (shown above) with interest as
my lawn has had a major infestation of crabgrass this year. Can I
overseed those areas and still use a crabgrass pre-emergent
herbicide next spring?
A. It is ideal
to have any reseeding done this fall – by October 15th - if you can.
Fall is an ideal time to plant new lawns from seed or to overseed
and repair damaged areas. Air temperatures are cooler and less
stressful for tender grass seedlings. However, the soil is still
warm, which speeds germination and root development. Finally, there
is much less weed pressure when you plant grass in the fall rather
than spring. Weed seeds are “programmed” to germinate when they have
the best chance of growing and producing a new generation (seeds) of
the species. A handful of winter annuals such as henbit (Lamium
amplexicaule), deadnettle (Lamium purpureum) or annual bluegrass (Poa
annua ssp. annua) germinate in fall, but the vast majority of weed
seeds germinate in spring.
Seedheads bearing next year's crop of crabgrass
If you can reseed in fall, you should be able to apply one of the
traditional crabgrass pre-emergent herbicides next spring with no
problem. These include Dimension (dithiopyr), Halts (pendimethalin),
Barricade (prodiamine), Team (benefin and trifluralin), or corn
gluten meal. If you cannot reseed until spring or find that you have
some patches to repair in spring, there is a product called Tupersan
(siduron) that will prevent most of the crabgrass seed from
germinating while allowing most of the good grass seed to germinate.
This product exploits the fact that crabgrass is a warm season
grass. The seeds of our cool season lawn grasses such as Kentucky
bluegrass, perennial ryegrass, fine fescue or turf-type tall fescue
are able to metabolize this herbicide while warm season grass seeds
cannot and are killed.
Buying grass seed
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