Large crabgrass (Digitaria sanguinal)

Crabgrass is the bane of many people seeking a ‘nice’ lawn. It is a weedy grass which will out-compete desirable grass species and take over in a short time. Crabgrass has a wider blade, is lighter in color and grows faster than the lawn making it obviously stand out as a weed. Its seed germinates earlier and at lower ground temperatures than other desirable turfgrasses giving it a jump in growing time.

The best defense against all weeds of lawns is to maintain a healthy stand of turfgrass by having soil pH and nutrients at the correct levels. Healthy soil supports healthy grass. Lawns mowed at a height of three inches or taller has less crabgrass and other weeds as the soil it shaded, excluding light from reaching the seeds which initiates germination. Crabgrass is an annual growing new plants from seed each year. None of last year’s crabgrass lived through the winter.

Low cut grass invaded by crabgrass.



Chemical control against crabgrass is applying a pre-emergent herbicide. They attack the newly produced tissue from the germinating seed up to young plants with a couple of leaves. Pre-emergent herbicides have no effect on seeds in the soil that do not break dormancy and start to grow, only the seeds which start to grow. Timing of application is before the crabgrass seeds start to germinate when to the soil temperatures are 50 to 55 degrees Fahrenheit. This is the same time the forsythia is just past its full bloom stage and starting to but out some green leaves.  The germination period ends when the lilacs begin to bloom. Just remember to apply after forsythia and before lilac flowers. Forsythia and lilac make a great plant indicator for applying the pre-emergent herbicide against crabgrass.

Forsythia bush

Forsythia bush.


Lilac, photo (

There are several different pre-emergent herbicide active ingredients with varying rates of how long they last in the soil. Products containing pendimethalin will last about four months out in the environment. Other active ingredients, dithiopyr, benefin+trifluralin, prodiamine, will last a shorter period of time. Read the labels for the residual rate for each formulation’s time it will last. Most pre-emergent herbicides will stop all seeds from continuing to grow after germinating. This means you will not be able to plant desirable grass seed after applying it. Products containing Siduron are the only pre-emergent herbicide that will allow cool season grass seeds to grow while eliminating crabgrass.

-Carol Quish


Crabgrass is overtaking some lawns by this time of late summer. It is an annual grass weed with wider blades and lighter yellowish green color than the preferred lawn grasses. Crabgrass seeds that germinated last spring are large spreading plants by September. The cooler soil temperatures at the end of summer trigger new plants to sprout, adding to the crabgrass population. All crabgrass plants will die with first the hard frost. As stated above, they are annual plants meaning they grow from a seed to plant, produce seed and die all in one year. Each year, new plants grow from seed make up the entire crabgrass population. Knowledge of the plant’s growth cycle is useful for using the correct control measure.

The first line of defense against crabgrass is healthy soil and a dense stand of turf. Soil pH for turf should be 6.5. Connecticut soils usually are in the below 6 range. A soil test will determine your particular yard’s pH level as well as reveal the nutrients available. Soil testing how to’s can be found at After receiving results by mail, make recommended additions to bring your soil into optimum grass growing condition. Turf needs oxygen and water also to support a healthy lawn. If soil is hard to dig and compacted, now is the time to core aerate. Aeration machines remove a small plug of turf and soil, depositing it on the lawn surface. Leave these plugs on top, do not rake them up. Rain or watering and wind will breakup the plugs, redistributing the soil microbes into the top layers. If you want to add more organic matter to the soil, spread a thin, (1/4 inch), of compost over the entire lawn. Compost will add rejuvenating microbial life to the root zone of the grass.

Mid September is the ideal time to overseed bare and thin turf areas. Choose a grass seed mixture that contains a high percentage of fescue grass. Some bluegrass and perennial rye grass is usually included in the mix. Do not chose one with annual ryes as these will die with the cold weather.  Rake the bare spots to break up any crust to give the seed good contact with the soil. Tamp down after spreading seed. Keep seed moist during germination occurs and new seedlings are two inches tall. Next spring, this new grass should fill in nicely.

To keep the crabgrass seed from germinating in the spring, use a pre-emergent herbicide. Crabgrass begins to germinate when the soil temperatures reach 38 to 40 degrees F. Forsythia blooms at the same soil temperatures making it a good plant indicator to help with remembering the timing of herbicide application. Pre-emergent herbicide does not kill the non-germinating seed, only the new little pip emerging from the seed. This new tissue, the pip, is very tender and susceptible to the chemicals in the pre-emergent herbicide. Not all the crabgrass seeds will germinate every year. Some will stay dormant for many years, creating a seed bank. Each year crabgrass is allowed to grow and produce seed, it adds more seeds to the seed bank. Read the label of the pre-emergent to see how long it will last. Some formulations will last two months, some last six months. These anti-germinating chemicals halt the germination of all seeds, broadleafed and grasses. Only one, Tupersan allows desirable grasses seed to germinate. Tupersan only works on the crabgrass seed, but it doesn’t last long, two months most labels advise. An organic option of pre-emergent herbicide is corn gluten meal. Timing is the same as with a synthetic pre-emergent.

Fertilize with the amounts indicated on the soil test report. Starter fertilizer may be used at the seeding stage.

So to recap the step needed to thicken the turf and reduce crabgrass:

1. Soil Test – apply lime as recommended

2. Core Aerate

3. Top dress with compost.

4. Seed with high fescue seed mix.

5. Keep moist.

6. Next spring when forsythia blooms, apply a pre-emergent herbicide.

Result is a lush healthy lawn.