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 www.soiltest.uconn.edu. 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.