Crabgrass is one of the most prevalent grassy weeds that you will find in your lawn.
It is most noticeable when lawns go dormant during dry periods, but it is probably there if you look for it.
Crabgrass thrives in full sun and high temperatures. It is often found growing along sidewalks, driveways and even in flower beds and vegetable gardens.
In mowed lawns, it grows low, lying stretched out on the ground, waiting to go to seed.
Crabgrass belongs to the genus Digitaria that has about 300 grass species. “Digitus” is the Latin word for finger and crabgrass has long, finger-like inflorescences (flowering part of the plant).
Large crabgrass (D. sanguinalis), sometimes called hairy crabgrass and smooth crabgrass (D. ischaemum) are prevalent in New York. It grows especially well in lawns that are lightly watered, under fertilized, poorly drained or where the grass is thin.
Large crabgrass can be identified by its light green appearance, hairs on all of its surfaces, and its swollen, zig-zag nodes. Smooth crabgrass, also called small crabgrass, does not have hairs on its leaves.
The foliage is light green when young, with parts turning reddish-purple as it ages.
Smooth crabgrass does not root at its nodes while large crabgrass does. Both have a fibrous root system and are low growing.
Smooth crabgrass is a more common lawn weed, as large crabgrass does not tolerate close mowing as well. Smooth crabgrass can produce seed even if you mow it as short as a quarter inch.
Large and smooth crabgrass are both summer annuals. The seed germinates in the spring, plants grow through the summer and then die with the first hard frost.
When plants die, they leave gaps in the lawn.
Plants produce a tremendous amount of seed in mid to late summer when the day length starts to shorten. One crabgrass plant can produce as many as 150,000 seeds in one season!
A large number of viable seeds can remain dormant in the soil, so do not expect to control crabgrass in one growing season. If seed production is prevented, you can significantly reduce the seed bank over time, which will help decrease this weed appearing in your lawn.
Crabgrass is a symptom of poor lawn health. If nothing is done to improve the health of the lawn, it will return every year.
It easily out-competes stressed turf, but a healthy lawn can win out against it.
Crabgrass seeds need open soil to germinate. It will often establish in areas damaged by grubs. Once established, crabgrass tolerates both high temperatures and dry weather conditions, which we have seen this summer.
What is the best way to control crabgrass?
A healthy, dense stand of turfgrass keeps crabgrass out. Proper mowing, fertilization and irrigation all impact the health of your lawn.
Lawns should have a slightly acid soil pH, between 6.0 and 6.8. If your lawn has a serious infestation of crabgrass, more than 40 percent, you may need to do some renovation.
Mowing at 3 inches or higher helps turfgrass shade out weeds and encourages a thicker, more competitive turf. Mowing at the optimum height also increases grass vigor and reduces the germination and establishment of crabgrass.
After mowing crabgrass-infested turf, thoroughly rinse the mower to remove seeds to avoid transferring them to uninfected sites.
Proper irrigation — when required — is important to encourage the development of turfgrass roots and shoots. Water should be applied to the depth of the roots.
Frequent and shallow watering encourages germination and establishment of many shallow-rooted annual weeds.
Avoid summer fertilization. Would you believe that crabgrass benefits more from fertilizer application — under high temperatures — than cool season grasses such as Kentucky bluegrass?
If cultural methods don’t work, you may decide to use herbicides.
Timing is very important when making herbicide applications. Pre-emergence herbicides should be applied in late April or early May, before seeds germinate.
If your pre-emergence herbicide fails to give you season-long control, it is likely due to weather conditions.
Some years the residual activity runs out, allowing the seeds in the soil to germinate.
Post emergence herbicides can be used when crabgrass is in the two to five-leaf stage. \
Repeat applications may be required. Follow all pesticide label directions.
Crabgrass plants are usually too large to control effectively with herbicides after mid-July.
In the garden, mulching, hoeing and hand pulling when the plants are young and before they set seed can help control crabgrass. Mulching with wood chips, compost, or landscape fabrics covered with mulch will reduce crabgrass in shrub beds, flower beds and around trees by blocking the sunlight that it needs for germination, establishment and growth.
Mulches will need to be replenished periodically. In ornamental beds, gardeners can also use herbicides to control crabgrass. Read the label to make sure the product you are using is labeled for use around the ornamental plants in your garden.
The best thing you can do for your lawn is to give it a longer haircut. Mowing to a grass height of 3.0 to 3.5 inches will greatly improve your lawn health.
Longer grass will reduce weed pressure, lawn diseases and fertilizer requirements. By keeping your lawn healthy, crabgrass will have a tough time invading it.
Have a gardening question? The Master Gardener office is open.
Wear a mask when visiting the Cornell Cooperative Extension office and check in at the reception window. Master Gardener volunteers are normally in the office Monday through Friday, from 10 a.m. until noon. You can stop in at our CCE office at 420 E. Main St.; call (585) 343-3040, ext. 127, or e-mail the program at firstname.lastname@example.org. Visit our CCE web site at genesee.cce.cornell.edu or like us on Facebook www.facebook.com/CCEofGenesee.
Garden Talk will be conducted at noon Thursday on Zoom. The topic will be “Colchicums for the Fall Garden.”
Colchicums are an underused fall blooming bulb that many gardeners have never heard of. If you want to add some pizazz to your fall garden tune in for this program.
Register at the CCE website events page at www.genesee.cce.cornell.edu/.