Warm early October weather has extended the “overseeding window” for fall lawn care. While the first week in October is usually the traditional cutoff for establishing new lawns or rejuvenating existing lawns, the warm early October weather has extended the window by 1-2 weeks.
You can read my September blog post for more information on overseeding and you can still do it, but this follow-up post will deal more with a question that has been coming into the Home & Garden Information Center as of late: “I established a new lawn (or overseeded) in September and some weeds are starting to come up — now what?”
While fall is a great time for lawn renovation, because there is reduced broadleaf weed competition, there can still be some winter annual broadleaf weeds to contend with. For a typical lawn in Maryland, these may include chickweed, henbit, purple deadnettle, and speedwell. From an integrated pest management (IPM) perspective, the best way to reduce the weeds in the long-term is to help the grass be as healthy and dense as possible. Over the course of the growing season, these practices include mulch mowing at the recommended height, fertilizing according to MDA guidelines, watering judiciously, and overseeding with varieties that have done well in University of Maryland (UMD) trials.
Over the next 4-6 weeks, you can increase density by starting to mow when the grass is ~3 ½” tall and mowing to about 3”. This may not seem very important to remove only ½” of leaf tissue, but by mowing you actually stimulate hormones that will cause the grass to produce tillers at the plant’s growing point (crown). These tillers grow from the bottom and help each plant become wider, increasing density.
In addition, “gently” pushing the new (or overseeded) lawn with small amounts of fertilizer will help increase growth and tip the scale of favor toward the new grass. While the maximum application rate for an application is 0.9 lb. N/1000 sq. ft., I usually recommend halving this for new lawns since they don’t have as well-developed roots as established lawns. By dividing this amount into two or three applications and applying more frequently, you are able to supply nutrients to the new seedlings just as they need them and can help to “push” your new lawn to fill in while still fertilizing responsibly and within state guidelines (be sure not to exceed the annual maximum of 2.7 lbs. N/1000 sq. ft./year). Obviously, this takes a little more effort, but the effort you put into a new lawn early will pay dividends for years to come.
Winter annual weeds germinate in the fall with the onset of shorter days, will grow as long as daytime temperatures are ~40° F or higher, bide their time during the parts of the winter that are colder than that, then resume growth with the onset of slightly warmer temperatures in late winter/early spring. Winter annual weed pressure is usually lower in years with a dry fall, and colder/longer winters. So far this fall has been dry, so you probably don’t have too many winter annual weeds germinating in areas of the lawn that haven’t been watered. However, if you overseeded or you have a new lawn and diligently kept the area moist, you may see some winter annual weeds. While these will die and fade away in the spring, you can usually apply a selective broadleaf control for these weeds once the new grass is a month old (be sure to check label directions for details about this and safe application).
Conventional herbicide active ingredients that are labelled for many of the common winter annual broadleaf weeds include dicamba, mecoprop, quinclorac, sulfosuluron, and others. More information about specific conventional pesticides for broadleaf weed control can be found in UMD publication TT-49.
By Geoff Rinehart, Lecturer, Turfgrass Management, Institute of Applied Agriculture, University of Maryland