It seems like ages ago, but during late spring and early summer we were in the midst of a long dry spell–and then things changed! It seems once the rain started it hardly ever stopped during late July and early August and all of this rain created its own set of problems. In particular, summer annual weeds and sedges were given new life with all of the wet conditions. For many homeowners, it has been a difficult summer keeping weeds like crabgrass, Japanese stiltgrass, kyllinga, and nutsedge at bay during the wet, humid weather. Even folks who had applied a second application of pre-emergent herbicide in late spring were seeing that product break down more rapidly with the inordinate amounts of rain the region experienced.
University of Maryland (UMD) research (and others) has indicated that the best way to deter crabgrass is to mow higher. Experiment plots mowed in the 3½-4” range have consistently had less crabgrass invasion than plots mowed at 2” or 3”. While this late summer weather has led to a lot of crabgrass and sedge invasion, homeowners can take solace in the fact that relief is in sight as far as the calendar is concerned. Late August/early September is the perfect time of year to re-seed with cool-season grasses like tall fescue to undertake a full-scale renovation or a lawn “rejuvenation.”
First, let’s define a few terms:
Complete renovation involves killing the existing sod to bare soil and re-seeding or installing sod.
Overseeding involves using an aerator or de-thatcher to open up the turfgrass canopy and then applying seed to increase density and sustain the stand.
Repairing bare spots involves raking up old debris by hand or loosening with a de-thatcher, then seeding.
More detailed information on these techniques can be found in Starting a New Lawn.
How do you decide what to undertake? If your lawn is thin, overrun with crabgrass, has a high percentage of broadleaf weeds, or is otherwise an “unsalvageable mess,” you should probably consider a full renovation. If your lawn is a little weak in places, but otherwise dense and relatively healthy, overseeding would be more appropriate.
Late-August through September is the perfect time for a lawn renovation. Be careful waiting beyond September, though. While it still may be warm in the beginning of October, there likely won’t be enough time for an October-planted lawn to grow in and get established before significantly colder nights and the chance for frost in the last half of the month. If you’re planning a full-scale renovation which includes killing out the existing lawn, between now and Labor Day is the time to start since it usually takes about 1 week for a non-selective herbicide to take effect fully.
As outlined by Bulletin HG 102, ensuring good “seed to soil” contact and maintaining adequate moisture to the seedbed is critical for successful seed germination. Another key element in lawn renovation and overseeding is seed selection. There are a number of varieties (cultivars) of tall fescue available, however, some have performed better than others in the UMD evaluation trials and these are listed as “recommended varieties” in UMD Extension Bulletin TT-77, Recommended Turfgrass Cultivars. Although these varieties may be difficult to find at “big box” stores, many local garden retailers seek them out to stock them and homeowners can often purchase them from local landscape professional suppliers or find them online at sites like seedsuperstore.com.
I would like to invite anyone interested to attend the Fall Lawn Care and Lawn Renovation Workshop I will be conducting at the U.S. National Arboretum in Washington, DC on Saturday, September 8 at 10 a.m. Registration is free, but space is limited and registration is encouraged. Call 301-405-4692 or e-mail rinehart@umd.edu to register.
By Geoff Rinehart, Lecturer, Turfgrass Management, Institute of Applied Agriculture, University of Maryland.