Turfgrass Research & Extension Associate
Grady L. Miller, PhD
Professor & Extension Specialist Turfgrass Science
Crabgrass is a very competitive and invasive weed in tall fescue lawns, especially during late summer and early fall. Large and smooth crabgrass are both summer annual weeds that typically germinate during late winter/early spring, grow vegetatively during summer, and die at the first frost in fall. One reason it is so prevalent this time of year is that pre-emergence herbicides applied in the spring begin to break down as summer progresses. In addition, fescue begins to thin out as summer progresses due to heat/drought stress and disease pressure. These 2 factors combine to result in crabgrass that thrives as fall approaches and present homeowners with the tricky scenario of how to handle the crabgrass without affecting fall seeding of tall fescue.
In areas where your yard is COMPLETELY crabgrass, non-selective herbicides like Glyphosate are the cheapest and most effective option. Glyphosate (sold under trade names such as Roundup) is very effective on mature crabgrass and has a very short re-seeding restriction of only a few days. After spraying the crabgrass, simply leave the plant carcasses in place, because they will actually provide an effective mulch for your new seed. In areas where crabgrass is intermittingly mixed with tall fescue this may not be desirable due to the glyphosate killing the tall fescue as well as the crabgrass. In areas such as these, it may be best to not treat the crabgrass and wait until late Sept or early Oct to seed. This is due to the fact that crabgrass may outcompete the tall fescue for light, water, and nutrients early in the fall (especially if it stays warm), but by late fall will be slowing its growth and will not be too competitive with your new seed. Again, the crabgrass carcasses will serve as an effective mulch after its growth ceases due to cool weather.
Whether you decide to ignore the crabgrass at this point, or treat it with glyphosate, definitely plan on applying a suitable pre-emergence herbicide next spring. Due to the high seed production of annual weeds like crabgrass they are best controlled with pre-emergence herbicides applied prior to germination. The catch is that you have to be sure to apply the product PRIOR to weed germination, because these products typically have very little or no effect at all on germinated weeds. Common pre-emergence products include active ingredients like prodiamine, pendimethalin, and dithiopyr. Their trade names may vary, but the label will indicate the active ingredient. As with any herbicide, always consult and follow the label for proper application rates and timing. In addition to pre-emergence herbicides, any ‘escapes’ can be treated during early and mid-summer by spraying post-emergence crabgrass products like 'Drive' (Quinclorac) or 'Acclaim' (Fenoxoprop). However, the later you spray these products during summer, the more difficult it is to achieve effective control due to crabgrass maturity. You also have to be aware of any potential re-seeding restrictions as indicated on the product label.
As far as fall-seeding of tall fescue during water restrictions, that is a tricky scenario. Ideally you would like to water newly established seed on a “light and frequent” basis in order to prevent it from drying out. Although common water restrictions of one or two days a week may make this more difficult, it should be sufficient to successfully establish tall fescue, particularly with some timely rainfall. Proper seeding of tall fescue includes soil preparation (aerification, tilling, etc), application of seed and starter fertilizer at adequate rates, straw mulching in bare areas to prevent drying out, and frequent watering.
After successful establishment, it is also important to follow proper management practices, for two primary reasons. First and foremost, growing healthy tall fescue in the fall and spring will help it persist throughout the summer months and remain more competitive with crabgrass during periods of heat, drought, and disease. Recent research has shown that mature, dense stands of tall fescue do an excellent job of shading out crabgrass and preventing its germination, even without herbicides. Second, when spring pre-emergence herbicides are applied it is necessary to make sure your fescue has been mowed several times and has produced enough tillers to avoid any injury from these products. If you seed too late in the fall, and especially next spring, your tall fescue may not be mature enough to avoid injury.
As always, consult the TurfFiles website or your local extension specialist for more information regarding proper turf management practices. Helpful links on this topic include:
Tall Fescue Lawn Maintenance Calendar: http://www.turffiles.ncsu.edu/Maintenance_Calendars.aspx#000017
Weed Management Strategies for Newly Seeded Lawns: http://www.turffiles.ncsu.edu/PDFFiles/004443/Weed_Management_Strategies_for_Newly_Seeded_Lawns.pdf