Fall Weed Control Options for Home Lawns
Leon Warren and Dr. Fred Yelverton
Fall is a great time for postemergence winter weed control. Each year is unique so there is no magical date set in stone that one should target for application. Scout areas to be treated anytime in late September through October, and when winter weed germination or sprouting is evident, determine which species are present so the proper herbicide will be selected. Actual application could be anytime from October through early December depending on various factors such as turfgrass or summer weed density, late season rainfall or air and soil temperature. The main constant each year should be that the winter weeds are young, small, actively growing and have an immature root system. At the risk of sounding like Jeff Foxworthy, if your lawn was a weedy mess this past winter (February – March) or every other winter (no you’re not a redneck), you are an excellent candidate for fall-applied herbicides for winter weed control. Controlling winter weeds in the fall before they become an eyesore allows the lawn to thicken or spread before cold temperature slows or stops its growth, thus creating a visually appealing turfgrass that could be the envy of the neighborhood!
Unfortunately, weeds do not grow in monoculture stands. Lawns are almost always infested with numerous weeds (summer or winter), and this makes herbicide selection extremely important. With luck and skill, one herbicide or herbicide tankmix application could take care of all weed problems. Luck is having a weed population that can be controlled with one herbicide application, skill is knowing you have a weed population that can be controlled with one herbicide application. Skillful weed identification removes the luck factor with herbicide selection.
Winter turfgrass weeds common throughout the state consist of numerous annuals, biennials and perennials. The main grassy winter weed is annual bluegrass. Annual bluegrass is probably the most common and troublesome turfgrass weed in North Carolina. Winter broadleaf weeds are more diverse. There are many common cool-season perennials such as dandelion, white clover, mock strawberry and mallow species. There are also many common winter annual broadleaf weeds such as henbit, chickweed, speedwell, knawel, geranium and hop clover. Wild garlic is another cool-season perennial weed that actively grows in the fall. Check actual herbicide labels for the most accurate information concerning turfgrass tolerances, rates, adjuvant requirements, timings and weed species controlled.
Annual bluegrass control: warm-season turf
Several postemergence options for annual bluegrass control exist for warm-season lawns, with bermudagrass and zoysiagrass having more control options than centipedegrass and St. Augustinegrass. Atrazine and Simazine formulations have existed for several decades and are labeled in all the listed warm-season turfgrasses. Generally, Atrazine is applied from November 15 to December 31, and Simazine is applied from November 15 to December 15. These herbicides provide preemergence and early postemergence control not only of annual bluegrass, but of many of the common winter annual broadleaf weeds as well. Perennial broadleaf weed control is usually weak. When applied early in November, Atrazine and Simazine can control small annual grass and broadleaf weeds with 1 lb ai/A. Rate increases up to 2 lb ai/A due to weed size are usually needed if applied to large weeds in December. Expect annual bluegrass to germinate and break through these herbicide treatments in March and April if the turfgrass stand is thin and spring rainfall is plentiful. To fight against this, Simazine can be applied at 1 lb ai/A from October 15 – 30 and then again from December 15 – 30.
In the early 2000’s, sulfonylurea herbicides such as Monument and Revolver were developed. These herbicides provide excellent postemergence annual bluegrass control and also control a fair amount of winter broadleaf weeds. Monument even controls perennial broadleaf weeds such as dandelion and white clover. Bermudagrass and zoysiagrass are tolerant to both herbicides. Unlike Atrazine and Simazine, annual bluegrass control is usually season-long, regardless of spring rainfall amounts. Apply Monument at 0.33 to 0.45 oz/A and Revolver at 17.4 fl oz/A.
Annual bluegrass control: cool-season turf
Ethofumesate (trade names Prograss or PoaConstrictor) is one option for postemergence annual bluegrass control in cool-season turfgrasses such as perennial ryegrass, Kentucky bluegrass and tall fescue. Prograss is typically used in high-maintenance bermudagrass overseeded with perennial ryegrass. Make the first application 30 to 45 days after overseeding, then reapply 21 to 28 days later. Apply 1 lb ai/A for each application. Do not apply Prograss after December 31 to bermudagrass or spring greenup will be delayed. Prograss will not control annual bluegrass that has extensively tillered so don’t wait until November or December to make the first application.
Xonerate (amicarbazone) is a new herbicide registered by Arysta LifeScience that can be applied to most cool-season turfgrasses established 6 months for annual bluegrass control. Apply 2 to 4 oz/A twice at two to three week intervals.
Broadleaf weed control: warm and cool-season turf
Broadleaf weeds and specific herbicide recommendations are too numerous to try to list here. There are dozens of trade names for broadleaf herbicides that are labeled for both warm and cool-season turfgrass lawns. Remember to scout the lawn to determine which weeds are present so the correct herbicide can be chosen. The majority of these herbicides belong to only four chemical families; 1) phenoxy 2) benzoic acid 3) pyridine 4) triazolinone. Phenoxy herbicides consisting of 2,4-D, mecoprop and dichlorprop have been labeled since the 1960’s and 70’s. Dicamba is a benzoic acid herbicide that has been labeled many years as well. In the 1980’s and 90’s, pyridine herbicides consisting of triclopyr, cloypyralid and fluroxypyr were registered. Triazolinone herbicides consisting of carfentrazone and sulfentrazone were registered in the 2000’s. In many cases, these chemistries are combined to form 2, 3 and even 4-way herbicides. Because of the wide spectrum of broadleaf weeds that are usually present in any given lawn, package mixes are the surest way to controlling all species that may be present with one application. This is why there are so many broadleaf herbicides to choose from.
Sulfonylurea herbicides such as Monument and Manor can also be used exclusively for broadleaf weed control. Monument is an excellent annual bluegrass herbicide and is actually a very good broadleaf herbicide as well. Many broadleaf weeds are controlled with rate a of 0.33 oz/A. Manor applied at 0.33 to 0.5 oz/A controls even more broadleaf weed species than Monument. Both herbicides are tolerant to bermudagrass and zoysiagrass, with Manor also tolerant to centipedegrass (up to 0.5 oz/A), St. Augustinegrass, Kentucky bluegrass and fine fescue. These herbicides can be tankmixed and applied to bermudagrass and zoysiagrass to create an outstanding broadleaf herbicide treatment.
Wild garlic control: warm and cool-season turf
2,4-D amine can be used in cool-season lawns such as tall fescue and perennial ryegrass. Bermudagrass and other warm-season grasses are tolerant as well. Apply 3 qt/A twice at yearly intervals. Wild garlic populations will be greatly reduced but not eliminated after one application. Bulblet reproduction will occur rapidly if a 2nd application is not made.
Manor herbicide also provides some control of wild garlic. Bermudagrass, zoysiagrass, St. Augustinegrass and centipedegrass show tolerance as do cool-season grasses such as Kentucky bluegrass and fine fescue. Apply up to 0.5 oz/A and do not exceed this rate for centipedegrass, Kentucky bluegrass and fine fescue.
General rules for fall seeded lawns
Always check specific herbicide labels for overseeding requirements, but here are a couple of general guidelines to follow.
If broadleaf weeds are present and an application is made before overseeding or reseeding, wait three to four weeks before overseeding or reseeding. Broadleaf herbicides can temporarily affect grass seed germination.
If the lawn is overseeded or reseeded first, broadleaf herbicides cannot be applied until the turfgrass has grown enough to withstand two to three mowings, or become extensively tillered and established. Many times, this means that if the lawn is seeded first, broadleaf herbicides should not be applied until the following spring when new growth resumes.
Exception to guideline #2. Tenacity herbicide controls various winter broadleaf weeds and can be safely applied to newly seeded tall fescue, Kentucky bluegrass and perennial ryegrass anytime before germination. After germination, wait four weeks or until turfgrass has been mowed twice.