Put the chill on winter weeds

If you're fortunate, the only scares you'll have this month are from the ghosts and goblins of Halloween and not the ineffectiveness of your weed-management program. With upcoming holidays and other fall-maintenance activities, you easily can forget about weed control. However, with appropriate action now, your weed problems will not come back to haunt you.

The weed-control strategy you should use depends on the types of weeds present. Therefore, the first thing you need to do is determine the major weed species at each property you maintain. You may need to use different strategies at the different sites, based on the predominant weed species. For example, wild onion may be the only serious weed at one site, while another location has common chickweed and annual bluegrass as the predominate problems. These two sites would certainly require different treatments.

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In some ways, it's easier to control winter weeds than summer weeds. However, if you neglect them, winter weeds can become a major burden in spring when you need to devote your time to other maintenance practices. Thus, treating for weeds now buys you time in the spring. And no matter what season it is, it is always easier to maintain a site in weed-free condition than it is to bring a heavily weeded site back into top condition. It pays to be proactive in your weed-control program.

Because we already have passed the peak period for winter-annual germination (which mostly occurs in September and October), your weed-management activities right now will have to address both the control of existing weeds and residual control for weeds that will germinate later. I hope that you applied pre-emergents in August because winter annuals germinate in early fall, with little germination during the coldest winter months. However, a second flush of germination can occur in early spring as temperatures start to rise. This is why some operators use split applications of pre-emergence herbicides: to control both flushes of germination. An August application for controlling fall-germinating weeds may no longer be effective by March, when more germination can occur.

Classification of winter weeds Most winter weeds fall into two categories: grasses and broadleaves. Weeds such as yellow nutsedge-neither a grass nor a broadleaf-are not a concern in winter because these plants go dormant with the onset of cold weather in the fall and do not emerge in spring until after temperatures have warmed up. Likewise, you cannot control warm-season grasses such as dallisgrass and bermudagrass in winter because these species also go dormant.

Broadleaf weeds consist of annuals, biennials and perennials. Examples of winter annuals include common chickweed, corn speedwell, vetches, rockets and henbit. Biennial weeds include certain thistles, such as musk thistle. Perennial winter broadleaves include bulbous buttercup and mouseear chickweed. In addition, seed of certain perennials that grow actively in warmer weather, such as dandelion and buckhorn plantain, often germinate in fall and early spring and therefore need attention in a fall weed-management program.

Grasses are either annual or perennial. Annual bluegrass is a common winter-annual weed. A perennial biotype of annual bluegrass exists, but this predominately is a golf-course problem. Quackgrass is a cool-season perennial grass that can invade turf and ornamental beds. If you maintain warm-season turf, you know that clumps of cool-season grasses such as tall fescue become quite apparent when the turf goes dormant. Cool-season grasses can be a conspicuous weed problem in such circumstances.

Other perennial monocot weeds (that are neither grasses nor broadleaves) of concern in fall and early spring include wild onion, wild garlic, grape hyacinth and star of Bethlehem. As in summer, perennial winter weeds are harder to control than annuals.

Weather conditions Weather conditions obviously are quite different in winter than summer, and this can impact weed management, especially herbicide applications. Temperature is an important factor for post-emergence herbicide application. Applicators frequently ask me about the minimum temperature in which they can apply post-emergents. This is not an easy question to address. I prefer air temperatures above 60 degreesF, with good soil moisture so weeds are actively growing. As temperatures drop, two effects occur. First, the development of injury symptoms is much slower compared to applications at higher temperatures. This does not necessarily mean that control ultimately will be worse. But if, for example, a herbicide takes 1 week in summer for effects to become visible, it may take 2 or 3 weeks in the winter. Thus, you may need to inform your clients not to expect fast results in the winter. Some weeds, such as henbit, seem to take an especially long time before complete control is apparent.

In some cases, however, control is less in winter because herbicides are not readily absorbed and translocated at low temperatures. This is a greater issue with systemic compounds than with contact products. Generally, I feel that ideal conditions for systemics are above 60 degreesF, while 50 degrees to 60 degreesF is acceptable, and 40 degrees to 50 degreesF is marginal. Avoid applications below 40 degreesF. Some applicators have told me that their cutoff temperature is 50 degreesF, but I have successfully used post-emergence herbicides at temperatures between 40 degrees and 50 degreesF. Nevertheless, applications below 50 degreesF increase the chance of erratic control, and you may have to retreat some sites. As in summer, it's best if no rain or irrigation occurs for at least 24 hours after applying a post-emergent.

Cold weather generally is not a concern for pre-emergence herbicides and can actually help certain products. Volatility losses are less in cold temperatures, a benefit for products such as trifluralin and dichlobenil which can volatilize during warm temperatures. Also, herbicide breakdown by microorganisms is slower in low temperatures, increasing herbicide longevity and, thus, the length of weed control for pre-emergence herbicides.

Rain and falling snow help incorporate applied pre-emergence herbicides. Further, due to cooler temperatures, the need for immediate rainfall or irrigation for incorporation can be less critical than it is in summer. However, do not apply pre-emergence herbicides to frozen ground or ground covered by snow.

Winter weed control in turf You may need different programs for warm-season and cool-season turf. You can take advantage of the dormancy of bermudagrass to use products that would injure the turf during active growth. This allows you to selectively suppress species such as tall fescue growing in dormant bermudagrass. Conversely, cool-season grasses are actively growing in cool conditions and are susceptible to these products. However, on overseeded warm-season turf, your control strategies must accommodate the presence of the actively growing overseeded species. * Cool-season turf. In cool-season turf, you can control emerged broadleaf weeds using combination products that contain two or more of the following active ingredients: 2,4-D, 2,4-DP, MCPA, MCPP, triclopyr, clopyralid and dicamba. Because deciduous trees and shrubs drop their leaves in the fall, the potential hazards from volatilization and drift are less at this time than in the summer. This especially applies to ester formulations, with which volatility on hot summer days can be significant. However, ester formulations penetrate the waxy coating of leaves better than amine formulations, so fall offers a good window of opportunity for weed control with ester formulations of broadleaf herbicides. Products containing 2,4-D also can control wild onion and wild garlic.

On newly seeded turf, the stand must be well established before you can treat it with a post-emergence broadleaf product. You usually can treat if the site has been mowed three times.

You can apply isoxaben in fall for pre-emergence control of broadleaves after you've treated existing weeds with a post-emergence product. Isoxaben not only controls broadleaf weeds as they germinate, it also reduces regrowth of certain perennial broadleaf weeds that your post-emergence product may not have completely controlled.

Some pre-emergence grass herbicides also control certain annual broadleaves from seed. However, the primary benefit of "crabgrass preventers" at this time of year is to stop germination of annual bluegrass, along with providing early season summer-annual grass control. Examples of these products include dithiopyr, prodiamine, pendimethalin, benefin plus trifluralin, bensulide and oxadiazon. Just remember that if you treat turf with a pre-emergence crabgrass herbicide in the fall, this might affect reseeding the following spring. Products vary in this regard, so check the label for specific information.

You can use ethofumesate for post-emergence control of annual bluegrass in a range of cool-season grasses. You may need multiple applications for acceptable control. In bermudagrass, you only can use ethofumesate when the turf is dormant. * Warm-season turf. In warm-season turf, greater options are available for winter-weed control. In addition to conventional pre-emergents, you can use products such as atrazine, simazine, metribuzin, pronamide and imazaquin. These herbicides provide both pre-emergence and post-emergence annual-weed control, including annual bluegrass and broadleaf weeds such as common chickweed. Check the labels for tolerant warm-season species and the weeds that these products control.

Glyphosate and diquat provide post-emergence annual-weed control in dormant bermudagrass. Glyphosate, pronamide and atrazine also control cool-season perennial grasses such as tall fescue, bluegrass and orchardgrass. Imazaquin and 2,4-D products control wild onion and wild garlic. Post-emergence broadleaf combination herbicides control existing broadleaf weeds, and you can add isoxaben to the application for residual broadleaf weed control.

Winter weed control in ornamental beds Winter weed control is more difficult in ornamental beds than in turf, especially broadleaf weed control. Essentially no chemical exists for selective control of emerged broadleaf weeds in ornamental plantings. You must treat existing broadleaf weeds with careful applications of non-selective products such as glyphosate, diquat or glufosinate.

A much better strategy is to use pre-emergence herbicides for control of annual weeds in landscape beds. In woody ornamentals, use a combination treatment, such as a sprayed application of isoxaben with a grass herbicide such as pendimethalin, oryzalin, oxadiazon, napropamide or prodiamine, or apply a granular pre-pack combination such as Snapshot, OH-II or Rout. Dichlobenil is another option for controlling a wide range of weeds, including certain perennials, in woody ornamentals. These combination treatments control a wide range of annual weeds if you apply them before germination (dichlobenil also has post-emergence activity). In bedding plants, the best option is to use one of the granular crabgrass products. Check the labels for these products to determine the tolerant ornamental species.

People frequently ask me how to control chickweed and vetch in beds. The preferred strategy is a fall application of a pre-emergence herbicide before these weeds germinate. In spring, your best option generally is hand-weeding.

Weed identification and knowledge of herbicide selectivity is important for ornamental as well as turf areas. Most of the post-emergence grass herbicides available for use on broadleaf ornamentals will not control annual bluegrass. Although clethodim will provide some control of Poa annua, the best option is to use a pre-emergence crabgrass preventer for annual-bluegrass control. Control cool-season grasses-such as quackgrass, tall fescue and orchardgrass-with post-emergence grass herbicides such as sethoxydim, fluazifop or clethodim. If you intend to add mulch, do so after application of a pre-emergence herbicide. Also, you can use landscape fabrics to improve weed control over mulch alone.

Assess the sites you maintain for the presence of weeds. Develop a program to control both the existing weeds, as well as weeds that will germinate later. Beneficial results from such a program will be apparent in spring, when instead of playing catch-up, you'll have time to devote to more pressing maintenance tasks.

Dr. Jeffrey Derr is an associate professor of weed science with Virginia Polytechnic Institute and State University (Virginia Beach, Va.).

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