A weed problem in newly established turf often means that one of the steps required to establish the turf has failed. To minimize competition from weeds, these steps are critical: soil testing and amendments; using the most appropriate turfgrass cultivar, blend or mix for the site; using proper seeding rates and methods; and providing good post-seeding care of the seedbed.
One of the most useful tactics is to establish turf during those times of the year when weed competition is minimal. Therefore, establishing cool-season grasses in the fall, and warm-season grasses in late spring and early summer, is particularly important.
However, sites differ in many ways, including the amount of weed seed present in the soil “bank.” If a site has had a history of significant weed infestations, the soil seed bank can be extensive. When this is the case, the disturbance caused by seedbed preparation can promote enormous amounts of weed seed to germinate. Thus, even though you may diligently take the proper steps, the site can still end up being a battle between plants that are desired and those that are not. In such cases, you must take steps before or after establishment to reduce weedy competition.
One way to reduce the competition from weeds in the seedbed is to fumigate the soil prior to seeding. Fumigation is relatively expensive and, with methyl bromide, it requires the use of a restricted fumigant (one which soon will be unavailable). Nevertheless, methyl bromide is still used occasionally by some landscapers and golf-course superintendents.
Methyl bromide is appropriate where the weed history is well known and would lead one to anticipate serious problems after establishment. For example, a site that has an abundance of quackgrass, which would be extremely difficult to control after establishment, would benefit from fumigation. The same is true for sites with a known history of annual-bluegrass infestation. Although annual bluegrass can be successfully controlled after the turf is established, a preventive approach may be easier than trying to eliminate it after it is present.
You should only fumigate where you can control access to the site and when you cannot otherwise effectively control the weeds present. If you decide to fumigate, give serious consideration to using a commercial applicator. Such a choice is particularly important if you wish to use methyl bromide because experienced applicators can produce superior results with this chemical.
One of the most effective tools for reducing weed competition is mowing. When the seedling turfgrass has grown to the point where you can mow it without scalping injury, do not hesitate to do so. Removing the top growth from the seedling grass plants will promote basal (daughter) tillers, which will increase the turf stand's density. Increasing stand density reduces the number of spaces that a weed seedling can occupy.
Also, some broadleaf summer annuals, such as pigweed, lambsquarters and galinsoga, that might invade the seedbed do not tolerate close mowing. The same is true of some of the upright-growing summer annual grasses, such as foxtails and barnyardgrass. Another important impact of early mowing is that these weeds will not be able to produce many seedheads, thus significantly reducing the amount of new seed being added to the soil. Therefore, you can effectively control these species in this manner, as well as reduce their potential for causing future problems.
If necessary, you can use a vertical mower adjusted to cut about 0.75 to 1 inch above the soil surface. This usually requires a little trial and error to find the height adjustment that maximizes removal of weed leaves without harming the grass plants. Remember that most seedling grasses have crowns that are located quite near the soil surface and will not be affected by vertical mowing at these heights. In addition, seedling grass blades are mostly vertically aligned, which allows the vertical mowing blades to pass through them without harm.
When forced by circumstances to establish cool-season grasses at less-than-desirable times of the year (spring or early summer), the competition from summer annual grasses (foxtails, crabgrass, goosegrass and barnyardgrass) can be severe. In most of the cool, humid regions of the country, the soil seed bank often contains abundant amounts of these summer annual grasses. For nearly 40 years, the herbicide siduron (Tupersan) has been the herbicide of choice when establishing cool-season grasses at a less-than-optimal time of the year. This herbicide can provide excellent pre-emergence control of foxtails and smooth crabgrass without affecting the germination of the desired cool-season grasses. It's ideal to apply siduron to the site following seeding but before any summer-annual grassy-weed germination. However, siduron does have some degree of early post-emergence control of these species.
Turf managers often apply siduron as a combination product with starter fertilizer. This approach allows the applicator to “kill two birds with one stone.” Another popular and effective way to use siduron is in a hydraulic-seeder mix. The seeding operation can therefore apply seed, fertilizer, pre-emergence herbicide, a little water and mulch all at the same time.
It is a good idea to provide establishing turf the best possible opportunity to compete with the weeds before applying any post-emergence herbicide. By doing so, you will allow the seedling turfgrass to become established, occupy space, increase in density and mature. Plus, allowing the new turfgrass community time to “equilibrate” with the competing weeds allows you to better assess which herbicide to use or whether one needs to be applied at all.
In addition, allowing the turf to become more mature will increase its tolerance to herbicide applications. By waiting until you have mowed the site at least three times, you can make a reasonable assessment of which herbicide, if any, to use.
The herbicide with the greatest safety on seedling turf is bromoxynil (Aventis' Buctril). This herbicide is safe on most seedling turf and is effective on an array of seedling broadleaf weeds (particularly the summer-annual types). Be aware that bromoxynil is much less effective on mature broadleaf weeds. Also, it is not registered for use on residential sites.
Quinclorac (BASF's Drive 75 DF) is a post-emergence herbicide that is effective against selected broadleaf and grassy weeds (notably crabgrasses, foxtails, clover and dandelion) that you can apply to most seedling turfgrasses (see product label — turfgrasses have varying post-germination application intervals). Quinclorac is labeled for both residential and non-residential uses.
Turf managers have many tools to ensure the successful establishment of turfgrasses, including cultural practices such as soil testing and amendments, proper fertility, appropriate seeding date, careful selection of cultivars, blends or mixes, and other post-seeding practices (especially irrigation).
Unfortunately, having done “all the right things” won't necessarily prevent the need for herbicides. However, their effectiveness will be enhanced when the seedling turf is competitive. In addition, a strong turf stand may mean that the herbicide application is not required “wall-to-wall.” Rather, spot applications may be adequate to address those areas where the turf has not competed successfully with the weeds.
Addressing weed infestations during establishment is a critical step. Starting off with a strong stand will result in a more competitive turf and reduce the severity of future infestations.
Dr. Thomas L. Watschke is professor of turfgrass science at The Pennsylvania State University (University Park, Pa.).
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