Winter Weeds

It would be nice to think that after struggling with weeds all summer, the fall and winter would provide a well-deserved break from the concerns of weed control. However, winter weeds begin their life cycle in the late summer through the early fall (when daytime temperatures are in the 70s), grow during the winter and die in the late spring or early summer, typically when temperatures exceed 85∞F. As weather conditions become favorable for winter weeds, the growth of warm-season turfgrasses begins to slow as they approach dormancy.

Winter annual weeds such as annual bluegrass, henbit, common chickweed and speedwell and winter perennials such as wild garlic and wild onion are unsightly and can be problematic for desired turfgrasses. These weeds compete for sunlight, water and nutrients that can slow green-up of warm-season turfgrasses during spring transition. Likewise, as dense mats of these weeds die in the spring, openings in the turf canopy allow summer weed species an opportunity to invade the turf.

IDENTIFICATION

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Proper identification of weed species is the first step to a successful weed management program. Control is most effective when weeds are immature (two to four leaf stage) and actively growing. However, proper identification is generally easier once weeds have matured. Many books and weed guides exist to aid in your identification of weeds. Also, make use of your county extension agent, state weed specialist or botanist at the local college or university. Take the time to become educated on weed identification. Knowledge is power.

The second step in managing problem weeds is understanding the biology, morphology and ecology of the undesirable plants. Annual weeds complete their life cycle (germinate, grow, reproduce and die) in a single growing season. Depending on when the annual weed does most of its growing, it is classified as either a summer or winter annual. Summer annual weeds germinate from seed in the spring and early summer, mature during the summer, produce seed in the fall and typically die with the first frost. Winter annual weeds germinate from the late summer through the early spring, grow during the fall, winter and spring, and die in the late spring or early summer.

Perennial weeds live multiple years, but may behave similarly to annual species. Winter perennials such as wild garlic flourish during the cool months and go dormant at the onset of summer heat. Perennial weeds produce reproductive structures (rhizomes, stolons, fleshy tap-roots, corms and tubers) that allow these species to survive from year to year and through adverse environmental extremes (cold, heat, drought etc.). It is difficult to control perennial species because these structures contain the stored food reserves and nodes for subsequent root and shoot growth.

PRE-EMERGENCE WEED CONTROL

Many turfgrass managers do not think of fall as the time to treat turf with pre-emergence herbicides. But for winter annual weeds, preemergence herbicides are a good management tool to prevent weeds from becoming established and are recommended in known areas with high weed pressure. Most of the pre-emergence herbicides labeled for turfgrass are effective for controlling grassy weeds and are generally thought of as crabgrass materials. The herbicides (benefin, dithiopyr, oryzalin, oxadiazon, pendimethalin and prodiamine) that you typically use in the spring for summer annual grass control will also provide control of most winter annual species and even are effective on some winter annual broadleaf weeds. Research conducted at several universities showed that when these herbicides were applied in mid-September, they gained excellent control of annual bluegrass during the subsequent winter and spring. These herbicides were also able to control broadleaf weeds such as common chickweed, henbit and corn speedwell when applied at a similar time.

If winter annual broadleaf weeds are your primary problem, isoxaben may be the best solution. Isoxaben provides the widest range of pre-emergence broadleaf weed control. For example, research conducted in Georgia showed isoxaben controlled lawn burweed, which the previously mentioned herbicides could not control. Also, isoxaben will control broadleaf weeds such as dandelion, white clover and plantain, which typically germinate in the fall or early spring. Like all pre-emergence herbicides, you must apply isoxaben prior to broadleaf weed germination, in the late summer or early fall for cool-season annual weeds. You can also tank mix isoxaben with other pre-emergence herbicides for control of grassy weeds, such as annual bluegrass (Poa annua).

The most problematic winter annual grassy weed is annual bluegrass. In non-overseeded situations, you have several options. The herbicides commonly used for crabgrass control also provide excellent pre-emergence control of annual bluegrass. Other herbicides such as pronamide, ethofumesate and rimsulfuron have shown to provide excellent pre- and postemergence control. However, you must take certain precautions with these products. For example, you should apply ethofumesate only to dormant bermudagrass; applying it to actively growing bermudagrass may cause premature dormancy. Other precautions you should take include not applying pronamide and rimsulfuron on or up-slope to desirable cool-season turfgrass species, as these herbicides may laterally move. Non-selective herbicides such as glyphosate, glufosinate and diquat are products that you can apply to completely dormant turf. After green-up in the spring, it is not uncommon to find dead spots with the carcass of a winter weed in the center of a warm-season turfgrass that you treated during the winter with a non-selective material. This can happen when, at application, you assume the turf is completely dormant when it is not, resulting in the death of the weed as well as the desirable turf.

Where warm-season turfgrasses are overseeded with perennial ryegrass or Poa trivialis, the number of available herbicides becomes limited. The challenge is trying to selectively control the weedy annual bluegrass that is trying to germinate at the same time as the desirable overseeding species. For the past 10 years, the best selective control of annual bluegrass during the overseeding process was a fungicide, fenarimol. Multiple applications were required in the late summer to early fall with the last application two weeks prior to overseeding perennial ryegrass and 30 days prior to seeding Poa trivialis or bentgrass. A follow-up application in early January was required for season-long control. Rimsulfuron, a new product for control of annual bluegrass in overseeded bermudagrass, was introduced last year, and you can apply it within 10 to 14 days prior to overseeding perennial ryegrass and Poa trivialis. These products have some strong advantages and provide good control of annul bluegrass.

POST-EMERGENCE WEED CONTROL

Due to factors such as application flexibility, low cost and overall effectiveness, turfgrass managers have relied on triazine herbicides such as atrazine, simazine and metribuzin in non-overseeded warm-season turfgrasses. This class of herbicides has pre-emergence and postemergence activity on a wide spectrum of weed species, including grassy and broadleaf weeds. These herbicides control previously mentioned weed species along with parsley-piert. Because these herbicides have pre-emergence and postemergence activity, you can apply these herbicides to actively growing weeds and expect residual control. However, when you use them exclusively over several years, annual bluegrass resistance can develop to this herbicide family on many turfgrass sites. If you suspect the development of triazine-resistant annual bluegrass on your site, rotation to a different chemistry of herbicides is the only way to achieve acceptable control. Also, annual bluegrass resistance to the dinitroaniline family (benefin, oryzalin, pendimethalin and prodiamine) of herbicides also has recently been confirmed.

Traditionally, the backbone of postemergence control of broadleaf weeds in established cool-season grasses has been a phenoxy herbicide (2,4-D, MCPP and others). For improved herbicidal activity on a wide range of species, you can add a benzoic acid herbicide (dicamba) to the phenoxy combinations. The result of these combinations is the two- and three-way mixtures (Trimec, Three-way, Triplet and many others). Recently, formulations that contain picolinic acid herbicides (triclopyr) have been released for broadleaf weed control in cool-season grasses. For adequate weed control of large mature weeds, you generally must repeat applications at an interval of 10 to 21 days.

It is important to note that the lower temperatures of winter can influence herbicide activity. It is not uncommon for the development of injury symptoms to be slower during the winter than at warmer times of the year. The same weed control is typically achieved, just delayed. However, there are cases where postemergence control is not as effective because less herbicide is being absorbed and translocated within the plant. Carfentrazone is a herbicide that is added to some formulations of these phenoxy combinations to improve and “speed-up” the control of broadleaf weeds during the cold winter months. Pre-emergence herbicides, on the other hand, are generally not adversely affected by cooler temperatures. In fact, they will typically have a longer residual in cooler soils due to less microbial activity and less volatility when you apply them in the late fall and winter.

BEST DEFENSE: GOOD OFFENSE

While herbicides are an option, the best defense against weed infestations is the promotion of a dense turfgrass stand. Begin with selecting a turf species adapted for the location (sun vs. shade) and intended use (low vs. high traffic). A turfgrass suited to its environment will require fewer inputs and generally will be less weedy. Secondly, perform the appropriate cultural practices (maintaining soil fertility, aeration and moisture, mowing at the correct height, etc.) to promote turfgrass growth. Preventing weed establishment and encroachment can be the result of a competitive turf. Generally, light is required for optimum germination of many annual weed species and, by using a healthy turf canopy to reduce the amount of light that reaches the soil surface, you can reduce weed seed germination. But, when it's necessary for you to use chemical control, using the appropriate herbicides at labeled rates is critical. As with all pesticides, read and follow the manufacturers' directions and recommendations on the label.

Clint Waltz is an assistant professor and Extension Turfgrass Specialist at the University of Georgia (Griffin, Ga.).

TIPS FOR SUCCESS WITH POSTEMERGENCE HERBICIDES:

  • Apply to actively growing weeds,
  • Avoid mowing 24 to 48 hours prior to or after application,
  • Irrigate 24 to 48 hours prior to application,
  • Avoid irrigation within 24 hours following application, and
  • Apply when the weeds are most susceptible, smaller weeds are easier to control than larger mature plants.

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