Unlike disease and insect pests, which tend to occur sporadically, weeds of some sort will almost certainly be a problem every year. In warm-season turf, winter weeds are highly visible and can be particularly unsightly due to the color of dormant or semi-dormant turf. As a result, pre-emergence or early post-emergence strategies for weed control are popular. The reasoning is simple: control the weeds before they become unsightly. In areas where warm-season species grow, the climate is conducive to winter activities. Therefore, winter weed control is all the more important. Unfortunately, the wide array of winter-annual weeds that are problematic in warm-season turf usually prevents a single pre-emergence herbicide from providing enough broad-spectrum control for acceptable weed control. As a result, successful weed control usually requires both pre- and post-emergence strategies.
It’s all in the timing As with any pest, successful management of weeds requires knowledge of their biology and ecology. For instance, is the weed an annual, biennial or perennial? When developing a weed-management program, it is absolutely necessary that you understand such factors. Why? Because annuals must come back from seed every year, whereas perennials regenerate from underground tubers, bulbs, rhizomes, etc. Therefore, pre-emergence herbicides, which prevent seedlings from developing successfully, are seldom effective against established perennial weeds. The vast majority of annuals, by contrast, are easy to control, at least to some degree, with a pre-emergence herbicide.
When considering pre-emergence strategies for annual weeds, it is critical to know when the weeds germinate. Unfortunately, soil-temperature data for weed germination exists for only a few species, and most are summer annuals. Crabgrass and goosegrass are two examples. Previous research has shown that when 24-hour mean soil temperatures average about 52 to 55oF in the upper soil surface for a few days, crabgrass can germinate. The critical average temperature for goosegrass is 60oF. Thus, you can obtain maximum control with pre-emergent application and activation when mean soil temperatures are in the high 40s or low 50s.
In the case of winter-annual broadleaf weeds, such as henbit, common chickweed, lawn burweed, Carolina geranium and hop clover, we know little about critical soil temperatures for germination. In addition, most broadleaf winter annuals are difficult to see when they are small. This is because most winter annuals germinate when the warm-season turf is still actively growing or, at least, still green. Most of the time, you won’t even see them until the warm-season turf goes dormant, which can be weeks or even months after germination. Many times, you will not even notice you have a problem until air temperatures start to warm in late winter and the weeds accelerate their growth. This often leads to the incorrect assumption that winter annuals germinate in warm-season turf in January through April. While some germination may occur this late in winter, most winter annuals germinate in the fall, some as early as late August. Therefore, if you plan to use a pre-emergent to control winter annuals, you must apply the product and activate it (water it in) in late summer before night temperatures start to cool.
Grassy weeds A common winter annual that is a significant problem in turf is annual bluegrass (Poa annua). This weed germinates over a range of temperatures, but it is not readily visible until winter when warm-season turf goes dormant. As temperatures approach a high of about 70oF for several days (which happens in January to April, depending on location), annual bluegrass produces seedheads. In areas with bentgrass or overseeded ryegrass or Poa trivialis (in other words, where turf stays green through the winter), only then does annual bluegrass become an apparent problem. Because of this, many turf managers assumed that much annual bluegrass germinates in the winter and spring. However, research conducted at North Carolina State University has shown that approximately 80 percent of annual bluegrass germinates in the fall. Some germination even can occur in late summer as soon as night temperatures start to drop. Therefore, if you use pre-emergence herbicides to control annual bluegrass, you must make the application in late summer before germination to realize maximum control.
Turf managers have relied heavily on the triazine herbicide family to manage winter-annual broadleaves and annual bluegrass in non-overseeded warm-season turf. These herbicides include atrazine, simazine and metribuzin, and are highly effective on a broad spectrum of broadleaf weeds. They also are active on a few grass species. For instance, annual bluegrass is sensitive to this family of herbicides. However, these herbicides are primarily broadleaf herbicides. The triazines have early post-emergence activity as well as pre-emergence activity. Therefore, application timing is not as critical with these products as with others. Atrazine and metribuzin (less so with simazine) have foliar activity that provides excellent control of sensitive species when they are small. As a result, you can apply these products later in the fall with excellent results compared to products that only exhibit pre-emergence activity.
Currently, the trend is away from using these herbicides for annual-bluegrass control. Many reasons exist for this. Resistance of annual bluegrass to this family of herbicides has occurred. Also, these herbicides can move laterally with water. If cool-season turf is growing near an area that you’ve treated with triazines, the possibility exists for lateral movement which can injure or kill cool-season turf. As the trend of using other products for annual bluegrass control continues, I expect a general increase in winter-annual broadleaf weeds. This is because, with the exception of isoxaben (Dow’s Gallery), few pre-emergence turfgrass herbicides provide a broad spectrum of control on annual broadleaf weeds.
Nevertheless, if application timing is correct, many products can provide good control of annual bluegrass in non-overseeded warm-season turf. Oxadiazon (Chipco’s Ronstar) and most of the dinitroaniline herbicides (such as prodiamine, oryzalin, dithiopyr, pendimethalin, triflualin and benefin) used primarily for crabgrass and goosegrass also will provide good control of annual bluegrass. At this study location, annual bluegrass can germinate as early as the 1st of September. Therefore, we applied pre-emergence products in late August. Note that, as with the triazines, pronamide (Rohm and Haas’s Kerb) has both pre- and post-emergence activity. Therefore, you can obtain excellent results with this product even if you apply it later in the fall. However, like the triazines, pronamide is water-soluble and can move laterally with water, injuring or killing cool-season turf that it contacts.
I must point out that all of the annual bluegrass we found during this study was the annual biotype that comes back from seed. No perennial biotypes were present. Also, in some areas annual bluegrass has developed resistance to dinitroaniline herbicides. Therefore, any long-term approach to pre-emergence control of annual bluegrass must involve rotation among herbicides with different modes of action. This will help prevent the development of resistant biotypes (see the September 1998 issue of Grounds Maintenance for a detailed discussion of herbicide resistance).
Broadleaf weeds As I previously mentioned, the products we studied do a good job of controlling annual bluegrass if you apply them in a timely manner. However, they primarily control grassy weeds. They control some broadleaf weeds as well, but the spectrum of control is not as wide as with the triazines. As a result, you should not rely on these products for broad-spectrum control of broadleaf weeds. In fact, few herbicides are available that provide good pre-emergence control of a wide range of broadleaf weeds. However, isoxaben is an exception, and the dinitroanilines also can be effective against specific broadleaf weeds (check product labels). Thus, they may be effective in certain situations.
One increasingly problematic weed deserves special mention. Lawn burweed or spurweed (Soliva pterosperma) is a winter-annual broadleaf weed that is increasing in warm-season turf. This weed occurs throughout the South, from the East Coast west to Texas. It is spreading rapidly and succeeds with mowing heights as low as 0.5 inch on golf-course fairways. It also occurs in home lawns, athletic fields and other recreational turf. Spurweed can be particularly troublesome because the spurs that form on the seed capsule can be quite painful when someone touches them. This species is extremely difficult to control with pre-emergence herbicides.
Of all the products we’ve tested, Gallery and simazine (or a combination of the two) at the highest labeled rates are the most effective controls of lawn burweed. Gallery provides broad-spectrum pre-emergence control of many other broadleaf weeds as well. However, unlike the triazines, Gallery is basically a pre-emergent only. If weeds have already germinated and emerged, Gallery will not provide control. Therefore, it is extremely important to apply this product before broadleaf-weed germination. Another successful strategy is to tank-mix Gallery with a broad-spectrum post-emergence herbicide that contains 2,4-D, MCPP, dicamba, triclopyr, clopyralid, etc. Remember that Gallery provides little control of grassy weeds.
When warm-season turf is overseeded with perennial ryegrass or Poa trivialis, the pre-emergence control options are severely limited. With the exception o f fenarimol (Rubigan and Patchwork) for Poa annua on overseeded bermudagrass putting greens, most weed problems are easier to manage with post-emergence herbicides.
Successful pre-emergence control strategies for winter weeds requires turfgrass managers to be diligent about applying products on time. You can easily misjudge when winter annuals germinate because warm-season turf is usually still actively growing during this time. However, careful selection of pre-emergence herbicides can form the foundation of successful management of winter weeds. Turfgrass managers should also plan to “clean up” winter-annual weeds with appropriate post-emergence herbicides. A comprehensive management program that includes both strategies will reduce winter weed problems over time.
Dr. Fred Yelverton is professor of turfgrass science at North Carolina State University.