Now that you’ve spent the season successfully managing your bermudagrass field, fall is here and it’s time for winter overseeding.

Wait — what? Why would you do that? Why would you spend all that time, money and effort to create a beautiful, perfect, weed-free monostand of bermudagrass and then turn around and introduce, via seeding (on purpose!), millions upon millions of weedy plants?

Well, aesthetics and better wear tolerance are two of the major reasons. In both golf and athletic turf, winter overseeding has been done for years, mainly in the Southwest but also in the Southeast, in order to have green grass to play on in the middle of winter.

One of the disadvantages of the savanna type (chloridoid) warm-season grasses, which include bermudagrass, is that during the winter they go dormant in cooler regions of their adapted range. (When dormant they turn a straw or tan color.) Not that we use them for sports fields, but this is perhaps a better outcome than what would happen with the tropical warm-season grasses (Seashore paspalum, St. Augustinegrass, etc.) because these grasses simply die where it’s too cold in the winter, rather than go dormant. For sports field managers, winter overseeding is a practice conducted mostly on dormant bermudagrass. The fact that the field is dormant, rather than green and actively growing, is a problem not only aesthetically, but from the standpoint of being able to tolerate wear and host sporting events. The concept sounds simple – interseed a cool-season grass in the fall that will grow and remain green during the winter, when weather conditions favor its growth. What follows is the cool-season grass goes away and the bermudagrass returns with the onset of summer.

Which grass?

Many different cool-season grasses are utilized for winter overseeding on golf courses, and naturally each has its own strengths and weaknesses. But only ryegrasses are usually considered for use when winter overseeding athletic fields. The other species have some potential advantages, but there’s usually a good reason for these grasses not to be used:

One example is rough bluegrass. It germinates quickly, has excellent cold tolerance and a fine texture that makes it aesthetically pleasing. Also, spring transition is quicker compared to other grasses. But, rough bluegrass is not commonly recommended for sports fields because it has comparatively poor wear tolerance and isn’t as competitive with annual bluegrass (Poa annua), which can be a major weed issue on an overseeded field.

Creeping bentgrass has fine texture and is very tolerant of cold. It also germinates and establishes fairly quickly. But it’s more likely to be used on golf courses because of its lack of traffic tolerance. Another limiting factor for using bentgrass is that it has excellent heat tolerance, which makes it more difficult to transition back to bermudagrass in the spring.

Fine fescues have a fine texture and transition easily in the spring. But they have less traffic tolerance compared to the ryegrasses. Also, fine fescue germinates and establishes more slowly than the other grasses. These grasses are mentioned here because, while they might not seem like good ideas today, breeding efforts constantly produce improved cultivars, which may one day make them viable choices.

Other cool-season grasses that are not often considered are tall fescue and Kentucky bluegrass. The turf-type tall fescues might seem like they would have some worthy agronomic characteristics. But the heat and drought tolerance of tall fescue makes it a very difficult grass to remove during spring transition. An article I read recently talked about the idea of managing bermudagrass as a mixed stand with a more heat-tolerant cultivar of Kentucky bluegrass. This is an interesting idea, because as breeding efforts for Kentucky bluegrass evolve, there might one day be cultivars that could be used for this purpose. But as of this writing, our choices for overseeding athletic fields are the ryegrasses.

Perennial ryegrass germinates and establishes very rapidly. Because of this it’s more competitive with annual bluegrass. It also has good wear tolerance and in most places where bermudagrass is grown, it won’t get cold enough for winterkill to be an issue. The texture and color are also both very good. Its disadvantages are that its heat tolerance can cause spring transition to be slower and you must use an herbicide to remove it. It’s also on the expensive side, and you should make sure to only use turf-type rather than pasture-type varieties.

Annual ryegrass germinates very quickly and is inexpensive (half as much, or less, compared to perennial ryegrass). Since it’s an annual, it will die on its own in early summer and thus doesn’t require any special management or additional herbicides. Annual ryegrass has some disadvantages, though. Sometimes it dies on its own in late June or early July, which is late enough to potentially limit the growth period for the bermudagrass. On the other hand, it might die too early in the spring, leaving bare areas on a field. Growth tends to be excessive, thus requiring more mowing. It also has a coarse texture and a noticeably lighter green color. Annual ryegrass has comparatively poor cold tolerance, meaning increased potential for some winterkill. Some improved varieties are making their way into the market.

Best management practices

Choosing the grass is only the first step in a successful winter overseeding program. Proper management of the bermudagrass prior to seeding and best management practices when overseeding, along with seedling establishment afterwards, is just as critical.

Practices such as scalping or verticutting that might slow bermudagrass growth may seem like a good idea. But research shows this doesn’t improve ryegrass cover, and it can result in a less healthy bermudagrass base. Research also shows that an application of herbicide that suppresses the bermudagrass doesn’t improve ryegrass cover and can be detrimental to the bermudagrass. If the bermudagrass is also subject to environmental stresses such as shade or poor drainage, or if there are pest issues, then the transition back to bermudagrass in the spring will be more difficult. For overall best success, it’s important to understand that while you want to slow the bermudagrass a bit to aid in seedling germination, you must maintain a healthy stand of bermudagrass.

Beyond the above, there is not a singular best way to overseed, and practices can vary by region. But we can generalize. For example, thatch management is important, because an excessive thatch layer can impede seed germination. A heavy coring several weeks in advance of overseeding can help improve bermudagrass health and decrease thatch. The major weed of concern in winter overseeding is annual bluegrass. To deal with it, a sulfonylurea herbicide such as DuPont’s Tranxit (rimsulfuron) can be applied prior to seeding. Naturally, you should consult and always follow label directions.

Rimsulfuron will also suppress the growth of the bermudagrass temporarily. If you don’t control annual bluegrass with growth-regulating herbicide (such as rimsulfuron) prior to overseeding, it may be beneficial to apply a growth regulator such as trinexapac-ethyl a couple of days prior to seeding, in order to slow the bermudagrass and make it less competitive against the seedlings. Another purpose of the growth regulator is to decrease the frequency of mowing required during seedling establishment.

Seeding date is important for successful establishment but varies depending on where the field is located. In the cooler parts of the country where bermudagrass is used, seeding around mid-September is advised – data shows that the overall cover of perennial ryegrass can be reduced if too late a seeding date is chosen. Generally, you want to seed by Sept. 21 in Northern areas and by Oct.21 in the South.

Besides thatch control and weed management, little other mechanical preparation is usually done. Bermudagrass growth tends to change in the fall and the canopy becomes a bit more open and less dense. This makes it easier for seeds to move into the canopy. But proper seed soil contact is critical for success. After seeding, it’s helpful to drag the field to help move the seeds down to soil level. Though more expensive, you can also topdress, which will enhance establishment by 50 percent or more. The actual seeding rate varies by cultivar and species but is usually much heavier than would normally be used during establishment (around 10 to 20 pounds per 1,000 square feet). The general idea is to give the field a green appearance, and the combination of overseeded ryegrass and a healthy bermudagrass base will contribute to the traffic tolerance.

Proper seeding practices afterwards are also very important. After seeding, watch for signs of nitrogen deficiency but avoid applying nitrogen until the bermudagrass has stopped growing and the risk of seedling diseases is reduced. Then, apply nitrogen at 0.5 to 1.0 pounds per 100 square feet. This can be repeated periodically to maintain color or promote recovery of the ryegrass, and at least one of the fertilizer applications should contain potassium. Irrigation should be light but frequent in order to keep the top soil layer uniformly moist, but not overly wet. After germination and over a couple of weeks, the irrigation should be reduced so that you’re only watering the seedlings when they wilt in order to decrease disease potential. You may need to apply fungicides for control of Pythium or brown patch. If this is, or has been, an issue, then fungicide- treated seed can result in better stand establishment. Note that mowing of ryegrass should be done with a sharp blade.

One of the most important factors for successful overseeding of bermudagrass is to maintain a healthy base of the grass. Practices that encourage seedling growth by damaging the bermudagrass will make the spring transition back to bermudagrass less successful.
PHOTO: KARL DANNEBERGER

The transition back

Spring root decline can be an issue with overseeded bermudagrass. As soil temperatures begin to warm, regrowth of stolons, rhizomes and shoots commences. If the soil warms too rapidly, then rapid growth of these shoots puts a stress on the energy reserves of the bermudagrass. Unfortunately, at about this same time conditions for ryegrass growth can be ideal and the competition results in less light reaching the bermudagrass. This can lead to die back of the bermudagrass roots. Therefore, proper management when transitioning the field back to bermudagrass in the springtime is very important.

Several practices are conducted in early spring to aid in transition back to bermudagrass. One is to lower the height of cut until new bermudagrass growth is noticed, after which the height of cut is raised to normal. Vertical mowing can be useful in high seeding rate areas but should not be done after bermudagrass begins to grow. Core cultivation can delay bermudagrass if done prior to green up. But if this is done after bermudagrass is actively growing, it can be helpful. Nitrogen fertilization should be increased, as initially this is of benefit to both the ryegrass and the bermudagrass. But as temperatures rise, the increased growth will become more detrimental to the ryegrass.

In addition to cultural practices to favor the bermudagrass over the ryegrass, postemergent herbicides can be used to remove perennial or intermediate ryegrass. There are several sulfonylurea herbicides, including Tranxit (rimsulfuron), PBI-Gordon’s Katana (flazasulfuron), Bayer’s Revolver (foramsulfuron), Nufarm’s Manor (metsulfuron) and Syngenta’s Monument (trifloxysulfuron). Combination products, including Quali-Pro’s Negate (metsulfuron plus rimsulfuron) are also marketed to aid bermudagrass transition. Annual ryegrass will die on its own, but you should mow at about 0.75 to 1 inch. After bermudagrass begins to green up, apply nitrogen to encourage bermudagrass growth and accelerate the annual ryegrass decline.

Should you do it?

There’s no right or wrong answer. It’s a business decision, but there are some factors to consider. Purposely introducing a “weedy” species into your field, if the field has a lot of events during times when the bermudagrass is dormant and the wear and tear is enough to cause loss of the bermudagrass, then certainly winter overseeding should be an option to consider. This is the case mainly because proper overseeding does provide a certain amount of protection for the bermudagrass. The improved perennial and intermediate ryegrasses can contribute to shear strength during winter. There are fields where overseeding isn’t done and the bermudagrass is then worn away during winter play. These fields are obvious candidates.

Another benefit of winter overseeding is that the soil temperature is decreased, which decreases weed seed germination. Also, the overseeded grass provides competition against germinating weeds.

In some cooler parts of the country where bermudagrass is used, the transition in the spring can be delayed sufficiently so that the bermudagrass declines and is eventually replaced by the ryegrass, which then does not perform as well in the summer as did the bermudagrass – hence why bermudagrass was chosen to begin with. Even if you don’t have this issue, winter overseeding can be an expensive process. In areas where water use is restricted, for example, winter overseeding might not be practical or possible. Even if this isn’t an issue, winter-overseeded bermudagrass is going to require more water, fertilizer and pesticide inputs compared to non-overseeded fields. On fields with less traffic, an option might be to dye the field instead. While you might have to apply the dye several times during the winter, this will still be a cheaper alternative.