Consider the options
There are several reasons to overseed. One primary reason is to provide good green color during the winter in warm-season and transition zone regions when bermudagrass is dormant. Nick Gammill, sports turf manager at American University in Washington, D.C., says, “Besides the color, the actively growing grass offers some protection from both cold and wear to the dormant bermuda.”
Another primary reason to overseed is to fill in worn or heavily used areas. As Steve LeGros, sports turf consultant at Turf & Dirt, Inc., central Pennsylvania and Hagerstown, Md., says, “Overseeding should be done whenever needed to assure a good stand. You need to be proactive and avoid losing the turf canopy.”
Selecting seed to fit the need
There are several different grasses you can use for overseeding. In some situations for fill in, you may want to overseed with the same variety or mixture that is growing on the field. Or, you may want to choose something that will fill in much quicker If you are overseeding with a cool-season grass to provide color and protection on a warm-season grass field, you can choose between numerous grasses. It is important to choose what is best for field use, establishment rate and transitioning needs the following spring.
Annual ryegrasses are among those with the shortest germination period. The problem with common annual ryegrass is it produces clumpy, light green, coarse-textured turf that doesn’t blend with other sports field turf, but there is a new annual ryegrass called Panterra that is more dwarf and darker green than common annual ryegrass.
NTEP trial results and university test plot trials help sports field managers assess the performance of turf varieties in climatic conditions that are similar to their own. The major turf climate zone classifications of warm season, cool season and transition have multiple variations in weather patterns and temperature fluctuations regionally, from one side of a city to another and even within the microclimates on a single field. Some turf managers experiment with different varieties on their own, overseeding them in small areas off the playing surface of the game field.
Tracking performance in these plots in conjunction with notes on daily weather conditions help pinpoint the type of grass and the varieties within that grass that best fit specific field needs. That may lead to the selection of a single variety, a blend (multiple varieties of one turf type) or a mix (multiple turf types with one or more varieties of each).
When the primary consideration is transitioning out the overseeded grass in the spring, LeGros has had success using the SOS (super overseeding) from Barenbrug. It combines the annual ryegrass Panterra with select perennial ryegrasses in a ratio that fits a designated climatic region for the desired transition speed. They split the U.S. into three zones based on the average number of days above 90 degrees Fahrenheit per year, but note that the field manager needs to take localized microclimates into consideration. Choices within each zone provide fast transition, gradual transition or slow transition. For example, on a transition-zone baseball field, you may want to keep the overseeded cool-season turf actively growing throughout the entire season, so you would choose a slow-transition mix.
Your seeding rate will depend on the variety or varieties of seed you choose. Gammill and LeGros both recommend seeding rates of 13 to 15 pounds per 1,000 square feet for overseeding with Panterra. That can drop to 2 to 3 pounds per 1,000 when overseeding wear areas weekly.
Overseeding warm-season turf
If your field is bermudagrass, zoysiagrass or a combination of warm-season grasses, it will go dormant when the soil temperatures go below 72 degrees at a 4-inch depth. The overseeding process needs to start well before the temperature drops.
The first decisions need to be made several months before the actual seeding. You need to establish the approximate date of seeding, which will depend on field use and an open window for preparation and seed establishment. The establishment could take five to 21 days depending on the seed used. You will need to make your seed choice at least partially based on the number of available days without play. If you are using preemergent herbicides, you need to determine when the latest application can be made to allow the germination of your selected seed.
Gammill says, “I overseed our high-end bermudagrass fields in mid to late October. I look for a window in field use that allows a minimum of five to six days to get it done. I’ll seed again at a lighter rate in late November or early December, and again in the spring if we have some areas that need to recover.”
As breeders have developed improved varieties of perennial ryegrasses, along with characteristics of color and density, they also have greater heat, cold and drought tolerance and wear resistance. These ryes have become more aggressive competitors of the bermuda and are harder to transition out in the spring. Even in the warmer areas of the transition zone, cultural practices alone—low mowing, reduced irrigation and a strong shot of nitrogen as soon as the bermuda begins to green up—may not take out all of the perennial rye. It may require the chemical removal with selective control products such as Revolver or Monument, which is typical in cooler sections of the transition zone.
Overseeding cool-season turf
Many turf managers with primarily bluegrass fields start overseeding early in the season with straight bluegrass, switch to a mix such as 50 percent bluegrass and 50 percent ryegrass midseason and close out the season overseeding with ryegrass. Generally, a perennial ryegrass or blend of two or more perennial ryegrasses has been used. Some field managers have worked with the intermediate ryegrasses, either alone or in combination with a perennial variety.
The improved perennial ryes have often been able to outcompete the bluegrasses, gradually changing the year-round percentages of blue to rye in the field. For heavy-wear field sports like football, soccer, rugby and lacrosse, the majority of play takes place in the early spring and late fall, when the faster germinating perennial ryegrasses have the advantage over the bluegrasses.
In heavy-wear areas, such as goalmouths, inside the hash marks, along the sidelines where players stand or referees pace, you may need to overseed on a regular basis. LeGros recommends applying some seed before weekly games to keep ahead of the wear factor. He says, “The athletes cleat in the seed, providing the needed seed-to-soil contact.”
No matter the reason, remember the basics whenever you overseed. Seed-to-soil contact is essential for seed germination and establishment. Remove any surface debris. Drop the mowing height of the established turf. Remove excess thatch. Time core aeration in conjunction with overseeding or about a month prior to it. Apply fertilizer as needed based on soil test results. Apply the seed in at least two directions by broadcasting, slitting or slicing. Topdress following seeding if the budget allows. Keep the seed moist from its first moisture contact through establishment with light syringing two to four times a day. After germination, reduce watering. Once the grass is established, switch to deeper, less frequent irrigation.
Mow the new turf once it reaches mowing height, following the one-third rule to encourage the grass to branch out and fill in more quickly, which creates the dense turf that is better able to withstand wear.
The author is a contributing editor for SportsField Management.