Improving playability and stability

Dense, sturdy turf forms the best playing surface on athletic fields. One method of increasing density is overseeding, the procedure of adding turfgrass seed to an existing turfgrass base. The primary objective on athletic fields is to improve the playability of the field with better stability due to the increased density and the elimination—or at least the reduction—of bare spots. The improved appearance of the field is a bonus.

Most athletic fields will benefit from overseeding, especially those that support multiple sports.

In cool-season grass regions and some areas of the transition zone, overseeding consists of adding new seed of cool-season turfgrasses to an existing field of cool-season turfgrasses. Generally, that means seeding more Kentucky bluegrass or perennial ryegrass, or a mix of the two, into a Kentucky bluegrass field or a field with a mix of grasses—bluegrasses and perennial ryegrasses or bluegrasses and turf-type tall fescues, or a mix of all three. This type of overseeding provides a better playing surface for current season play and increased turf density for future seasons.  

In warm-season grass regions, and some areas of the transition zone, overseeding generally refers to the addition of a cool-season turfgrass seed, such as perennial ryegrass, to an existing field of warm-season turf, such as bermudagrass. This provides actively growing grass that, in combination with the dormant grass base, supports late fall, winter and early spring play. The overseeded, cool-season turf also brings green color to the otherwise dormant field. The overseeded grasses are a temporary addition to the field. They will be transitioned out through field management procedures or chemical applications, or a combination of the two, to allow the bermudagrass to take over when warmer temperatures spur its growth in the late spring or summer.

The type and amount of seed and the timing and methods of application are based on existing field conditions and the desired outcome. Current and anticipated weather conditions and field use schedules further impact the overseeding process.

There are numerous resources available to help in the selection of the turfgrass type and the specific cultivars to use for overseeding. Considerations include quality; color; density; disease and insect resistance; heat, cold and drought tolerance; establishment rates; and performance under athletic field-specific conditions. Results of the nationwide testing program of The National Turfgrass Evaluation Program (NTEP) are posted on the Web site www.ntep.org. Sports field managers also can tap into their networking resources to find out what cultivars perform most effectively in field conditions similar to their own.

Overseeding basics

A key to success in overseeding is establishing good seed-to-soil contact. That requires some method of working the seed through the canopy of the existing grasses and thatch layer to reach the soil. There are multiple methods to achieve this. Core or spike aeration creates holes within the soil. Seed applied following aeration and topdressed afterward has double protection. Slit seeding makes a cut through the turf into the soil and deposits the seed within the slit. Any type of equipment that can be set to scratch or scarify the surface of the soil will form a better seedbed. Verticutting slices through the turf canopy, creating passages for the seed. Even a leaf rake can be used to open up the turf canopy and thatch layer and lightly scratch the soil. Light brushing that helps work the seed down through the grass blades will be beneficial. Many sports field managers broadcast seed over the turf surface prior to practices or games, counting on the players to work it into the soil with their spikes.

Use soil tests to determine existing conditions. Moderate pH levels as needed. Prior to, or soon after, seeding, apply adequate fertilizer to supplement any deficiencies, and provide the nutrients necessary to support seedling establishment and growth.

Topdressing following full-field or large-area overseeding provides protection for the germinating seed and then for the young seedlings as they develop. Low mowing prior to overseeding helps prepare the turf surface to receive the newly applied seed.

Moisture is needed from the point the seed is applied, through germination and seedling establishment. Frequent, light irrigation cycles that keep the seed and young plants from drying out is another key to success. Avoid too much moisture, which would increase compaction as the field is used, reducing the air and root space needed by the young plants.

Soil temperatures need to be within the range for germination for the type of seed selected. The natural temperature fluctuations during the late fall, with warm days and cool evenings, are well suited to seed germination for cool-season turfgrasses. Heavy evening dew often remains overnight and well into the morning, which may provide adequate moisture for overseeding on non-irrigated fields.

Overseeding warm-season grasses

It takes aggressive procedures to get seed to the soil through a dense bermudagrass field. Verticutting in mid and late summer helps open the canopy and reduce the “puffiness” of excess thatch. Mow at the lowest recommended rate prior to overseeding. Core or spike aerate if temperatures allow and if compaction is an issue. Apply between 10 and 20 pounds of perennial ryegrass seed per 1,000 square feet, depending on the cultivars selected. Apply half the seed, covering the field in one direction. Apply the remaining half in a direction parallel to the first to ensure even seed coverage. Use a mat drag to help work the seed through the remaining turf to the soil. Topdress with 1/8 to.25-inch of material. Irrigate lightly until the seedlings are well established, then gradually resume the regular deep, but infrequent, irrigation cycles.

Overseeding cool-season grasses

Kentucky bluegrass works well for overseeding during the early fall in most cool-season regions. It takes longer to germinate; six to 30 days if a wide range of cultivars are considered, 10 to 21 days for most cultivars used on athletic fields. Perennial ryegrass germination rates range from four to 10 days for most cultivars. So, as the temperatures drop, switch to a mix of bluegrass and perennial ryegrasses. For late-season over­seeding, use a blend of perennial ryegrasses.

Calculate the applications based on pure, live seed. That’s the purity percentage of the seed cultivar as listed on the package label multiplied by the germination percentage on the label. You’ll use a slightly greater amount of seed by weight than a pound to reach a pound of pure, live seed.

Also, be aware of the number of seeds of a specific cultivar per pound. Bluegrass seeds are small (ranging from 1.3 million up to 2.2 million seeds per pound), compared to the larger perennial ryegrass seeds (ranging from 220,000 to 270,000 seeds per pound). Adjust the ratio of the amount of each type of seed used to correspond with the ratio of grass plants you want to overseed. At 220,000 seeds per pound, it would take 10 pounds of perennial ryegrass seed to equal the seed count of 1 pound of bluegrass at 2.2 million seeds per pound.

Prepare the field for the seed application based on the density of the existing turf and the underlying soil conditions. Follow the same application and establishment procedures as used on warm-season turf fields.

Pregerminating seed

Pregerminating the seed can cut germination time on the field in half. It’s a fairly simple process and can be done in any size batch from enough to overseed the entire field to small amounts to use in the divot mix.

The process generally takes four to five days from the start until the seed is ready for application. Plan to use the pregerminated seed within 24 hours, the sooner the better. If necessary, it may be preserved for about a week if refrigerated.

A 50-gallon plastic trash can or a 55-gallon barrel work well as the container. Insert a spigot at the base to drain away the water if you wish. The water will be replaced frequently throughout the process.

Any tightly woven, yet porous, material can be used for the bag to contain the seed. For large amounts, use the bag in which the seed was shipped. Measure the amount of cool-season turf seed you want in the ratio you wish to use. Place it in the bag and seal the bag.

Test plots used for NTEP trials provide sports field managers with the opportunity to observe the turfgrass and gain performance information, which helps in making the cultivar selections for overseeding.

Conduct the process in a heated area, at least 70 degrees. Water can be cool to start with, but not cold, and should reach room temperature within a short time. Add gibberellic acid or an organic fertilizer or biostimulant to the water, with the amount based on the formula for that product and the amount of water used. In the fall, add some fungicide to prevent “damping off.”

Place the sealed bag of seed in the container and add the treated water to entirely cover the seed. The best timing is a 12-hour soak period, followed by a 12-hour period where the water is drained from the wet seed. Soaking from the start of the workday until the end—a period of eight to 10 hours—will work. Lift the bag out of the water momentarily every few hours, and then replace it, to keep the water oxygenated. Running air into the water through a tube from a generator could accomplish the same thing.

Following the soak period, lift the bag of seed out of the water and drain away the water. Allow the seed to drain overnight (or for 12 hours). Prepare a fresh batch of water each day and repeat the soak and drain cycle on days two and three. Examine the seed on the morning of the fourth day. If light colored fuzz is apparent, the seed is ready for use. If not, repeat the cycle one more day.

Clean an area on a concrete or asphalt surface to use to prepare the seed. If temperatures are cold outside, do this indoors. For broadcasting the seed over the entire field, mix it with a carrier of topdressing material or a fine-textured organic fertilizer such as Milorganite. For divot mix, combine the seed with your regular material, such as a combination of sand and calcined clay. If green dye will be used in the mix, do the drying and mixing process on plastic to avoid staining the concrete.

Remove the seed from the water and allow it to drain for several minutes. For large quantities, spread the seed over the concrete and allow it to dry for a few more minutes. Carefully turn it with a shovel if necessary to reach the damp, but not soaked, stage. Alternating shovel loads, add the seed and the carrier in the proportions you want into a cement mixer and mix slowly. Test a small amount of the mixed material to make sure it will flow easily through a spreader. Adjust the proportions as needed and mix just until blended.  

For smaller batches, mix the seed with the carrier using the pile method or the layering method. Mix one shovel full of seed with the desired shovel loads of carrier, placing both materials in a pile. Gently turn the pile with the shovel to mix it well. For the layering method, spread a batch of the carrier over the floor, spread the desired amount of seed over the top and gently mix it with a shovel. Test the flow of the mix if using a spreader for application, and adjust as needed. For divot mix, fill enough 5-gallon plastic pails to equip the divot-walk crew.

Whatever the budget level of the athletic field management program, overseeding can be a valuable resource for increasing turf density.

Suz Trusty is a partner in Trusty & Associates, a communications and market research firm located in Council Bluffs, Iowa.