Overseeding Bermudagrass Turf
Texas Cooperative Extension
Text and images copyright © Richard Duble.
To some, overseeding is a simple operation-just scatter some seed and
wait until they germinate. But to the professional turfgrass manager or
golf course superintendent, whose success depends on the quality of his
product, overseeding is a complex operation that requires preparation, timing
and luck. Unfortunately, knowledge and experience do not always suffice.
Just plain bad luck, usually unfavorable weather conditions, can sometimes
ruin sound procedures.
The objective of a skilled turfgrass manager is to minimize the chance for failure by proper seedbed preparation, planting adapted grass varieties at the proper time and careful management during the seedling stage.
Preplant PracticesSeedbed preparation is just as important for overseeding as for establishment of a new turf. Perhaps one of the greatest causes of a poor stand of winter grasses is poor seedbed preparation. Thatch, compacted soils and weeds in the seedbed can result in seedling diseases and thin stands of grass during the early stages of overseeding.
Seedbed preparation does not begin two weeks before the date of seeding. It begins several months prior to seeding. Light vertical mowing during late summer and fall helps to reduce thatch in bermudagrass turf. Aeration and topdressing also help control thatch, provide a smooth surface and create favorable conditions for germination of winter grasses. Timing of each of these practices is crucial to their success.
Light vertical mowing should begin in midsummer and continue through fall when bermudagrass growth slows. Frequent vertical mowing so that the effects are not noticeable after several days is the most successful program to follow. Vertical mow golf greens weekly and sports fields monthly during this period. The use of grooming mowers several times each week may substitute for vertical mowing on golf greens. Infrequent and severe vertical mowing results in unsightly turf and poor playing surfaces during late summer.
Aeration is also important to seedbed preparation. Late spring and summer are ideal times to aerify to alleviate compaction, reduce thatch and help develop a seedbed. Overseeded sites should not be aerated after September 1 because it promotes germination of annual bluegrass. Core aeration within a month of the date of planting also results in the seed emerging in clumps rather than in a uniform stand over the site.
Light and frequent topdressing also helps prepare a bermudagrass turf for overseeding. Topdressing materials may vary between sites, but topdressing for sand based rootzones should be free of sand particles larger than 1.0 mm in diameter and should contain less than 10% of particles (sand, silt and clay) below 0.1 mm in diameter. Organic amendments may be added to the sand to "soften" the mix and increase its moisture and nutrient retention. Where organic amendments are high in fine sand, silt and clay, the final mix should not contain more than 10% (by weight) of particles below 0.1 mm in diameter.
Topdress turf growing on native soils with soil similar to that which exists at the site. Generally, sandy loam soils are ideal for topdressing such sites. More important than helping to prepare a seedbed, all of these practices (vertical mowing, aeration and topdressing) help maintain an attractive, smooth, resilient surface up to the time of overseeding. Of course, routine mowing is the unwritten requirement that must go along with the other practices to maintain fine bermudagrass sports fields and putting surfaces.
Where these cultural practices are followed, there is little else that needs to be done at the time of overseeding. Disease control prior to the date of planting is the only other requirement for seedbed preparation. A broad-spectrum fungicide should be applied one to two weeks prior to planting. The fungicide application will reduce populations of soilborne disease organisms that attack seedling grasses. Fungicide treated seed can also be planted to reduce seedling disease such as Pythium.
Annual Bluegrass ControlAnnual bluegrass (Poa annua) can be controlled (but not eliminated) through cultural practices. If overseeded sites are prepared as previously described, perennial ryegrasses are the dominant overseeded grasses, planting date is properly timed and seedling diseases are controlled, annual bluegrass should not be a serious problem on overseeded sites. On the other hand, where bermudagrass is severely thinned by vertical mowing immediately prior to planting, the site is aerated in September or October when annual bluegrass is germinating and a dense stand of overseeded grasses does not develop rapidly, annual bluegrass readily invades the winter grasses.
Where turf managers find it necessary to use preemerge herbicides or Rubigan for annual bluegrass control, they should be applied 60 days prior to the expected planting date. Preemerge herbicides should not be used on poorly drained sites or on sites that are moderately shaded. And, when used, herbicides must be applied uniformly at recommended rates. Rubigan can be used on sites not suitable for other preemerge products. Ideally two applications of Rubigan 60 days and 30 days before seeding, should be made. Finally, perennial ryegrasses should be the dominant overseeded grasses planted where preemerge herbicides are applied prior to seeding. Poa trivialis, fescues and bentgrasses are more sensitive to preemerge herbicides than the perennial ryegrasses.
The use of preemerge herbicides, proper seedbed preparation and perennial ryegrasses can effectively eliminate annual bluegrass as a problem on overseeded sites.
Grass VarietiesThe choice of grass varieties for overseeding influences cost, texture, color, time required for a complete cover, cold tolerance, wear tolerance and, to some degree, spring transition. But, as for the success of the overseeding program, the choice of grass varieties is secondary to seedbed preparation.
The most expensive grasses in terms of seed cost per 1,000 sq. ft. are the perennial ryegrasses. Mixing fescues, bentgrasses and/or Poa trivialis with the ryegrasses reduces seed costs per 1,000 st. ft. and may improve the characteristics of the mix in terms of cold tolerance, texture and spring transition. Perhaps the least expensive overseeding grass that produces attractive and good playing surfaces is Poa trivialis . The only weakness of pure stands of Poa trivialis are wear tolerance and annual bluegrass invasion.
Even with higher seed costs, most turfgrass managers prefer the perennial ryegrasses or mixtures with a high percentage of perennial ryegrass. Their fast establishment, wear tolerance and competitiveness with annual bluegrass give the turf manager greater opportunity for success. And, in overseeding operations, reducing opportunities for failure is important to the manager and the facility.
Varieties of perennial ryegrass that consistently perform well include Birdie, Caravelle, Citation, Delray, Derby, Fiesta, Gator, Goalie, Loretta, Manhatten, Omega, Palmer, Pennant, Pennfine, Prelude, Regal and Yorktown. Grass mixtures or blends containing perennial ryegrasses have also performed very well.
Poa trivialis in combination with the perennial ryegrass improves the density, texture and cold tolerance of overseeded sites compared to ryegrasses alone. Usually, 15 to 20 percent Poa trivialis is required to make a noticeable difference. The addition of as much as 20 percent Poa trivialis also reduces the cost of seeding. Varieties of Poa trivialis that have been consistently outstanding include Sabre, Laser and Cypress.
Fine fescue, bentgrass and bluegrass can also be added to mixtures with perennial ryegrasses to improve the color, texture and cold tolerance of overseeded golf greens. Fine fescues are desired because of their very fine, stiff texture and good wear tolerance. However, the fescues do poorly under wet conditions and do not persist in hot weather. Improved varieties of fine fescues that have performed well include Atlanta, Banner, Dawson, Highlight, Jamestown, Marker, Scarlet, Southport, Vista and Warick.
Creeping bentgrasses are often used in overseeding mixtures because of their fine putting quality and persistence into early summer. Cobra, Seaside, Pennfine, Pennlinks, Putter, SR1020, Penneagle, and Emerald creeping bentgrasses are most often used in overseeding mixtures. A relatively slow rate of establishment is the major drawback to creeping bentgrasses for overseeding purposes.
Kentucky bluegrasses are used in overseeding mixtures to provide a dark green color. They also have excellent cold tolerance and retain their green color under freezing temperatures. However, the Kentucky bluegrasses are very slow to establish a cover and contribute very little to an overseeding mixture for the first several months. Varieties of Kentucky bluegrass that have performed well in overseeding trials include Alble-1, Arista, Baron, Mystic, Nugget, Pennstar, RAM, Touchdown, Vantage and Victa.
Planting ProceduresMost golf courses and sports fields are played straight through the overseeding operation. Thus, it is important to maintain good playing conditions throughout the period of overseeding. Playing condition is only one reason to begin preparing for overseeding several months in advance of planting time. Other reasons include maintaining an attractive appearance and giving the overseeding operation every opportunity to succeed.
If the site has been prepared as described above (aerated, dethatched and topdressed during late summer), only regular mowing is necessary immediately prior to seeding.
Distribute seed in several directions to obtain uniform distribution. Water lightly for several days to work the seed into the turf. Then, topdress with sand or a topdressing mixture and smooth with a brush or carpet drag.
Watering is critical during the establishment period. But, avoid overwatering. Wet, watersoaked sites are not playable and are prone to disease problems during seedling establishment. On the other hand, the surface must be lightly watered at frequent intervals to obtain rapid germination. Uniform application of water is essential to uniform emergence of seedlings. Light watering two to three times a day for 7 to 10 days is ideal. After seedlings emerge, watering frequency can be gradually reduced to your regular schedule. Do not continue the light, frequent irrigation schedule past the germination period. Thorough irrigation at less frequent intervals is important to leach salts below the rootzone and to promote root development.
Planting date plays an important role in the success of an overseeding program. Planting too early increases problems with seedling diseases and with bermudagrass competition. These two factors can seriously weaken and thin overseeded turfgrasses. On the other hand, planting too late can prolong the time required to obtain a complete cover because of low temperatures. The ideal planting date occurs when soil temperature at the 4-inch depth reaches 72°F and remains at that temperature or below for 48 hours. Generally, the time to plant is after bermudagrass has nearly ceased growing, but before freezing temperatures are expected. A more specific date would be two to three weeks before the average first frost date. In the northern half of the bermudagrass belt (North Texas, Oklahoma, Arkansas, Tennessee, North Carolina and the northern regions of Mississippi, Alabama and Georgia) this date would be October 1-15. In the southern half of the zone the optimum dates would be October 15-November 1 and in southern extremes of Texas and Florida, November 1- November 15.
Seeding rate is also important to establishing a fast, dense cover of overseeded grasses and to competing with annual bluegrass. Low seeding rates result in thin stands of winter grasses and high populations of annual bluegrass. On golf greens, perennial ryegrasses should be planted at 30 pounds of pure live seed per 1,000 sq. ft.; Poa trivialis, at 10 pounds; fine fescues, at 30 pounds and bentgrass, at 4 pounds. Mixtures of these grasses should be planted at rates according to the percentage of each grass in the mixture. For example, an 80-20 mix of perennial ryegrass and Poa trivialis should be planted at 26 pounds per 1,000 sq. ft. (80% of 30 pounds plus 20% of 10 pounds, or 24 pounds of ryegrass and 2 pounds of Poa).
Sports fields are planted at about 30% of the seeding rate of golf greens. Sports fields require 8 to 10 pounds of perennial ryegrass per 1,000 sq. ft. or 3 pounds of Poa trivialis. Only the infield of a baseball field needs a higher seeding rate. The infield needs about 75% of the seeding rate of golf greens.
Maintenance PracticesMowing, watering, fertilization and pest management are all critical to the successful establishment of newly overseeded winter grasses. Mistakes, or poor judgement, with any of these practices can lead to poor stands of winter grasses and heavy infestations of annual bluegrass.
Since play is continuous on newly overseeded golf greens, superintendents must mow greens daily. Raising the mowing height to -inch and removing grass catchers for about two weeks after planting will help develop a fast cover. It is absolutely necessary to keep the mower razor sharp during this period to prevent pulling up the young seedlings. Daily mowing at -inch during this time will produce slow, but playable greens. After two weeks the mowing height can be reduced at 1/32-inch increments to about 3/16-inch four to six weeks after planting. Daily mowing with sharp mowers is essential throughout this period. Greens should be mowed when the grass is dry to prevent tracking seed onto the collars. It may be helpful to lightly water the greens at daylight to wash dew off the foliage. Not only will that speed drying, but it may help reduce disease development and leaf tip burning attributed to leaf exudates.
Newly overseeded sites should not be kept wet and should not be allowed to become excessively dry. Close attention to watering is important for the first several weeks after planting to establish the grass and provide playable conditions. Avoid late evening watering that keeps grass moist all night. Diseases can develop and spread rapidly when grass remains moist overnight.
Fertilize overseeded sites with a complete fertilizer such as 12-4-8 at about 1 pound of nitrogen per 1,000 sq. ft. Apply fertilizer immediately after seeding so as not to "burn" the young seedlings. After seedlings emerge, light applications of nitrogen will help produce a dense, healthy stand of grass. Soluble nitrogen sources such as urea or ammonium sulfate can be applied most effectively as a foliar spray at rates not exceeding ° pound of nitrogen per 1,000 sq. ft. per week. Nitrogen sources with a low "burn" potential such as Nutralene, IBDU or Milorganite can be applied in dry applications at rates between ° and 1 pound of nitrogen per 1,000 sq. ft. Very low rates of soluble nitrogen, such as 2 to 4 ozs. per 1,000 can be applied as a foliar spray along with fungicides.
Seedling diseases such as Pythium and brownpatch must be controlled to maintain a thick, healthy stand of winter grasses. Application of a preplant fungicide and the use of treated seed will go a long way toward producing a disease-free turf. Diseases are much more effectively controlled on a preventative rather than a curative basis. Once a disease becomes a problem it can set overseeded grasses back several weeks. Strict attention must be given to spray schedules for disease prevention for the first several weeks after planting.
Where overseeded fields are covered with tarps during wet conditions, a fungicide such as Subdue or Aliette must be applied before covering to reduce Pythium.
Broadleaf weeds such as lawn burweed, chickweed and clover can be controlled after winter grasses are established. Products such as Trimec, Weedone DPC, Turflon II Amine and Confront can be used if label directions are followed.
Set-Up ScheduleIt is important to develop an overseeding schedule well in advance of planting time. A suggested program might include the following:
- Measure overseeded sites to determine the quantity of seed needed
- Decide on a variety or mixture and order seed in midsummer
- Select several new grasses or mixtures to observe on a practice putting green or test location
- Vertical mowing schedule (begins during midsummer)
- Aeration (late summer)
- Topdressing (prepare topdressing material ahead of time)
- Preemerge herbicide or Rubigan (optional)
- Preplant fungicide
- Seeding rate (calibrate spreaders)
- Seeding date (soil temperature at 72°F at 4-inch depth)
- During seedling emergence
- After seedling emergence
- Fungicides (Pythium and brownpatch)
- Herbicides (broadleaf weeds)
- Before seeding
- After seeding
- Poor seedbed preparation
- Planting too early (too late)
- Seedling diseases
- Herbicide injury (pre- and post-emerge)
- Overwatering (or excessive rain)
- Fertilizer burn
- Mowing with dull mowers