Overseeding warm-season turfgrasses
As temperatures drop and day lengths get shorter in the southern United States, the growth of warm-season turfgrasses slows to the point of total dormancy (if temperatures drop low enough). However, if you are a warm-season turfgrass manager, especially if you manage a golf course or sports turf, you need to provide green, actively growing turfgrass cover during the winter season or your job may be in jeopardy. What should you do? Overseed with cool-season turfgrasses, of course.
Climatic adaptations To better understand overseeding, a brief discussion of climatic adaptation of turfgrasses is helpful (see Table 1, facing page). Grasses fall into two groups based on their climatic adaptation to temperature. A cool-season grass is one for which optimum growing temperatures range from 60 to 75 degrees F, whereas a warm-season grass is one with optimum growing temperatures from 80 to 95 degrees F. Thus, cool-season grasses are adapted to the northern United States, and warm-season grasses are adapted to the southern United States. The most difficult region to grow turfgrasses in the United States is the Transition Zone, which lies between the northern and southern regions. In the Transition Zone, neither grass type is as well adapted.
The growth-intensity curve in Figure 1 (page Golf 16) shows that unadapted cool-season turfgrasses succeed as winter ground covers in the southern United States, but only during the cool winter months. Overseeding takes advantage of this by using these unadapted, perennial cool-season grasses to provide an attractive, live ground cover for golf courses, home lawns and sports turfs in winter.
Overseeding species choices An overseed mixture is a combination of two or more species of grass. Many years ago, when turfgrass management was more of an art and less of a science than it is now, turf managers used complex mixtures of several species at extremely high rates in the hope that something would grow. Today, simple two-way overseeding mixtures such as perennial ryegrass + rough bluegrass or rough bluegrass + creeping bentgrass are popular.
A seed blend is a combination of two or more cultivars of the same turfgrass species. Seed blends of up to three cultivars of perennial ryegrass are quite popular for overseeding.
To attain high stand density and uniform ground coverage, turf managers use much higher rates for overseeding than for seeding bare ground, especially on putting-green surfaces. Normal seeding rates on bare ground are based on establishing an average of 10 seedlings per square inch of soil surface. However, overseeding into an established warm-season stand requires 5 to 10 times more seed. This means planting 50 to 100 seeds per square inch. Seeding at a rate of 30 pounds of perennial ryegrass per 1,000 square feet on putting greens actually results in a "mulch" layer of seed on the putting surface. Such high rates result in a juvenile seedling stand with few tillers and fine, narrow leaves. This juvenile stand never fully matures during the entire winter growing period. All resulting seedlings are thus vertically oriented which results in a better putting surface on greens.
Table 2 (below) lists the number of cultivars and characteristics of cool-season turfgrasses that are available on the market. Thanks to active and productive breeding programs by universities and private seed companies, your choices of for overseeding are almost endless. This should make consumers happy because the abundance of suitable varieties makes it easier to choose an overseed cultivar, mix or blend on the basis of cost.
* Ryegrass. By far, perennial ryegrass is the most popular turfgrass for overseeding in the South. Its advantages are rapid germination (3 to 5 days) and establishment, ability to compete with and mask annual bluegrass, excellent traffic and wear tolerance, good color, fine texture and high density. Its disadvantages are its high costs (due to high seeding rates) and its competition with emerging bermudagrass during spring transition (due to improved heat tolerance of newer cultivars). Annual ryegrass has many of the same attributes as perennial ryegrass, but lacks fine texture, tends to be yellow-green in color and is susceptible to disease. Annual ryegrass is, however, a good, inexpensive choice for overseeding home lawns and general purpose turfs.
* Rough bluegrass. This species ranks second in popularity for overseeding. It has a moderate establishment rate and is less competitive than ryegrass. Plant breeders have improved the yellow-green foliage color of the 'Danish common' cultivar to the blue-green color of current cultivars. Rough bluegrass has good spring transition, but it lacks wear tolerance and disease resistance. Its small seed size allows turf managers to work seed into a dense bermudagrass canopy with minimal disruption of the playing surface. For this reason, rough bluegrass is becoming especially popular for overseeding the new high-density, ultradwarf bermudagrasses that are coming on the market.
* Bentgrasses. Bentgrasses, especially creeping bentgrass, are popular on putting greens because they provide excellent putting surfaces. Like rough bluegrass, the small seed of bentgrass is easy to work into a bermudagrass canopy. Unfortunately, bentgrasses are slow to establishment and do not provide a good playing surface until late in the growing season. Due primarily to its improved heat tolerance, bentgrass has slow spring transition and is the last cool-season grass to decline in late spring or even early summer.
* Kentucky bluegrass and fescues. These species were popular overseeding choices several years ago. Interest has waned not because they were poor grasses but because breeding programs with increased emphasis on perennial ryegrass and rough bluegrass improved these species and increased their popularity. However, because of serious transition problems experienced throughout the South during the last two years, the popularity of bluegrasses and fescues may again be on the upswing.
Calculate your seed needs Because of the high seeding rates used for overseeding, a frugal buyer should purchase seed based on its pure live seed (PLS) content and number of seeds per unit of weight. A seed lot's PLS content is calculated from purity and germination information that, by law, is listed on the seed tag. In the example in Table 3 (above), you can see that Seed Lot B is a better buy even though both lots cost $1.50 per bulk pound. This is because it has a higher percentage of PLS due to its germination rate. Thus, it requires less bulk seed to equal 1 pound of PLS. In this case, you save 15> per pound when you base seeding rates on PLS. This may not sound like a lot, but in this case a golf-course superintendent could save $405 on overseeding costs for 18 holes by basing a perennial-ryegrass seeding rate of 30 pounds per 1,000 square feet on PLS.
Table 2 is a generalized comparison of cool-season grasses that may be used for overseeding purposes. As it shows, average seed weight differs greatly between grass species. Thus, you should not try to compare per-pound seed costs between grass species. For example, 1 pound of perennial ryegrass contains 240,000 seeds, whereas 1 pound of rough bluegrass contains 2,000,000 seeds. This is an eight-fold difference in potential seedling stand. Note that bentgrasses average 6 million seeds per pound! A smart shopper does not buy according to the size of the bag or price per pound.
Average seed weight also varies between cultivars within a species. Last year in our annual overseed trials, the number of seeds per pound varied between cultivars of perennial ryegrass from 191,500 for '30G' to 281,800 for 'Paragon'. Similarly, the number of seeds per pound varied between cultivars of rough bluegrass, from 1.5 million for 'Snowbird' to 2.9 million for 'Fuzzy'. Wise shoppers will adjust their seed needs and seeding rates based on seed number and correct for this difference between cultivars. Although purity and germination percentages are listed on a seed tag, seeds per pound is not. You should request this information from the seed company when making seed-purchasing decisions. This step, along with the proper use of the PLS concept in overseeding decisions, usually makes "cents."
Dr. Al Dudeck is professor of turfgrass science at the University of Florida (Gainesville, Fla.).
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