Turfgrass seed selection is one of the most important decisions to be made when establishing a playing surface suitable for athletics. Since the field is meant to be permanent, it is important to select a grass species adapted to the area and to the intended level of management.
The most important criterion when selecting grass seed for athletic fields can be summarized as:
- Ability to tolerate heavy traffic/wear and to rejuvenate or “fill-in” any worn areas quickly.
- Quick seedling germination and establishment.
- Good color and density
- Drought, heat and cold stress tolerance
- Moderate mowing height tolerance within species
- Pest, disease and weed resistance
Only a few grass species are useful for athletic fields in the cool-season zone. Each species has relative advantages and disadvantages based upon the above criteria. In essence, no one grass species is perfect. The recommended species are Kentucky bluegrass, perennial ryegrass and tall fescue. Individual cultivar performance can be evaluated on the National Turfgrass Evaluation Programwebsite. In the warm-season zone bermudagrass is the most common athletic field grass since it is a fast-growing grass with aggressive creeping stolons and rhizomes. Several varieties of bermudagrass (Riviera, Sunsport, Veracruz, Yukon) produce viable seeds.
Timing the seeding operation
The ideal time to do any kind of renovation of cool-season turf is in the fall (August and September), when soils are warm, there is a good chance of rain, and weed pressure is low, but there are other times when renovation can be done. For example, some fields are used February through May with downtime in June and July before fall sports resume in August. Renovating cool-season turf fields in June and July is far from ideal. Unfavorable weather conditions and pressure from weeds like crabgrass, goosegrass and nutsedge make it almost impossible to get cool-season grasses established, especially slow-growing species like Kentucky bluegrass.
Warm-season turf species like bermudagrass grow best at soil surface temperatures from 80 to 95 degrees Fahrenheit, but they can be established as soon as soil temperatures reach 50 degrees Fahrenheit. Bermudagrass establishment by seed is typically carried out in April or May, no later than June, so it is mature enough by fall to withstand traffic, wear and the onset of frost.
Dormant seeding is the distribution of seeds during a period outside the normal growing season so the seeds will be in place and ready to germinate when conditions allow. Ideally, the weather is persistently cold to prevent premature germination. Typically, dormant seeding is done into a prepared soil after soil temperatures have dropped below 40 degrees Fahrenheit. Research on dormant seeding practices has revealed the following:
Advantages of Dormant Seeding:
- Soils are generally drier and easier to work in fall than in spring, especially native soils (silt clay loams).
- The seed is in place to take full advantage of warming soil temperatures the following spring.
- There is more personnel time available
- Dormant seeding requires less irrigation than spring or summer seedings. However, seeds will dry out if there is no winter precipitation at all.
- Dormant seeding requires less robust weed and disease control strategies.
- Dormant seed can emerge up to 15 days earlier than conventional spring seeding.
Disadvantages of Dormant Seeding:
- Dormant seeding would most likely not be as successful as an early fall planting.
- A spring warm-up could initiate germination only to be followed by an extreme cold period, which could kill the seedlings.
- Dormant seeding with quick-germination species, such as perennial ryegrass, has a higher risk of cold damage.
- Situations in which dormant seedings fail include areas of soil erosion and the use of dark colored mulches, which raise surface temperatures.
- Increased seed application rates (30 to 50 percent) are recommended because seed mortality rate is higher in dormant seedings.
Overseeding during the season
Grasses used for overseeding during the playing season need to germinate and establish quickly. “Quick” grasses can be listed as:
1. Perennial ryegrass
2. Annual ryegrass
3. Transitional ryegrass
4. Festulolium – a forage grass that does not like low mowing heights or traffic, but grows rapidly.
These grasses typically germinate in three to five days. Festulolium and annual ryegrass can germinate in as little as two days if environmental conditions are favorable. If these grasses are irrigated and fertilized, they can provide green cover in a few days. The process can be enhanced even more if growth blankets are used.
Seed application rates are guided by the recommendation that there be 1,000 to 2,000 grass seedlings per square foot, which is why turf species with very small seed (Kentucky bluegrass and bermudagrass) are applied at much lower rates than species with larger seed (perennial ryegrass and tall fescue). Other factors that influence rate include seedling vigor, growth habit, cost of seed, weed and disease potential, and level of post-seeding care. Research has suggested that the density of a mature turf stand is influenced by the carrying capacity of the environment more than the seed rate, so applying seed in much greater numbers may seem pointless (and actually detrimental since overcrowded seedlings are more susceptible to disease), but for high-traffic athletic fields under constant renovation higher seed rates are often preferred.
Typical seed rates for athletic turf are as follows: Kentucky bluegrass at 1 to 3 pounds of seed per 1,000 square feet; perennial ryegrass and tall fescue at 6 to 8 pounds of seed per 1,000 square feet; and Bermudagrass at 1 pound of hulled seed per 1,000 square feet.
Seeding rates during overseeding are typically higher than seeding rates used to establish a permanent stand of turf. There has been success with rates at 10 pounds per 1,000 square feet per week. Research by turfgrass specialist Dr. David Minner found that annual ryegrass had a seeding rate ceiling of 90 pounds per 1,000 square feet, or 18 times the normal seeding rate of 5 pounds per 1,000 square feet. The conclusion for all overseeding research would suggest that higher rates than normal are appropriate during traffic. The question of whether a “seed bank” can be built up is not quite as clear-cut.
Perennial ryegrass has a short dormancy in that it is viable and ready to germinate without extended periods of chilling or other preconditioning. The dormancy of perennial ryegrass is broken when the seed is moist and it has gone through a short cycle of diurnal temperature variations over three to five days. Once the radicle (first root) has emerged and cell division is taking place, the seed has officially germinated. If moisture has caused some cell expansion but then the seed dries, it could possibly survive, but once there is cell division, moisture must be continually provided or the seed will die. For a viable seed bank to build up, the seed must be in the soil but not subject to moisture or the other germination triggers, which is highly unlikely if the seed has been broadcast on the surface and subjected to daily irrigation. To that end, the seed should ideally be applied on a weekly or biweekly basis rather than just once at the beginning of the season.
Kentucky bluegrass seeded at 1.5 pounds per 1,000 square feet in two directions (3 pounds total).
Overseeding dense turf is a waste of time and money. Many years of research in the golf course management industry have shown that there is little to be gained by overseeding one type of grass with another in the hope of changing the composition of the sward because the mature, existing grass prevents the seed from germinating. Thus, overseeding should be restricted to high-traffic areas that have thinning turf or bare ground visible. Broadcast seeding by hand or spreader prior to games has become a standard method of seed delivery, based mainly on testimonials among sports turf managers. There is also some data to back this up. Our research showed that slit-seeding during heavy traffic caused significant surface damage and was probably not the best seed delivery method. Broadcast seeding appears to offer the best results without causing soil damage, though ideally the seed should be lightly raked in to maximize seed-to-soil contact.
Some rules of thumb when applying seed include:
- Choosing seed that has shown greater resistance to summer diseases like Pythium blight, brown patch, dollar spot and gray leaf spot.
- Increasing sward diversity by including three to five cultivars.
- Using seed coated with fungicide (“Apron” treated), especially on perennial ryegrass seed.
- Sticking to the seed rate. Excessive seed rates will result in crowded, weak plants that are more susceptible to traffic and disease problems.
Do seedlings mature?
There is little evidence to suggest that seedlings survive under intense traffic. Young seedlings can offer some green cover if there is adequate time between games for it to establish, but if daily games are taking place, even the most rigorous of species will succumb. That is not to say that overseeding programs do not work. The seed is viable and will germinate, so as soon as there is a rest period the seedlings will have the chance to produce tillers and mature into plants that can withstand traffic.
Seedbed preparation for major renovations
Preparing soil prior to planting seed is the most important step in a successful renovation project. There are several steps to the preparation process.
First, conduct a soil test to determine if there are nutrient deficiencies or corrections to be made with soil pH and/or organic matter content. Follow recommendations from the lab in relation to fertilizer rate and frequency.
If doing a total renovation, kill the existing vegetation with a nonselective herbicide and remove as much plant material/organic matter as possible by close mowing, scarifying and removing debris from the site. The type of seedbed preparation will then be determined by the quality of the soil, whether it needs improving or not.
If the soil is poor quality or the field needs to be regraded, till the soil to a depth of 4 to 6 inches and remove deleterious material like rocks and glass. The extent to which the soil is remedied or amended will depend upon the soil test, soil quality desired and budget. The goal is to maximize drainage and root growth, so ideally the soil should contain at least 75 percent by weight sand and greater than 5 percent organic matter, but this will depend on available funds. At the very least, tilling in 1 to 2 inches of organic material like compost will help to improve soil properties. Total renovation also offers an opportunity to install drainage tile and irrigation pipe, as well as re-establish proper grades.
Since weed pressure is high in the spring and summer months, it’s important that the seedbed be treated with an herbicide that does not affect germination of the desired grass seed. Examples of herbicides for cool-season turf would be Syngenta’s Tenacity (mesotrione) or FMC’s SquareOne (quinclorac and carfentrazone). In our research, applications of Tenacity resulted in a clean, weed-free seedbed. A seedbed tilled during the summer months without Tenacity resulted in greater than 90 percent weed cover, namely crabgrass, yellow nutsedge and goosegrass. Research from the University of Wisconsin has also shown that Tenacity can be added to a hydromulch slurry to prevent and control grassy and broadleaf weeds. The hydromulch slurry already offers a way to conserve moisture, so adding a weed control product will allow for much better establishment of the desired turfgrass.
If the soil doesn’t need to be amended or graded, do not till the soil, as it will bring huge numbers of weed seeds up to the surface. Avoiding soil disturbance will prevent aggressive weeds like crabgrass and annual bluegrass from taking over. Prior to seeding, a heavy-duty scarifier can be used to create shallow slits and grooves on the soil surface so seed-to-soil contact is achieved. Alternatively, a slit-seeder will place the seed directly into the soil surface. Research at Ohio State University (OSU) has shown that this particular seedbed preparation with minimal soil disturbance, and therefore minimal weed competition, results in less than 30 percent weed cover, which is easily controlled with a postemergence herbicide once the new grass seedlings are mature enough that they have been mowed three times. The conclusion here as far as soil disturbance is twofold: try not to disturb the soil and bring weed seeds to the surface; and if the soil is disturbed, use an herbicide at the time of seeding to prevent weed seed germination.
If total renovation is not needed, then an overseeding operation can improve the turf stand. Thin turf/worn areas on the field are remedied by removing thatch and other surface organic matter with a scarifier/dethatcher, and then slit-seeding diagonally in two directions.
Seeding operation and post-seeding care
Once the seedbed is ready and a granular starter fertilizer has been applied at a rate of 1 pound of nitrogen per 1,000 square feet it is time to seed. Using a slit seeder is ideal, as it places the seed directly into the soil surface. Slit seeding in two directions diagonally ensures uniform seed distribution and prevents the grass from establishing in stripes. It is important to check the seed hopper to ensure you don’t run out of seed.
Grass seed needs moisture, oxygen and warm soils (above 50 degrees Fahrenheit) to germinate. Moisture is the key. If there is moisture stress/drought during early germination, when the seed is taking on moisture and the tissues swell (imbibition), then it may not damage or kill the embryo. However, if there is drought stress during the later stage of germination, when there is cell division and growth, then the seed is more likely to die and the renovation will fail. It’s critical that the seed stay moist until green grass cover is visible. With fast-germinating turf species like annual and perennial ryegrass, the germination period is around three to five days; tall fescue and bermudagrass germinate around seven to 10 days; and Kentucky bluegrass takes seven to 21 days, depending on the variety.
Keeping the seed in a constant state of moisture until green tissue is visible requires the sports turf manager to syringe the seed several times daily. There are no hard and fast rules on the amount of syringing cycles needed, as it depends on weather conditions. On sunny, windy days with a high evapotranspiration (ET) rate seed may need syringing as much as five times per day, while on cloudy, calm days with a low ET rate seed may only need to be syringed three times per day. An example of a syringing cycle could be: 9 a.m., 11 a.m., 1 p.m., 3 p.m., and 5 p.m. During late June, July and August, when there are more daylight hours, a 7 p.m. cycle may also be required. Syringing is not the act of replenishing water lost through ET or applying water deeply and infrequently to encourage growth. Syringing is a light application made purely to wet the seed. Syringing is done either by a quick rotation of an irrigation head or by hand. Syringing by hand involves light watering with a hose equipped with a syringing nozzle. The purpose is to lightly wet the seed, not blast it off the soil surface, so a syringing nozzle is essential.
As mentioned previously, a starter fertilizer is applied with the seed. Repeat applications of starter or maintenance fertilizer are also important for the renovation process. Typical rates would be 0.75 to 1 pound of nitrogen per 1,000 square feet per calendar month until there is 100 percent sward density. Lighter, more frequent rates of 0.5 pound nitrogen every two weeks are sometimes preferred by field managers.
Left: Perennial ryegrass at four weeks after seeding. Right: No seed. Perennial ryegrass is quick to germinate and provide some ground cover.
To prevent seed movement and soil erosion after a heavy rain, the seed should be lightly covered with topdressing sand, a growth blanket, hydromulch or straw. A cover helps to retain moisture and heat and prevents seed and soil movement until seedlings are established. It might also deter field users from using the area until the renovation is complete.
Turfgrass disease prevention
Applying starter fertilizer and overwatering will increase the likelihood of diseases such as Pythium blight and brown patch. A preventative approach is to use fungicide-coated or “Apron” treated seed and to apply a liquid or granular fungicide at the time of seeding. It is crucial to have a fungicide program planned if renovation is taking place over the summer, because environmental conditions are favorable for disease to occur. There are several fungicide families that target Pythium blight and brown patch, and they should be rotated to discourage fungicide resistance.
The fungicide effect
During previous studies at OSU there has been some evidence to suggest that fungicides could have a beneficial, nontarget effect on turf health, particularly during the summer establishment period. In 2008, newly seeded perennial ryegrass turf that had been treated with Syngenta’s granular Subdue Maxx (mefenoxam) fungicide at the time of seeding showed improved color, density, biomass, sward height and overall establishment quality compared to perennial ryegrass that had had no Subdue applied. The treated turf also contained greater (0.5 percent) tissue nitrogen compared to the untreated turf. Neither showed symptoms of disease, so the fungicide appeared to have a nontarget effect on turf health. In 2009, both granular Heritage (azoxystrobin) and granular Subdue Maxx were applied during seeding. Results were similar to those seen in 2008 in that fungicide-treated turf provided better quality and quicker establishment.