Seeding during dormancy
Dormant seeding can mean different things, depending on whether you are primarily working with cool-season or warm-season turfgrasses. Most people are familiar with winter overseeding of dormant warm-season grasses, but many are unfamiliar with dormant seeding of cool-season turfgrasses.
Dormant seeding of cool-season turf has slightly different meanings among turfgrass specialists. Some equate it to seeding after the soil is frozen. Others consider it acceptable to dormant seed with the soil temperature below 50oF, which is low enough to inhibit seed germination. Either can work.
When should you dormant seed? Successful turf establishment requires that you select the appropriate turfgrass species, apply the seed at the proper rate and with a suitable method, and use optimal timing. This usually means seeding in early fall or spring. Sometimes, however, you have to seed when temperatures have become too low for germination. Perhaps you cannot seed in early fall due to contract-schedule restrictions or because the turfed area is in use during the fall. This latter situation occurs with sports fields used in the fall and again in spring. This is when you should consider dormant seeding. In fact, dormant seeding may be your only option for establishing or re-establishing a field or for overseeding thinned turf.
Another situation in which dormant seeding may be useful occurs with wet, poorly drained soils. Some soils do not dry out enough in the spring to permit working the soil during the optimum period for seeding. Seeding later, however, would not allow adequate time for turf to mature before the onset of harsh summer temperatures. By contrast, dormant seeding may occur between November and March, depending on local conditions.
Winter is the most challenging time to seed. This is why dormant seeding is not recommended for novices. Most turfgrass specialists do not suggest this as an option to homeowners, leaving it only to professionals to attempt. Even then, the oddities of nature can determine success or failure. For example, an unusually mild winter-which could result in premature germination-reduces the chances of obtaining a good stand of turfgrass with dormant seeding.
How do you dormant seed?
* Timing. To ensure that seeds do not germinate immediately, it is vital to wait until temperatures are low. Thus, proceeding only after the ground is frozen offers the greatest chance of success with dormant seeding.
Dormant seed no later than early spring, before frosty weather has passed. Seed benefits from frost heaving and cracking, which allows the seed to settle into the surface for closer contact with the soil and increases chances for successful establishment. Once spring arrives, keep the area moist, as you would with any turf establishment.
Properly dormant-seeded grass should not germinate until the spring, when soil temperature rises above 50oF on a regular basis. By dormant seeding, you avoid some of the problems you encounter when seeding in spring. Spring tends to be wet. This is good for seed germination, but you may not be able to prepare the soil and plant because of these wet conditions. Also, by the time the soil is dry enough for you to prepare it and plant, you'll have to contend with a flush of weed-seed germination.
Any of the cool-season grasses are suitable for dormant seeding, although tall fescues generally do not germinate until air temperatures are in the 70s. This may delay their germination later than other cool-season grasses.
* Methods. First, you must perform standard seedbed preparations before the ground freezes. This includes a soil test, deep tilling (at least 4 to 6 inches deep), incorporating any necessary amendments, final grading and smoothing the surface to make a firm, but not compacted, seedbed. If you're overseeding an established turf, make sure you've reduced the thatch to less than 0.5 inch.
One successful strategy is applying seed on a frosty morning. When the sun emerges, the moisture from the melting frost "grabs" onto the seed, causing it to sink nicely into the soil for good seed-to-soil contact.
Another method involves seeding prior to a predicted snow. This reduces the chances of soil thawing and seed washing and blowing. In regions where snow remains throughout the winter, this provides the best protection of dormant seedings. It also is possible to seed over a snow cover if the ground is firm enough to support seeding equipment and the snow depth is 2 inches or less. When the snow melts, the seed drops to the soil surface. The melting snow then provides moisture needed for seed germination. You can speed snowmelt by mixing dark-colored fertilizer with the seed. The seed then drops more rapidly through the snow to reach the soil surface.
You can perform the actual seeding process by hand (for small areas) or with a seeder, drop spreader or hydroseeder, depending on the terrain. Application of the seed is similar to typical fall seeding. If you use a hydraulic seeder, apply the fertilizer first at no more than 0.5 pound nitrogen per 1,000 square feet, followed by the seed and then the mulch. Mixing all three may be easier, but performing each step separately ensures the best use of the fertilizer and proper placement of the seed.
Seeding rates are the same as those for early fall seeding. If erosion is a potential problem, you may consider increasing the rate of seed to compensate for expected losses. However, preventing erosion with adequate mulching is preferable.
Dormant seeding failures Cool-season turfgrasses germinate most rapidly with air temperatures of 60 to 85oF and soil temperatures above 50oF. Seeding before the temperature drops enough to prevent germination is not truly dormant seeding. Seeds may germinate if temperatures rise above the minimum germination level and remain there long enough for germination to occur. These tender seedlings risk being lost to frost heaving or exposure to temperatures that can kill them outright. Heaving pushes seedlings out of the soil, which may lead to desiccation before they are adequately established. If rooting is insufficient to cause interweaving of roots and stems, undesirable frost heaving is likely.
Soil erosion is another consideration when dormant seeding. Sloped areas are of particular concern. Be sure to mulch seeded slopes to reduce the potential for seed washout by surface water. Some experts suggest avoiding dormant seeding altogether where slopes are greater than 3 percent.
Finally, avoid dark-colored mulches, which can increase the chance of germination of your dormant seeding by raising surface temperatures.
Dr. Bridget Ruemmele is associate professor of turfgrass science at the University of Rhode Island (Kingston, R.I.).
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