Cool-season turf benefits from fall fertilization

Management practices during fall and late fall have major effects on the quality of cool-season turfgrasses the following spring. Fertilization is one of the most important of these practices. It's easy to let other projects demand your attention in the fall, but you shouldn't overlook the basics that help prepare turf for winter and the next spring.

For cool-season grasses, fall is that period when the heat of summer has given way to cool nights, more uniform rainfall and sunny, warm days. This typically commences in September. Turfgrass plants respond with improved density and growth, recovering from thinning that may have occurred during the summer. Fertilization at this time aids this process.

As temperatures continue to cool during mid- to late fall, grass growth begins to slow and mowing frequency drops to once a week or less. This is the time for late-fall fertilization. In more Northern regions, the appropriate time for this application may occur in late October. As one moves farther south, it occurs later, of course. The appropriate timing for late-fall fertilization in a given region varies by as much as 2 or 3 weeks, depending on when frost and freeze events occur. A hard freeze usually causes cessation of turf growth, but the plant is still physiologically active, so photosynthesis will continue until snow covers the ground or the ground freezes. The plant continues to accumulate carbohydrates during this time when little or no growth occurs.

Phosphate and potash In the past, some turf consultants suggested that applying phosphorus and potash was the key to fall fertilization, but most now acknowledge that nitrogen (N) management is the most important aspect of fall fertilization.

If soil-test results recommend phosphate, you should apply it as necessary via several applications throughout the year. However, because potassium is so important in stress tolerance, you should be sure to apply potash in your fall and late-fall programs on high-maintenance turfs that receive heavy traffic, such as greens, tees and athletic fields. Potassium easily leaches from sands, so regular applications are essential on sandy soils throughout the year, especially in the fall.

Without the benefit of soil-test recommendations on finer-textured soils, apply potash at about half the rate of N. On sands, use equal rates of N and potash. However, to be confident you're applying adequate potash for medium- and fine-textured soils, it's better to test your soils. If tests suggest potash would help, apply appropriate rates based on recommendations and common sense. Recommendations from soil labs for phosphate (P2O5) and potash (K2O) given in soil-test reports are the amounts for an entire year.

Fall nitrogen Both fall and late-fall N fertilization have beneficial effects on cool-season grasses. During the shorter days and cool nights, the plants not only fill in open areas, improving turf density, but they also begin to accumulate carbohydrates more efficiently. Higher carbohydrate levels make the plant more tolerant of stressful winter conditions. Unfortunately, some turf managers overlook fall fertilization because they are busy with other projects and do not want to encourage more growth that might require more frequent mowing.

How much N should you apply in the fall? This depends on a number of factors such as turf density, intensity of use, type of turfgrass and rainfall. Recovery of density after summer stress should be a top priority. To determine the appropriate rate of N, evaluate the density of the turf. If it is thin, use 1 pound of N per 1,000 square feet. If the density is good, you can apply a lower rate. However, you should use a minimum of 0.5 pound of N. For new turfs that need extra N, you can make another application 2 to 3 weeks later.

On putting greens, fall N rates should be light-about 0.5 pound per 1,000 square feet. Some superintendents continue to spoon feed every 7 to 10 days. Either way, your objective should be to reach the desired turf density without excessive growth.

For turfs that receive heavy wear in the fall such as football and soccer fields, application of N in the fall is especially important because it encourages growth and some recovery of turf density. However, apply fall N at modest rates. Applications of 0.5 to 0.75 pound per 1,000 square feet per month during the playing season are adequate.

During late fall when turf begins to harden off, we normally suggest avoiding N applications. As the plants harden off, they become more tolerant of frost injury. N at this time followed by warm, rainy weather could result in turf losing its hardened state by producing excess growth and increasing its susceptibility to a hard freeze. If leaf tissue turns brown as a result, it loses its potential to conduct photosynthesis.

You can use many different fertilizer sources for fall fertilization. I prefer products that have predominantly soluble N-the plants can take up and use the soluble N quickly-but that also contain up to 25 percent of their N in slow-release form.

Late-fall nitrogen Different ideas exist as to how and when to use late-fall N applications. In part, this is because of differences in climatic zones and variations from one season to the next. However, the more important reason for variation in late-fall fertilization strategies is that objectives vary according to the site.

In my opinion, the most important objective of late-fall N fertilization is to supply N to the turf after growth has ceased but when photosynthesis still can occur. This normally takes place anywhere from late October in northern Michigan to mid- to late November farther south in Ohio and Indiana. This timing can vary 2 weeks or so depending on the weather of the particular year.

Some additional brief growth spurts may require mowing after the late-fall fertilization, but regular mowing is no longer necessary during this time of the year. At this time, the root system is still active because the soil is warmer than the air. The plant can still take up and use nitrate N. If you've practiced proper N fertilization during the fall period (September), the turf should still be green and physiologically active in the late fall. This permits the plant to continue photosynthesis whenever modest temperatures and adequate sunlight conditions occur. The plant does not "burn off" carbohydrates it manufactures during this time with growth and clippings but stores them instead. This builds up the plant for next year and permits early spring root growth.

Late-fall N also reduces the need for early spring N (which only enhances growth and mowing requirements at a time of year when growth is likely to be rapid anyway). Carbohydrates lost as you remove clippings in the spring are obviously no longer available to the plant. Doesn't it make sense to delay spring N and keep those carbohydrates in the plant as long as possible? High carbohydrate levels in the plant enhance stress tolerance and keep some reserves in the plant for recovery of turf density if needed later.

As with timing, the appropriate N rate to use in the late fall varies with turf conditions and your philosophy. For greens, 0.5 pound of N per 1,000 square feet may be sufficient. If tees are still thin from traffic, especially par-3 tees, 0.75 to 1.0 pound may be necessary. Fairways should receive 0.5 to 0.75 pound, while lawns and general grounds benefit from 0.75 to 1.0 pound of N. Some agronomists encourage even higher rates as a general practice, but the increased potential for leaching of nitrates suggests caution unless a significant portion of the fertilizer is in a slow-release form. An exception is football and soccer fields that are thin from fall play and need the extra boost from N, which may justify rates as high as 1.5 pounds.

Which carrier for late-fall nitrogen? The choice of carrier is important for late-fall N fertilization. To accomplish the objective of getting a significant portion of the applied N into the plant right after application necessitates that most of the N is from fast-acting soluble sources. Some of the N can be slow-release, as much as 25 percent or so. This slow-release source will provide a small amount of N next spring but will not result in any major response or flush of growth. Any of the slow-release carriers should be acceptable for this portion of the fertilizer.

However, if you applied only slow-release N sources during this late-fall period, not enough N would be available to the plant to provide the desired response of enhanced photosynthesis and carbohydrate storage. Although some golf-course superintendents use Milorganite right after Thanksgiving and have been pleased with the responses the next spring, this approach does not accomplish the objective of carbohydrate storage during the late fall.

Is late-fall nitrogen for all turfs? Late-fall N reduces the need for early spring N. Many turf managers do not fertilize again until just before Memorial Day because the residual effects from fall and late-fall applications provide good color and density without the spring growth flush caused by early spring applications. However, some lawn-care companies cannot justify the cost of late-fall N for customers who may not continue with their services the next year. Nevertheless, for continuing customers turf quality the next spring should be excellent about the time that spring sales begin. This may encourage customers to stay with your service. If you used late-fall N and still feel that the turf needs some spring N, apply it along with your early pre-emergent applications, but reduce the N rate to 0.5 pound of N rather than the higher rates you would normally use at that time of the year if you hadn't applied fall N.

Some turfs may perform better without late-fall N. If a turf site is normally so wet in the early spring that it restricts spring mowing until significant drying occurs, you probably should not apply late-fall N because this will enhance early spring growth. Evaluate this on a site-by-site basis.

Also, late-fall N may enhance snow-mold activity. If turfs are hit hard by snow mold nearly every year, and you have no snow-mold preventive program, it may be best for you to avoid late-fall N. Research by Dr. Joe Vargas at Michigan State University found that late-fall N increased snow-mold levels. However, it also resulted in a much quicker recovery from disease damage. Either the snow-mold damage may be more superficial with the late-fall N or the recovery simply is quicker.

Other potential problems with late-fall N fertilization exist, including the potential for N leaching, late-fall growth that could require more mowing, increased potential for desiccation and faster thatch accumulation. However, for most of these concerns, the potential is small for most turfs.

A commonly asked question is whether late-fall N results in leaching of nitrates. In a study we conducted here at Michigan State University, we found no difference in the amount of nitrates that leached from either fall or late-fall N applications. Nitrate levels were very low in both cases. If you apply soluble N in late fall while the plant is still physiologically active, the plant should take it up and use it efficiently. Therefore, it will not be in a form that can leach over the winter.

While it may cause a small increase in growth during the fall or spring, most turf managers are satisfied that the benefits of late-fall N are far greater than the potential negative effects. No evidence exists that late-fall N increases susceptibility to low-temperature or crown-hydration injury. With many advantages apparent for late-fall N and few disadvantages, it is clear why so many turf managers have adopted this practice. I have not talked to anyone who has tried late-fall N fertilization who has not continued to use the practice for agronomic reasons. This is the best testimonial for late-fall fertilization. Along with fall fertilization, it's the most important nutrition you can give to your turf.

Dr. Paul Rieke is professor of turfgrass science at Michigan State University (East Lansing, Mich.).

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