The keys to a successful late-season fertilization program

After a long, hot, stressful summer, a recovery period can be long overdue … both for people and turf. Traditionally, fall has been the recuperative season, a time for mending and revitalization.

The combination of cooler temperatures, consistent rainfall patterns and applied nutrients contribute to improve turf’s appearance, density and playing condition. Late-season fertilization is crucial to the process.

But just what defines late-season fertilization? The name would seem to imply that it refers to the very last applications that are made to the turf during the growing season. However, a broader consideration is helpful: For most cool-season grass regions, late season normally means mid-September through November, and, for warm-season regions, that could be late July through mid-October.

Early-fall fertility will set the stage for later applications by allowing carbohydrate storage to be more efficiently accomplished prior to the final fertilizer applications of the season. This allows the final application to be more of a “topping off of the tank” in preparation for winter dormancy and spring emergence.

Soil testing

Any discussion of fertilization should begin with a stated goal for a desired outcome and a determination of that which exists in the soil. Late summer is a good time to test the soil, especially if tests have been conducted at this time in the past, so that results can be compared appropriately. For the most accurate results, year-to-year comparisons should be made from tests conducted in the same month.

Comparing test results from spring analyses to ones made in summer is often misleading in that influences such as the microbial population of the soil and the release rate of previously applied fertilizer products can distort the concentration levels of various nutrients and percentage of organic matter.

Areas should be tested according to turf performance. Those considered to be homogenous should be tested separately from ones with mixed appearance and growth. In identified areas, samples should be pulled to a depth of 6 inches. The turf blades, crowns, rhizomes and thatch should be discarded; the target soil for testing resides in the 2- to 5-inch depth.

Collect 10 to 15 samples per section of the course, mix them together in a plastic bucket and send them to a reputable soil-testing lab. In the overall consideration of soil testing, consistency is key. Changes in sample depth, laboratory used, time of year and testing method can cause distorted results.

Specific results differ for each nutrient and other soil test features. For example, tests for phosphorous are usually more reliable than for nitrogen, as it is much more stable in the soil from year to year. Nitrogen is a mobile and transient nutrient, thus test results should be thought of as “snapshots” rather than hard facts. Potassium is generally considered to be intermediary.

When trying to answer the question of whether or not to apply a “winterizer” containing potassium and/or phosphorus, it is best to refer to your soil test. There is no real benefit from additional potassium and phosphorus unless your soil test indicates a deficiency. Often, these two nutrients are the most expensive in your product and can be minimized unless an application can be justified by a needy turf.

Turf and tree roots share the same space; always keep this in mind when fertilizing in either spring or fall.


Old school, new school

Within the science and art of golf course management, one must stay true to what has worked well in the past, but keep eyes and ears open to new developments and research from manufacturers and university investigators. In most cases, studies conducted close to your course tend to be more applicable than ones a couple of states away, but never say never.

Considering cool-season grasses, early- and late-fall applications have provided many benefits for cool-season grasses, including lengthening the late-fall and early-winter green period, extending the period of time of photosynthetic activity in fall and early spring, reducing the incidence of leaf spot and melting out in spring as opposed to heavy spring applications, and allowing plants to maintain higher carbohydrate levels that encourage growth of roots and rhizomes and increase winter stress tolerance.

Considering warm-season grasses, the routine for many for managing bermudagrass and other warm-season turfs has been to ease into winter without stimulating plant growth that could be damaged by the onset of cold winter temperatures.

While this is true in some circumstances, research at several universities has demonstrated that fertilizing with soluble nitrogen late in the growing season enhanced fall and spring color of bermudagrass without directly increasing the potential for winterkill. Slow-release nitrogen fertilizer sources provided little fall or spring response, however.

Foliar applications of iron in conjunction with biostimulants, like cytokinin, have been able to relieve stress-related chlorosis and extend greening into the fall for warm-season grasses. Combinations of iron and nitrogen (N) applications during the late season also can enhance metabolic activity before and after dormancy. For both cool- and warm-season turfs, these studies support the recent recommendation to apply low to moderate rates (0.2 to 0.4 lb. N/1,000 square foot) of quick-release N approximately two weeks before the onset of dormancy to provide needed nutrients for root and rhizome growth without stimulating excessive top growth. Previous recommendations for high rates of slow-release N are being questioned due to the question of eventual fate of the N – if 1.5 pounds are applied, and 0.5 are recovered in the spring, where did the 1.0 go?

Volatilization to the atmosphere and leaching to the groundwater are undesirable potential occurrences over winter. Some relatively recent data out of Wisconsin (Lloyd and Soldat, 2009) has shown that cool-season grasses take up more than 70 percent of September-applied nitrogen regardless if it is applied at 0.5 to 2.0 pounds N/1,000 square feet, but only takes up less than 50 percent at a 0.5 pounds N/1,000 square feet in November. Uptake only decreases in November if the rate is increased.

The balance with this approach is applying just enough that will be immediately absorbed and utilized in the plant, but not more than is needed, as it is likely to be wasted or lost from the site. On greens that have been spoon fed throughout the summer, where application rates have hovered around 0.10 to 0.15 pounds N/1,000 square feet, rates could be bumped up to about 0.25 pounds N/1,000 square feet with a soluble fertilizer every other week until about the last mowing.

On fairways, September applications of 1 pound N/1,000 square feet should be made up of 25 to 50 percent slow-release N to allow it to be taken up into October. Then, near the time of the last mowing, a half-rate of the previous application could be applied using primarily soluble sources. This could be a time to clear out those extra bags of urea still sitting in the shop.

Balancing act

The challenge of fall and late-season fertilization is to reap the benefits without increasing the susceptibility to problems such as winterkill, diseases and desiccation. As such, it’s quite a balancing act. Slow-release applications of slightly lesser rates of potassium may be recommended in an effort to increase winter stress tolerance. Applications of soluble iron also have been demonstrated to provide benefits in some, but not all, studies.

After late-season fertilization applications have been made, it’s important to wash the product off the blades and into the thatch. A light irrigation should be sufficient.

This is recommended for two reasons. First, removing the fertilizer pellets from the leaf blades greatly reduces the chances of foliar burn, which may be more pronounced in late fall than in other times of the year. Secondly, the sooner that the product can be moved into the thatch, the better. The thatch layer will act as a temporary binding agent, which will reduce the potential for off-site movement.

Additionally, do not apply any fertilizer product after the root zone has become frozen or a majority of the shoots have turned brown. Applications made after this point, no matter the source or composition, likely will be lost due to leaching or surface runoff or both.

Trees and turf

A common fall fertilization timeframe occurrence is the sound of the fertilizer pellets hitting fallen tree leaves. Depending on the layout of the golfscape, tree roots often overlap with turf roots. In locations where this occurs, potential exists for woody plant root absorption.

There are possible positive and negative consequences of fertilizing under trees in fall. On the positive side, when trees are in need of nutrients, one of the best times to apply them is late fall after leaves drop, as the roots absorb nutrients until roots stop growing when soil temperatures cool to 40 degrees Fahrenheit.

However, if a tree is in poor condition because of an insect infestation or other malady, the response from a nutrient application (especially nitrogen) is to produce new leaves at the expense of other tree functions such as building up sugars and carbohydrates, compartmentalization of fungal decay organisms and overall defense. In such instances, evaluate the tree health before applying fertilizer to both trees and turf.

Additionally, trees and other woody plants generally require a third to half as much fertilizer as turf. Over-fertilization often leads to the development of extensively succulent foliage and stems, which are weaker in structure and more attractive to sucking insects than ones that haven’t been excessively fertilized.