SPECIAL FOCUS- FALL FERTILIZATION: Cool-Season Clues

Except for the deep South, most U.S. homes and office complexes have cool-season turfgrass lawns because most cool-season turf stays green much longer than warm-season turf, which turns brown and goes dormant after the first killing frost. Often these species remain brown for six months or more, which is undesirable to many clients. Therefore, lawn care operators need to continually refresh their knowledge of contemporary fall fertilizer strategies for cool-season turf to maximize plant health, minimize mowing requirements and limit nutrient losses.

A HISTORICAL PERSPECTIVE. Twenty years ago the standard fertility programs for a cool-season lawn consisted of applying nitrogen (N) to match seasonal shoot growth. This philosophy was very much analogous to the way farmers fertilized row crops like corn. Corn, however, is a warm-season grass and benefits from summer N applications. Unlike row crops, turf managers are not trying to maximize yield in order to ensure profits. Slowly, the shortcomings of this strategy became evident. Landscape managers began to notice there was an overabundance of growth in the spring months, which necessitated frequent mowing. Additionally, many cool-season lawns declined dramatically during the summer months, especially under drought conditions. Turfgrass scientists began researching how turfgrasses grew and responded to different fertilizer programs. Their discoveries evolved into the industry’s current fertilization recommendations.

For a high-quality established lawn, this consists of light N applications (< 1 lb. of actual N) in the late spring and almost no N during summer, unless the area is irrigated and clippings are removed. The majority of N (60 to 75 percent) is applied from late summer until the plant goes dormant, sometime between Nov. 1 and Thanksgiving. The annual N required may range from 1 to 5 pounds per 1,000 square feet. The goal of these fertilizer applications is to supply just enough N after the spring growth flush to keep the turf from becoming yellow or chlorotic and minimize stored carbohydrate use and conserve the root system. The summer goal is to minimize disease incidence and the late-summer/fall goal is to promote maximum root growth and carbohydrate storage.

Ultimately, this program enables a healthy, mature plant to emerge from dormancy with plenty of carbohydrates available to regulate foliar growth during spring growth and ensure maximum summer root system survival.

TIMING TIPS. While it is difficult to predict specific environmental conditions from year to year throughout the cool humid region, some general N application guidelines can be made. These guidelines may vary by about two to three weeks depending on yearly weather conditions and when the first hard frost or freeze occurs in a particular region.

Regardless of geographic location, the most important time to apply N to cool-season turfgrasses is late summer/early fall. In most regions, this will occur sometime around Labor Day. Throughout the region there are still at least 60 to 90 days of optimum growing conditions. During this time the less intense sun, shorter days, more favorable air and soil temperatures combined with adequate available N, allow the turf to recover from summer stress. Additional benefits to fertilization during this period are that the turf greens with a minimal unwanted growth surge, density increases and the plant stores carbohydrates.

With fall fertilization, there is a common misunderstanding between late-summer/early fall fertilization and late-fall fertilization. Technically, fall does not begin until the third week of September and lasts until late December. Environmental conditions change dramatically through this period, which affects the turfgrass plants response to fertilization. The second most important time to apply N is about one week after the final mowing for the season, normally between November first and Thanksgiving, depending on geographic location. This is referred to as late fall fertilization. At this time, relatively heavy quantities of water soluble N (1 to 1.5 pounds of N per 1,000 square feet) are recommended. The major benefit to this application is that the leaves are still green and the plant is still photosynthesizing, but carbohydrates are rapidly stored rather than being used for shoot growth. Other benefits include prolonged winter color retention, earlier spring green-up without a large growth flush, and maximum root growth.

With increasing water quality concerns, the late-fall N application is being more closely evaluated with respect to rates and timings. Although the soil is still relatively warm at this time of year and roots are still active, it is unclear exactly how efficient different turf species are in extracting soluble N. Therefore, in the future expect to see some modifications in late-fall fertilization recommendations with respect to timing and rates. In the meantime, however, 1 pound of a water soluble N source like urea applied no later than the first week of November is probably more responsible than 1.5 pounds applied in late-November.

NITROGEN SOURCES. Traditionally, N sources have been divided into two broad categories, quick-release or readily water soluble and slow-release or water insoluble. Quick release N sources like urea or ammonium sulfate dissolve easily in the presence of water, and a rapid greening response may occur in a matter of hours as the N is translocated from the roots to the shoots. These N sources cause a short-lived growth flush that normally lasts only a few weeks. Compared to the slow-release sources, quick-release N sources are rather inexpensive, which explains their preference.

Anytime water-soluble N sources are used, one thing to consider is the turf burn potential. If you apply too much in one application essentially what happens is an osmotic stress. When used at relatively high application rates, the soluble salt level in the soil around the turf plant can increase dramatically, resulting in decreased water uptake, which may cause the plant to shrivel. These symptoms are very common during August and September, especially in dry years on seedling and drought-stressed turf. To minimize injury and maximize plant uptake, water fertilizer applications into the soil. Damage may occur when fertilizers are applied to turf with heavy dew and not watered into the soil. Depending on the salt index of the fertilizer, ammonium sulfate and potassium nitrate have a higher salt index than urea, tip burn may be minor to severe. Remember, anytime foliar burn occurs, photosynthetic capacity is reduced.

The traditional rule of thumb for quick release N sources has been that you should never apply more than 1 pound of N per 1,000 square feet. This guideline was established to avoid significant growth surges, minimize burn potential and also limit unwanted environmental losses. With this in mind, quick-release fertilizers by themselves are best used at reduced rates, applied frequently or as a smaller part of a fertilizer blend used at other times of the year.

The slow-release N sources include, sulfur coated urea (SCU), polymer coated urea (PCU), methylene ureas, isobutlydiurea (IBDU) and the natural organics. Slow-release N sources require more than just water to release their N. Several mechanisms like microbial decomposition, slow particle solubility and protective coatings all control how quickly the N is available. Thus, if you are trying to encourage rapid growth and carbohydrate storage in the late-season it is important to make sure that N source being used does not rely on microbial release. The N in organic sources will not be available when soil temperatures fall below about 50 degrees F. SCU or IBDU could be used instead, but they are controlled-released products and may not release enough N at this time to be effective. Additionally, these products will continue to release N so long as soil moisture is present, which may contribute to environmental N loss during winter when turf roots are not actively growing. Slow-release N sources can be applied at higher N application rates, 2 pounds of N per 1,000 square feet without the risk of foliar burn or significant environmental losses. Additionally, they do not cause rapid greening but provide extended feeding, often for two to three months, which sometimes make them more economical than the quick-release sources due to less application labor costs. Regardless of the N source used, plants take up two main N forms from the soil, nitrate (NO3-) and ammonium (NH4+) and roots do not discriminate between a natural organic fertilizer, SCU or urea N. Ultimately, your choice of N fertilizers should be made based on your intended growth and aesthetic goals, application rates and desired release characteristics.

Frequently, in middle to late fall you may see bags of "winterizing fertilizers." These products combine water-soluble N sources with higher percentages of phosphorous and/or potassium, than traditional maintenance fertilizer products. A common fertilizer analysis for a maintenance fertilizer might be 16-4-8, or 28-3-7 vs. an 18-0-18 which might be marketed as a winterizing fertilizer. Companies that sell these fertilizers claim the additional nutrients enhance winter hardiness. This is true if you have a soil nutrient deficiency but in most cases where the turf is mature and healthy the additional nutrients are unnecessary. The only way to know if you have a phosphorous or potassium deficiency is to sample the soil and send it to a testing laboratory. Between potassium and phosphorous, potassium is more likely to be deficient in a mature turf system since, like N, it can leach with heavy rain or frequent irrigation. Where potassium deficiency is a concern, supply about one half the quantity of N applied. So, if you are applying 2 pounds of N annually you should also supply 1 pound of actual potassium.

FALL IS THE TIME. As a rule of thumb, if you can only fertilize an established lawn once per year, the late-summer/early fall timing with a mixed N source product at 1 to 1.5 pounds of N per 1,000 square feet will give you the most benefits. Late-fall fertilization, although beneficial in most situations may not be appropriate for all turf areas. For instance, turf areas that stay rather wet during the spring, may perform better without late fall N. The early spring growth flush might necessitate frequent mowing and the extra traffic may do more harm than good. In these circumstances, use lower annual N rates and use late-spring and again in the late-summer timing.

For all the benefits of fall fertilization it is important to mention there are some risks. Occasionally, increased disease incidence from snow mold or winter desiccation are reported, but faster recovery is also associated with turf that was well fertilized during the fall. Overall, most experienced turf managers will attest that the benefits of two to three fall N applications are worth the very minor risks.