August 2017

SportsTurf provides current, practical and technical content on issues relevant to sports turf managers, including facilities managers. Most readers are athletic field managers from the professional level through parks and recreation, universities.

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www.spor 10 // August 2017 help turf resist winter injury and aid in dis- ease and environmental stress resistance the following spring and summer. Because carbohydrates are tapped for energy by roots and shoots during periods of rapid growth, forcing excess growth with early spring fertilizer applications can deplete carbohydrates quickly, leaving turf vulnera- ble to spring and summer stresses. Late fall nitrogen fertilizer applications tend not to produce as much leaf growth in spring as an equal amount of early spring fertilizer, thus carbohydrates are not exhausted as quickly. The result is a slight advantage to the turf in the form of better stress tolerance. Another reported benefi t of late fall fertil- ization is an increase in rooting, though pre- cisely when and how this increase occurs is a source of some debate. Maximum root growth of cool-season turfgrasses occurs in spring and fall. Some root growth will occur in winter if temperatures are above freez- ing, whereas little if any growth occurs in summer. Fertilizer applications are typically made in spring and late summer in attempts to promote root growth. One problem in using this approach is that much of the fertilizer is used by the shoots, sometimes preferentially over roots. One reported ad- vantage of late fall fertilization is that roots are still growing at a time when shoot growth has ceased or slowed dramatically, thus allowing the roots to make full use of the fertilizer. However, root growth is very slow during this period, and if the soil is fro- zen, they do not grow at all. Consequently, the benefi t of increased root growth in re- sponse to fall fertilization is questionable. One study at Virginia Tech showed that moderate rates of soluble nitrogen (1 lb. nitrogen/1000 sq. ft.) in late fall increased rooting of turfgrass without a noticeable in- crease in shoot growth. In contrast, a study at Ohio State showed no increase in root growth during late fall or winter following late fall fertilizer applications. However, when compared to early spring applications of nitrogen, late fall applications allowed more rooting in spring. Presumably, this benefi t was due to early spring green-up from late fall applications, which alleviated the need for early spring fertilization. When nitrogen fertilizer was not applied in late fall, but instead in early spring, ex- cessive shoot growth occurred, depleting carbohydrate reserves that would have otherwise gone into root production later in spring. The take-home message from the Ohio State study is that while the net effect of late fall fertilization on rooting is slight, application in late fall may be more bene- fi cial with respect to rooting than an early spring application. Late fall nitrogen fertilization is oc- casionally blamed for increased winter injury, snow mold, and annual bluegrass encroachment. A few studies have been designed to examine the infl uence of late fall nitrogen applications on winter injury. But to my knowledge, none have conclu- sively demonstrated detrimental effects in the sports turf arena. Heavy fertilization in mid-fall, when grass shoots are actively growing, can sometimes enhance snow mold diseases (presumably due to reduced pre-winter hardening and increased suc- culence of plant tissue). While some stud- ies have shown increased annual bluegrass populations in fall, there is little evidence to show that this increase is related to late fall fertilization.

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