Winter traffic stress on a Transition Zone bentgrass green

Many factors affect the durability of turfgrass under traffic stress, including the turfgrass species and cultivar, timing and type of traffic, root zone composition and construction type, soil moisture levels, recovery time and maintenance regiments. A 2008 study noted that both mower and foot traffic at 7 a.m. had a negative effect on turfgrass quality when creeping bentgrass (Agrostis stolonifera L.) had a canopy temperature lower than 32 degrees F. Vehicular or human foot traffic can be extremely damaging to turfgrass when temperatures are below freezing, resulting in injury or even death to the crowns.

Traffic inevitably causes turfgrass stands to lose quality. It can negatively affect the turfgrass by compacting soil compaction or causing wear damage. Sand-based root zone mixes for putting greens have minimized the effects of compaction, but wear from traffic still causes problems, especially in extreme temperatures with low mowing heights. Wear injury on turfgrass shoots is characterized by crushing, tearing, stripping or abrasion of leaf tissue followed by a degradation of chlorophyll and subsequent photosynthesis reduction.

The prevailing theory among researchers is that wear is the primary form of traffic-related injury on sandy soils or soils with water contents below field capacity. Conversely, soil compaction may be the primary form of traffic injury on finer-textured soils with higher water contents when greens see heavy loads. Because traffic stress most often occurs on closely mowed turfgrass stands on sand-based root zones, it’s reasonable to conclude that wear rather than compaction is the primary stress. In addition, a 2009 study identified wear stress as the most important influence on turfgrass quality, and that compaction is less important on sand-based or silt-loam root zones. The cushioning effect of surface organic matter likely is a reason for the low importance of compaction in various studies.

Potassium effects on tolerance

Potassium (K) is a required nutrient often applied to turfgrasses, especially creeping bentgrass, to help plants tolerate stresses. Fall K fertilization is thought to aid in creeping bentgrass winter traffic tolerance, but research on the subject is limited and inconsistent.

Research done in 1975 found that creeping bentgrass wear tolerance increased with higher K fertilization levels. The greatest increases were at 5.5 to 7.4 lbs. K per 1,000 sq. ft. per year (27 to 36 g K/m2). The study also indicated a low relationship between K fertilization and total cell wall content, but K fertilization increased the concentration of K in leaf tissue and improved load-bearing capacity and leaf tensile strength.

Additionally, K fertilization reduced tissue water content. This and other research led to the conclusion that K plays a critical role in the improvement of wear tolerance in creeping bentgrass. Other studies, however, have shown contradictory trends on other turfgrass species (and selected studies on creeping bentgrass). K had little to no effect on Kentucky bluegrass, seashore paspalum, perennial ryegrass or creeping bentgrass wear tolerance. As with the trend demonstrated with nitrogen (N) alone, the effect of different N/K ratios (1:1 and 2:1) on wear tolerance varied among species.

Temps versus ice

Low temperatures don’t necessarily cause freezing injury. Instead, ice formation resulting from low temperatures is responsible. Intracellular freezing (the formation of ice crystals within cells resulting from a rapid temperature drop) often is lethal because the ice crystals rupture cellular membranes. And more often, temperatures drop more slowly and ice forms between cells (extracellular freezing). This freezing can slow cellular reactions and metabolic processes but usually is not lethal unless the ice persists for extended periods of time or the ice is manipulated in a manner so that crystals rupture cell membranes, such as when traffic occurs on frost-covered turfgrass.

Our study concerning the performance of a creeping bentgrass putting green was conducted to investigate the effect of fall-applied K in combination with traffic applied at 8 a.m. when frost was present during the winter.

Materials and methods

A field experiment was conducted from Oct. 1, 2014 to April 30, 2015, and Oct. 1, 2015 to April 30, 2016, on a Crenshaw creeping bentgrass putting green established in 1997 to USGA guidelines with an 85:15 sand-and-peat moss root mixture at Clemson, S.C. Treatments consisted of three K rates (0, 0.75 and 1.5 lbs. K per 1,000 sq. ft.), three morning traffic rates (zero, four and eight passes with a modified water-filled push turfgrass roller weighing 165 lbs., and an applied force of 0.27 lbs. per sq. ft.), and two afternoon traffic rates (zero and six passes with the same roller). The K fertilizer used was 0-0-30 Brandt 30K (Brandt Consolidated, Inc.), and the fertilizer contained 30-percent soluble potash (K20). We applied fertilizer treatments three times over the course of a one-month period (to minimize potential for fertilizer burn from a single application) at rates of 0.25 and 0.5 lbs. K per 1,000 sq. ft. each fall to reach the total desired supplemental K rates of 0, 0.75 and 1.5 lbs. K per 1,000 sq. ft. K fertilizer application dates for year one were Oct. 10, 2014, Oct. 24, 2014 and Nov. 12, 2014, while fertilizer application dates for year two were Oct. 8, 2015, Oct. 22, 2015 and Nov. 5, 2015. We made K fertilizer applications as foliar applications in the late afternoon with a CO2 backpack sprayer, and they were lightly watered in after application. Traffic was applied only on the days that canopy temperatures were at or below 32 degrees F at 8 a.m. We did traffic treatments 19 times in 2014 and 18 times in 2015.

Throughout the study the green was maintained at a 0.125-inch mowing height and mowed five days weekly using a walk-behind mower with solid rollers. We implemented weed and disease control programs on a preventative and curative basis, while irrigation was applied as needed at various levels to prevent wilt. Fertilization (not including supplemental K applications used for this research) was applied at a rate of 7 lbs. N, 1.75 lbs. P and 3.5 lbs. K per 1,000 sq. ft. per year. Study-by-year interaction did not occur, so we averaged data over 2014 and 2015 (Table 1).

Results

Overall, traffic treatments, especially those applied in the morning at 8 a.m., had a significant detrimental impact on creeping bentgrass health and appearance (Table 1). Supplemental K did not improve turfgrass health (data not shown). The turfgrass recovered to acceptable quality by spring when temperatures warmed, even with the damage incurred in the colder winter months.

This study revealed uniform and higher turfgrass quality ratings in December through early January as well as in April. Turfgrass quality ratings (Table 1 and Figure 2) dropped during the colder part of the season, and even more so with morning traffic treatments. When morning traffic (at four or eight passes with the roller) was in combination with afternoon traffic (six passes with the roller), ball-roll distance increased, reflecting wear injury to the turfgrass. Afternoon traffic affected surface firmness, but only in the month with the most traffic treatments (February). In addition, soil moisture, soil bulk density, soil organic matter, soil K concentration and tissue K concentration were not affected by traffic or K treatments. Despite convergent or divergent findings of this study compared with some of the research we’ve mentioned, the vast majority of the previous research was conducted during periods of vigorous growth, not during the cooler months.

Winter traffic at 8 a.m. when creeping bentgrass canopy temperatures are below 32 degrees F can be a damaging force causing a significant decrease in visual turfgrass quality (and subsequent appeal to managers and golfers). Injury from winter traffic, however, is generally not long-term, and golf course greens should return to acceptable quality levels by the spring months, which is similar to results from a 2008 study.

At rates of 0, 0.75 and 1.5 lbs. K per 1,000 sq. ft., fall-applied K had little effect on creeping bentgrass appearance or health, and it did not improve turfgrass winter traffic tolerance, whether we applied traffic in the morning or afternoon.

Winston Nickles Mirmow is a graduate student, Haibo Liu, Ph.D., and Bert McCarty, Ph.D., are turfgrass scientists, and James Rieck, Ph.D., is a statistician at Clemson University. You can reach Nickles Mirmow at wmirmow@g.clemson.edu for more information.

Acknowledgements
Our thanks to Don Garrett, Gib Bethea, Ph.D., Bob Cross, Ph.D., Nate Gambrell, Adam Gore, Jacob Taylor and Caleb Patrick for assisting with turfgrass plot maintenance and sample collections. We also thank the USGA Green Section for partial funding of the research, as well as Patrick O’Brien, USGA Green Section agronomist, for the study objective development and use of the TruFirm meter.

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