Preparing for Winter
Bermudagrass is the most widely used turfgrass in the southern United States for golf courses, athletic fields, parks, roadsides and private grounds. Native to eastern Africa, bermudagrass has aggressive stolons and rhizomes that form a vigorous turf with high shoot densities and rapid growth capabilities. Bermudagrass has excellent wear, drought and salt tolerances but necessitates high nitrogen fertility, full sunlight and routine cultivation. As a warm-season grass, bermudagrass requires temperatures from 80°F to 95°F for optimum growth and is widely distributed throughout warm humid, warm subhumid, and warm semiarid climates. One of the characteristics of managing bermudagrass in the transition zone is winter dormancy. As temperatures approach 50°F, bermuda-grass begins to lose chlorophyll and turns brown. Recent research offers new perspectives on bermudagrass dormancy mechanisms to help practitioners better manage fall fertilizations.
PGRS AND DELAYED FALL DORMANCY
Turfgrass scientists for years have studied the effects of fertilizer rates, timings and sources on bermudagrass fall color. In recent years, plant growth regulator (PGR) use throughout the growing season has shown to delay the onset of winter dormancy. This is believed to be a morphological response from more compacted leaf tissue with greater amounts of chlorophyll per unit leaf area, which prolongs a darker green leaf color. However, delayed dormancy of PGR-treated bermudagrass may result from nutrient allocation to leaf tissue from increased reserve nutrients stored in roots and rhizomes.
Across a two-year field study, ‘Tifway’ bermudagrass had enhanced spring greenup from fall applications of Primo. It was noted that variable nitrogen (N) inputs with Primo did not enhance ‘Tifway’ bermudagrass spring greenup; however, PGR treatments increased overall green period by 20 to 25 days. Increased ‘Tifway’ bermudagrass stolon freezing tolerance has also been observed in late fall from repeated PGR applications in the summer or a late-season application.
Experiments in South Carolina on a ‘TifEagle’ bermudagrass putting green receiving Primo during the growing season had higher visual quality as dormancy was initiated. Bermudagrass visual quality also increased with nitrogen rate throughout the fall and early winter. ‘TifEagle’ bermudagrass treated with Primo retained green color in December, but all plots were completely dormant by the first week in January. In these experiments, nutrient allocation between leaves and belowground tissues was a major parameter and offers a new perspective on bermudagrass fall color.
DELAYED WINTER DORMANCY: A MORPHOLOGICAL OR NUTRIENT ALLOCATION PHENOMENON?
Excessive nitrogen levels during the late-season may deplete nutrients stored in rhizomes as shoots compete for plant reserves. Because PGRs effectively suppress bermudagrass shoot growth, applications may reduce nutrients removed through clippings and minimize nutrient partitioning to leaf growth. Minimizing nitrogen supply to new leaves may increase nitrate assimilation in roots and redirect additional nutrients for storage in belowground tissues. Nitrogen distribution away from leaves may also stimulate root growth as nitrate directs carbohydrates for amino acid synthesis.
‘Tifway’ bermudagrass treated with Primo at 12 ounces per acre every four weeks has shown to reduce nitrogen allocation to leaf tissue with increased retention in belowground tissues. Increased total nonstructural carbohydrates in ‘Tifway’ bermudagrass roots have also been noted following Primo applications at 12 and 18 ounces per acre. Therefore, routine PGR applications may be a practical method of balancing photosynthate and nutrient allocation within the plant to increase plant use efficiency of applied nutrients.
A two-year field experiment at Clemson University investigated nutrient reallocation of ‘TifEagle’ bermuda-grass when treated with four nitrogen rates and Primo. Turf quality generally increased with N rate; however, ‘TifEagle’ bermudagrass required 0.375 to 0.5 pounds N per 1,000 square feet per week from May to June and 0.25 pounds N per 1,000 square feet from July to August to maintain acceptable quality without Primo. When treated with Primo, initial discoloration resulted but turf soon recovered and displayed 10- to 25-percent enhanced quality. Furthermore, bermudagrass treated with Primo had 10-percent reduced leaf N concentrations after three months with 20-percent increased rhizome N concentrations. Similar trends were observed with phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sulfur (S) and iron (Fe) concentrations. Inhibiting bermudagrass leaf growth with Primo reduced total nutrients removed through clippings by 70 percent. By late August, Primo-treated turf had greater N, P, K, Ca, Mg and S retention in rhizomes.
These differences were not observed in leaves and rhizomes in the following spring as Primo-treated ‘TifEagle’ bermudagrass had similar nutrient concentrations to the untreated. Increased nutrients stored in belowground tissues throughout the summer may have been reallocated back to the shoots as a mechanism to delay dormancy and promote spring greenup. The synthesis of chlorophyll molecules requires a source of Mg, N and Fe, which were all found in great quantities in rhizomes during the fall. This reallocation of nutrients has not been previously correlated before to delayed fall dormancy of warm season grasses.
Similar nutrient allocations have also shown to prevent leaf nutrient deficiencies on dwarf-type bermudagrass in greenhouse experiments. ‘TifEagle’ bermudagrass treated with four rates of Primo — 0, 3, 6, and 9 ounces per acre every three weeks over 12 weeks — had 50 to 90 percent fewer nutrients removed through clippings with increased PGR rates. Rhizome and roots had higher N concentrations and retentions with increased Primo rates, suggesting greater leaf growth inhibition caused increased N storage in belowground plant tissue. After 180 days in another greenhouse experiment, ‘TifEagle’ bermudagrass treated with Trimmit (paclobutrazol) averaged 10 percent higher N retention in rhizomes compared to non-PGR treated turf.
PUTTING BERMUDAGRASS DORMANCY INTO PERSPECTIVE
Dormancy is a natural mechanism warm-season grasses experience when temperatures drop below optimum for shoot growth and chlorophyll retention. As day lengths become shorter and temperatures drop below 50°F, bermudagrass loses chlorophyll and begins allocating reserves for storage in belowground tissue. Maintaining green bermudagrass shoots during winter may be physiologically unfavorable for plant health. For example, if reallocated nutrients to leaf tissue are correlated with delayed dormancy, could this reduce storage of plant reserves or root growth during this critical period? Further investigation is warranted regarding mechanisms of bermudagrass winter dormancy, carbohydrate allocation and nutrient use especially for dwarf-type bermudagrass putting greens. However, it can be surmised that routine PGR use has significant influences on bermudagrass leaf morphology and nutrient allocation.
Patrick McCullough is a program associate at Rutgers University (New Brunswick, N.J.). Haibo Lui, Ph. D., is an associate professor and Bert McCarty, Ph. D., is a professor of turfgrass science in the Department of Horticulture, both at Clemson University (Clemson, S.C.).
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