Late-season bermudagrass nutrition and cold tolerance ... - GCSAA

081-092 Sept04Techwell 8/17/04 5:20 PM Page 82RESEARCHBefore we review this evidence, let’s frame thediscussion by reviewing bermudagrass growthduring the hardening period. Specifically, is itreasonable to think that lush shoot growthcan be promoted on transition zonebermudagrass (Cynodon species) once hardeningtemperatures are occurring? What arethese temperatures?When average daytime temperatures ([dailylow + daily high]/2) drop below 60 F (15.6 C),bermudagrass growth slows dramatically, withleaf discoloration (and the onset of dormancy)occurring in earnest once they drop below50 F (10 C) (5). In an upper transition zone climatesuch as in Roanoke, Va., average temperaturesof 60 F (15.6 C) are reached aroundSept. 30, with the first hard frost occurringabout Oct. 15, leading to almost complete leaftissue browning by early November.Thus, this mid-September to early-November hardening period is when textbooksadvise greatly reducing or actuallyeliminating the use of nitrogen. How doesbermudagrass respond to the lower temperaturesand shorter days of this period? Untilone or two hard frosts have occurred, thebermudagrass is still green and photosynthesizing,but little leaf elongation is occurring.But where is this photosynthate going?We contend that, as in cool-season grassesin late fall, most photosynthate is goingtoward root, stolon and rhizome growth orstorage. During this hardening period, itwould be almost impossible to promote alarge flush of succulent growth with an applicationof soluble nitrogen fertilizer. Try it, andthe result is most often a greening responsewithout significant vertical growth.If the turf remains green later in the fall,the results should be greater photosynthesisand net energy gain. Loss of this re-greenedleaf tissue to a hard frost most likely has littleconsequence for overwintering becausethe status of the stem tissue (crowns, stolonsand rhizomes) determines winter survival.Based on this reasoning, we hypothesizethat applying nitrogen late in the season inthe transition zone on bermudagrass that hasnot been overseeded will:• enhance color retention before frost events• encourage re-greening after mild frostevents• have no effect on cold hardiness• promote early spring green-upBelow we present supporting data fromfour late-season nitrogen studies conductedover the last 15 years.Study 1: Schmidt and ChalmersAt the Virginia Tech Turfgrass ResearchCenter in Blacksburg, mature Tifgreenbermudagrass was managed at 0.625 inch(15.88 millimeters) on a silt loam (pH 6.2and moderately high soil-test level of potassiumat 274 pounds/acre [307.1 kilograms/hectare]).[Researchers (5) havepresented the following potassium sufficiencyrankings: very low = 0-60 pounds/acre (0-67.3 kilograms/hectare); low = 61-120pounds/acre (67.3-134.5 kilograms/hectare);moderate = 121-280 pounds/acre (135.6-313.8 kilograms/hectare); high = greater than280 pounds/acre (313.8 kilograms/hectare).]The bermudagrass was treated with 0, 2.5pounds or 5.0 pounds nitrogen/1,000 squarefeet (0, 122.1 or 244.1 kilograms/hectare)from urea in split applications on Aug. 30and Sept. 27 of 1989, 1990 and 1991.The results in Table 1 clearly show thatlate-summer through early-fall nitrogenapplications to Tifgreen bermudagrass, amoderately winter-hardy cultivar, providedextended color in fall 1989. Similar resultswere found in 1990 and 1991. At no timeduring the three years of the study wasbermudagrass recovery from dormancyimpaired because of late-season nitrogen(13). This was true even though conditionsin December 1989 were unusually cold,with a record low for Dec. 23 of -10 F(-23.3 C). Temperatures were normal inBlacksburg, Va., during the winters of 1990-1991 and 1991-1992.Study 2: Goatley et al.Mature Tifgreen bermudagrass, managedat 0.18 inch (4.57 millimeters) on aloam (pH 6.0 and a high soil-test level ofpotassium of 300 pounds/acre [336.3kilograms/hectare]) in Starkville, Miss.,received 0, 1.0 or 2.0 pounds nitrogen/1,000 square feet (0, 48.8 or 97.6 kilograms/hectare)from ammonium nitrateand 0, 0.8 or 2.0 pounds potassium (0, 39.1or 97.6 kilograms/hectare) from potassiumchloride (in all combinations) during the secondweek of October in 1989, 1990 and1991. On this soil, which contained sufficientsoil-available potassium (5), fertilizingwith potassium did not measurably changebermudagrass color or levels of total nonstructuralcarbohydrate (TNC) at any timefor any level.LATE NITROGEN ON TIFGREENNitrogen application rate 1989 color rating (9 = darkest) % greenhouse regrowth*Pounds/1,000 sq. ft. Kilograms/hectare Oct. 3 Oct. 16 Oct. 23 Feb. 15, 1990Control (no nitrogen applied) 5.8 b 4.4 b 4.3 c 28.6 a2.5 122.1 7.2 a 5.3 a 4.9 b 24.7 a5.0 244.1 7.3 a 5.3 a 6.3 a 26.1 a*Percentage of greenhouse regrowth of dormant plugs that are subjected to controlled freezing to 23 F (-5 C). Plugs were sampled and frozen on Jan. 25, 1989.Table 1. Influence of 1989 late-summer and early-fall applications of nitrogen on fall color and cold hardiness of Tifgreen bermudagrass.Values followed by the same letters within a column are not significantly different from each other.82 GCMSeptember 2004