Summer Drought Effects on Warm-Season Turfgrass Canopy Temperatures

Kurt Steinke, David R. Chalmers, James C. Thomas, and Richard H. White, Soil and Crop Sciences Department, Texas A&M University, College Station, TX 77843-2474

Corresponding author: Kurt Steinke. ksteinke@ag.tamu.edu

Abstract

Little information exists detailing the impact of water restrictions on urban amenity grass systems. Through the use of an automated rainout shelter, a two-year field study was conducted to evaluate how water restrictions, drought, and warm-season turfgrass species influence the changes in radiant heat accumulation within warm-season turfgrass systems. Canopy temperatures and leaf firing characteristics of 3 turfgrass species and 24 total cultivars were monitored through a 60-day summer drought in San Antonio, TX, in two successive years. Zoysiagrass maintained significantly higher canopy temperatures compared to bermudagrass and St. Augustinegrass. Across species, midday canopy temperatures ranged 1.0 to 49.0°F warmer than maximum daily air temperatures. During the final 30 days without water, midday canopy temperatures averaged 22 to 43°F and 15 to 38°F warmer than maximum daily air temperatures for 2006 and 2007, respectively. Bermudagrass maintained a significantly lower overall leaf firing response and rates of developing leaf firing compared to St. Augustinegrass and zoysiagrass. Results indicate that the benefits of water conservation through a mandated complete water restriction during drought must recognize potential negative impacts of turfgrass systems losing the ability to buffer radiant heat as plants desiccate from low levels of soil moisture.

Introduction

Regulations limiting water applications to outdoor landscapes during periods of prolonged drought or water shortages are frequently enacted as a water conservation tool. To increase home consumer water conservation, the San Antonio Water System (SAWS) has drafted a list of drought-tolerant turfgrass species and cultivars approved for planting but few data are available documenting the secondary impacts (i.e., heat accumulation) of water restrictions on the ecology of urban landscapes (18). Beard and Green (1) discussed the ability of turfgrasses to dissipate radiant heat from urban environments, but little information is available pertaining to differences in individual microclimates created between multiple warm-season turfgrass species and cultivars once water becomes limiting. Differences in turfgrass temperature moderation with respect to decreasing amounts of water have been reported in cool-season turfgrasses (8).

Turfgrass canopy temperatures have been used to determine transpiration and management differences between species. The transpirational cooling process allows non-stressed grasses to maintain decreased canopy temperatures which then increase as soil water becomes limiting. Kneebone and Pepper (12) found that at equivalent soil moistures, infrared thermometry could distinguish differences in transpiration for bermudagrass and St. Augustinegrass. Under well watered conditions, cool-season turfgrass canopy temperatures may increase an average of 4 to 15°F above ambient air temperatures (7). Danielson et al. (6) found Kentucky bluegrass canopy temperatures to increase 3.0°F with every 10% decrease in evapotranspiration. However when water becomes limiting, turfgrasses do not possess the thermal moderation provided by evaporation and transpirational cooling (13). Canopy temperature may also be influenced by net radiation (10,14), canopy density (9) and air flow (14). These characteristics led to the development of the Crop Water Stress Index (CWSI) which deemed turf to be under stress and require irrigation once the difference between canopy and ambient air temperatures exceeded a preset value (20,21). Despite the introduction of canopy temperature monitoring techniques, many turfgrass managers continue to rely on qualitative visual assessments of water stress in determining irrigation needs.

As identified by Steinke (19), vegetation growing in urban environments must tolerate a variety of anthropogenic activities that may impact plant performance. Oke et al. (17) noted that the urban vegetation canopy had the potential to modify heat accumulation but a variety of human activities may limit a plant’s ability to moderate temperatures within these habitats. Byrne et al. (4) found that a combination of aboveground plant composition, arrangement, and density influenced a ground cover’s ability to alter ecosystem properties. Johns and Beard (11) indicated that maintained green turf landscapes in conjunction with buildings can decrease energy inputs for interior cooling through the cooling transpirational stream of turfs. In climates experiencing prolonged periods of elevated summer temperatures and frequent periods of drought, the contributory cooling effects of turfgrasses may increase in importance considering that irrigation of turf and surrounding landscapes may be prohibited. Few reported measurements of warm-season turfgrass canopy temperatures exist and no data are available pertaining to turfgrass canopy temperatures during drought conditions. The goal of this work was to measure the turfgrass canopy temperatures of 3 species and 24 cultivars of warm-season turfgrasses during periods of 60-day drought. The objective of this research was to compare the ability of 24 warm-season turfgrass cultivars to mitigate heat accumulation during a prolonged 60-day summer drought.

Evaluating Turfgrass Impacts on Heat Accumulation

The experiment was conducted 3 mi south of San Antonio, TX on an area of Lewisville silty clay (fine-silty, mixed, thermic Udic Calciustolls) previously used for sod production. An area approximately 100 ft wide by 500 ft long was cleared of sod and irrigation equipment. No visual barriers (i.e., vegetation or infrastructure) existed around the area to impede climatic conditions. A 50- by 350-ft area was laser graded to provide two 50- by 100-ft experimental areas at each end of a 17,500-ft² rectangle. In between the two experimental areas, a 1% slope toward the plot center was graded to aid drainage. A trench 2 ft wide and 1.5 ft deep was cut along each side of the 17,500-ft² rectangle to accommodate construction of a concrete footer and wall on which the tracks for a rainout shelter were mounted. Each experimental area was divided into four 20- by 20-ft blocks. Blocks were bordered on all sides by a 2-ft alley. Each block was sub-divided into 25 individual 4- by 4-ft plots. All plots consisted of native soil with an unrestricted rooting potential.

Irrigation was applied using a two-zone automatic irrigation system controlled by an Irritrol Systems, KwikDial automatic sprinkler system. Each block was equipped with a corner pop-up rotor spray head (Hunter Industries, San Marcos, CA, model PGJ-06) and a two gpm nozzle that provided head-to-head coverage. Plots were pre-treated with granular Basamid G (tetrahydro-3,5,-dimethyl-2H-1,3,5-thiadiazine-2-thione, Certis USA, Columbia, MD) at the labeled rate of 350 lb/acre. Basamid was raked into the upper 2 to 3 inches of topsoil, irrigated, and covered with a clear plastic tarp for 7 days. Upon tarp removal, the soil surface was scarified to break up surface crust, and plots remained fallow for 4 days prior to sodding to allow gaseous residues to dissipate.

Grasses evaluated in this study included 8 cultivars of bermudagrass [Cynodon dactylon (L.) Pers.] (Common bermudagrass, ‘Celebration,’ ‘GN-1,’ ‘Grimes EXP,’ ‘Premier,’ ‘TexTurf,’ ‘TifSport,’ and ‘Tifway 419’; 7 cultivars of St. Augustinegrass [Stenotophrum secundatum (Walt.) Kuntze.] (St. Augustine Common, ‘Amerishade,’ ‘Delmar,’ ‘Floratam,’ ‘Palmetto,’ ‘Raleigh,’ and ‘Sapphire’); and 9 cultivars of zoysiagrass [Zoysia japonica Steud.] (‘El Toro,’ ‘Emerald,’ ‘Empire,’ ‘Jamur,’ and ‘Palisades’) and [Zoysia matrella (L.) Merr.] (‘Cavalier,’ ‘Y-2,’ ‘Zeon,’ and ‘Zorro’). Cultivars were chosen based on commercial availability in the San Antonio market.

Sod was harvested and washed by producers to remove residual soil. Pallets were labeled and stored overnight in a refrigerated semi-truck trailer. Washed sod was delivered to the experimental site on the following morning for planting. All turfgrasses were allotted a 10- to 10.5-month establishment period before initiating the 60-day drought. Year one of the drought evaluation was planted on 20 September 2005 with the drought period extending from 23 July through 21 September 2006. The second year drought evaluation plots were established on 22 September 2006 with a drought period of 5 July through 3 September 2007. Year one evaluations were conducted on the east end of the rainout shelter in 2006. Year two evaluations were repeated on the west end of the site during 2007. To allow comparisons between turf species and cultivars, all plots were mowed weekly with a rotary mulching mower at a height of 2.25 inches. Although a 2.25-inch height of cut may not be optimum for all cultivars, the SAWS ordinance targeted new home construction and the stated height of cut satisfies these conditions across a majority of cultivars.

A 5,000-ft² movable rainout shelter was constructed to protect the plots from precipitation during the drought period. The rainout shelter automatically deployed and covered the research plot area within 90 sec upon detecting 0.02 inches of precipitation using a tipping bucket rain gauge. After 30 min of no precipitation, the rainout shelter returned to its centrally-located position between the year one and year two experimental areas.

Canopy temperatures were measured every 7 to 13 days throughout the 60-day drought periods across both years. Temperatures were only recorded on sunny days with little to no cloud cover and during the daily solar zenith. Canopy measurements were discontinued during the recovery period due to turfgrass transpiration and sufficient soil moisture. Canopy temperatures were measured using a Model MT4 MiniTemp non-contact thermometer (Raytek Corporation, Santa Cruz, CA). Leaf firing was visually evaluated according to the National Turfgrass Evaluation Program optimal environment and management standard using a scale of 1 to 9 with 1 being 100% leaf firing and 9 being 100% green (16).

All data were subjected to analysis of variance using the general linear model, multivariate test procedure in SPSS, ver. 15.0 (SPSS Inc., Chicago, IL) to determine statistical significance of the results. Mean differences were separated using Tukey’s HSD (honestly significant difference) procedure.

Turfgrass Species Performance

Heat accumulation profiles differed significantly among the turf species. Mean turfgrass canopy temperatures averaged across 6 sampling dates over each 60-day drought period in successive years significantly differed among the species (Table 1). To demonstrate differences in weekly heat accumulation, data for individual species canopy temperatures at each rating date are shown (Figs. 1 and 2 and Table 2). Zoysiagrass canopy temperatures were significantly warmer than bermudagrass or St. Augustinegrass on 5 of 6 sampling dates in 2006 and 4 of 6 sampling dates in 2007 (Figs. 1 and 2). Mean canopy temperatures significantly differed among the turf species with zoysiagrass maintaining a 6 to 23°F and 7 to 16°F warmer temperature than bermudagrass or St. Augustinegrass during the 2006 and 2007 droughts, respectively. The sensible heat load placed upon all three turf species increased throughout the 60-day period in both years as plots became significantly warmer as drought conditions progressed (Table 2). During evaporation, energy absorbed in the form of latent heat allowed water to overcome attractive forces between water particles to result in increased atmospheric evaporation and a drop in surrounding temperature. However as the quantity of water diminished, lower rates of plant transpiration resulted in a diminished cooling response and produced an accumulation of latent heat (i.e., heat island). These data embody the general trend seen throughout both years that zoysiagrass accumulated and retained heat more quickly than the other turf species used resulting in a significantly greater canopy temperature throughout both drought periods. Despite differences in leaf morphology and width, St. Augustinegrass and bermudagrass maintained similar canopy temperature readings on 4 of 6 sampling dates during both 2006 and 2007.

Table 1. Mean turf species canopy temperature (°F) during 60-day
drought periods in 2006 and 2007.

Turf species	Drought year
Turf species	2006	2007
Bermudagrass	115.4 a^x	106.3 b
St. Augustinegrass	120.6 b	104.4 a
Zoysiagrass	129.4 c	114.4 c

^x Column means followed by the same letter are not statistically
different (P ≤ 0.05) using Tukey’s HSD.

Table 2. Turf canopy temperature (°F) for three grass species in full sunlight measured on 6 dates as turf progressed through the 2006 and 2007 drought periods.

Fig. 1. Mean turf species canopy temperatures (°F) during the 60-day drought period in 2006.

Fig. 2. Mean turf species canopy temperatures (°F) during the 60-day drought period in 2007.

Increased temperatures, decreased humidity, and increased potential evapotranspiration rates culminated into a more severe drought in 2006 than in 2007 (5). The synergistic effects of both heat and drought stress in 2006 altered the turfgrass microclimate and caused canopy temperatures to rise much earlier as compared to 2007, where drought stress alone played the major role in the presence of lower temperatures and higher humidity when compared to 2006 (Table 3). As a result, plots began to display increased rates of leaf firing earlier into the 2006 drought period than 2007 (Table 4). Across both study years and all three warm-season grasses, canopy temperatures were frequently lower than bare soil temperatures up until 40 days into the drought period when a cool front reduced air temperatures allowing canopy temperatures to remain greater than soil temperatures.

Table 3. Mean turf canopy temperatures (°F) across all cultivars on six measurement dates during the 60-day drought in 2006 and 2007.

2006		2007
Date (and days into drought)	Temp (°F)	Date (and days into drought)	Temp (°F)
4 Aug (13)	107.5 a^x	5 Jul (1)	100.8 b
11 Aug (20)	123.3 c	12 Jul (8)	96.8 a
24 Aug (33)	128.3 d	23 Jul (19)	104.2 c
31 Aug (40)	135.8 e	3 Aug (30)	107.8 d
7 Sep (47)	118.5 b	13 Aug (40)	125.6 f
15 Sep (55)	119.7 b	24 Aug (51)	115.4 e

^x Column means followed by the same letter are not statistically different (P ≤ 0.05) using Tukey’s HSD.

Leaf firing trends were significant between all cultivars of the three species and similar between years. The overall leaf firing severity over both study years was bermudagrass < St. Augustinegrass < zoysiagrass (Table 4). Zoysiagrass had the fastest and greatest extent of leaf firing on 9 of 12 rating dates over both the 2006 and 2007 60-day drought periods. Bermudagrass experienced significantly less leaf firing than either St. Augustinegrass or zoysiagrass on 8 of 12 rating dates during the two year study. Intermediary in leaf firing response, St. Augustinegrass displayed greater leaf firing than bermudagrass on 8 of 12 rating dates but less leaf firing than zoysiagrass on 9 of 12 rating dates. Correlation coefficients between leaf firing and canopy temperatures indicated a strong inverse relationship for St. Augustinegrass (-0.53, -0.67), zoysiagrass (-0.43, -0.81), and bermudagrass (-0.57, -0.58) during the 2006 and 2007 drought periods, respectively. These data suggest that canopy temperatures will begin to increase at the onset of visual leaf firing, soon after water resources become restricted or withheld. As indicated by leaf firing, data suggest zoysiagrass enters into a dormant or quiescent state much earlier than bermudagrass or St. Augustinegrass and thus may accumulate heat at a greater rate and much earlier in the drought process as compared to bermudagrass or St. Augustinegrass.

Table 4. Leaf firing rated on a scale of 1 to 9 (1= 100% fired, 9 = green) for three grass species measured on 6 dates as a measure of drought stress during the 2006 and 2007 drought periods.

Cultivar Influence

Few statistically significant cultivar comparisons occurred amongst each of the three species throughout the two-year study (Table 5 and Table 6). ‘Premier’ bermudagrass did maintain the warmest canopy temperature amongst the bermudagrasses over both study years and maintained a leaf firing profile similar to the profiles of zoysiagrass and St. Augustinegrass as compared to the remaining bermudagrass cultivars. Amongst the St. Augustinegrass cultivars, ‘Floratam’ exhibited a pattern of delayed leaf firing symptoms when compared to the other cultivars over both study years. Cultivars of Zoysia matrella tended to accumulate heat more quickly than cultivars of Zoysia japonica over both study years.

Table 5. Turfgrass mean canopy temperature (°F) for 24 cultivars measured on six measurement dates during the 60-day drought in 2006.

Table 6. Turfgrass mean canopy temperature (°F) for 24 cultivars measured on six measurement dates during the 60-day drought in 2007.

Conclusions

Prolonged summer drought, in the absence of irrigation, has the capability to alter the urban ecology of amenity grass systems. Turfgrass species selection had the greatest effects on temperature moderation and leaf firing in this experiment. Similar to Beard and Johns (2), current data demonstrate substantial heat accumulation from dormant desiccating turf canopies. Zoysiagrass maintained significantly higher canopy temperatures during drought conditions as compared to bermudagrass and St. Augustinegrass. Differences between St. Augustinegrass and bermudagrass may have dissipated during the final 30 days of drought in years one and two due to a reduced evapotranspiration rate or similar leaf firing characteristics between the two species. Lower canopy temperatures recorded from both bermudagrass and St. Augustinegrass may indicate enhanced heat dissipation through greater evapotranspiration. The stiff, dense growth habit of zoysiagrass may better resist air movement through the turf canopy potentially creating a dormant boundary layer that may be located to a greater height above the zoysiagrass canopy than for bermudagrass or St. Augustinegrass.

The overall delayed leaf firing response of bermudagrass may be attributed to the ability to pull soil moisture from deeper within the soil profile as bermudagrass has been documented to be one of the deeper rooting warm-season turfgrass species with mature rooting depths of up to 85 inches recorded (3). While the turfgrass root systems in this study may not have been fully mature, a 10.5-month establishment period is sufficient to establish a healthy plant community across many turfgrass cultivars. The ability to access a larger pool of soil moisture may have increased the water potential or transpiration rate of bermudagrass compared to the other turfgrasses. The increased tissue water content may have decreased the canopy temperature of bermudagrass and aided in the delayed leaf firing response. The delayed leaf firing response and associated delay of bermudagrass color loss did not appear to cause increased heat absorption as is often seen in dark-colored inorganic mulch materials.

All grasses survived the 60-day drought period over both years of study. However, the inherent differences in the rate of heat accumulation between warm-season turfgrass species could impact a number of factors associated with species-specific drought performance. Zoysiagrasses appear to display increased rates of leaf firing and attain greater canopy temperatures than bermudagrass or St. Augustinegrass and may enter a quiescent state much earlier than the other species. Though mandated complete water restrictions during drought may increase water savings, the question to be answered is whether these water savings translate into increased energy usage for infrastructural cooling. As areas not receiving water lose transpirational cooling abilities, increased heat absorption will further the desiccation of plant biomass due to decreased moisture levels but the heat accumulation and leaf firing alone are not to be interpreted as predictors of drought tolerance. Municipalities may achieve greater benefits by considering a whole energy and water spectrum in lieu of water savings alone. As turfgrass acreage continues to increase in the United States (32 million acres) (15), data on the ecology of amenity grass systems with and without water are needed to more accurately design and manage sustainable urban ground covers to benefit the overall urban ecosystem.

Acknowledgements and Disclaimer

This work is a contribution of Texas AgriLife Research and Texas AgriLife Extension Service. Financial support from the San Antonio Water System and the Turfgrass Producers of Texas is gratefully acknowledged. This project would not have been possible without the assistance of Dr. Guy Fipps, Kendall Chilek, Chris Braden, and Dr. Wayne LePori who were responsible for the design, construction, and maintenance of the rainout shelter.

Mention of trade name does not constitute endorsement.

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