Figure 1. A field study evaluating centipedegrass rooting under four mowing heights (1 to 4 inches) and nitrogen fertility versus no fertility from April to July 2016 in Baton Rouge, La. Photo by Mike Adams
The Gulf Coast of the United States sometimes undergoes droughty periods during summer when evapotranspiration rates exceed soil moisture availability. In fact, the Gulf Coast experienced this during 2023 when above average temperatures, often exceeding
100 F (38 C), were accompanied by extended droughty conditions. Those types of climatic conditions leave unirrigated grassed areas prone to lower turfgrass quality and potentially death. Turfgrass professionals must rely on cultural practices such
as higher mowing heights and proper nitrogen fertility to ameliorate drought stress for greater turfgrass performance and survival.
Higher mowing heights increase leaf extension and thus leaf surface area and photosynthetic capacity, so increased energy captured can be allotted to rooting. This relationship between higher mowing heights supporting increased rooting has the benefit
of deeper root penetration to access soil moisture reserves (4). Turfgrass transpiration continues as the soil surface dries and until the next rainfall replenishes soil moisture. Therefore, deep root penetration is a significant component of drought
avoidance (5), with studies showing its importance across cultivars within species such Kentucky bluegrass (2). It is why Extension articles suggest increasing mowing height as a simple strategy for increasing turfgrass drought survival, particularly
for nonirrigated areas (6).
Materials and methods
To understand the relationship between these cultural practices and centipedegrass drought avoidance, a study was designed to evaluate mowing heights from 1 to 4 inches (2.54 to 10.16 centimeters) in 1-inch increments along with sequential nitrogen applications
in early April and July at 1 pound nitrogen per 1,000 square feet (48.82 kilograms per hectare) per application versus no nitrogen applications in 2016 at two locations in Louisiana (Figure 1). All treatment combinations were implemented 10 weeks
prior to drought simulation. Roots were periodically harvested from April until July, with roots analyzed using scanning software to measure architectural parameters including root length and surface area at soil depths of 0 to 3 inches (0 to 7.62
centimeters) and 3 to 6 inches (7.62 to 15.24 centimeters) as well as biomass recorded. Total root length, surface area and biomass were aggregated across the two soil depths for presentation. In July, cores of centipedegrass were harvested for each
treatment in the field and saturated before being kept under greenhouse conditions to simulate drought over 36 days.
Table 1. Changes in centipedegrass rooting in Louisiana from April until July 2016. All root parameter means are across all mowing heights and fertilities because these factors were not statistically significant.
Results
The results of the study raised some interesting questions. First, increasing mowing height did not prolong centipedegrass drought survival during the simulation period when measured as visual quality, nor did increasing nitrogen fertility have a positive
effect. In fact, centipedegrass that was not fertilized fared slightly better than fertilized centipedegrass, or a difference in quality of 5.9 versus 4.8 for unfertilized and fertilized centipedegrass, respectively, after 19 days of drought simulation.
Centipedegrass is a slow-growing species that generally does not require frequent fertilization, plus the increase in nitrogen leads to higher density that most likely increases transpiration on a per-area basis to hasten soil moisture depletion.
Second, when viewing centipedegrass field root growth data, rooting was robust in the initial weeks of spring, followed by a stagnant to slight decline in rooting in the summer months regardless of mowing height and nitrogen fertility. For example,
total rooting length declined from 854.8 to 740.5 inches (2,171.2 to 1,880.9 centimeters) or a 13% decline from spring into summer, along with declines of 15% and 13% in root surface area and biomass, respectively, when averaged across all treatments
(Table 1). Rooting was greater in the top 3 inches of soil versus the lower 3 to 6 inches given the origination of fibrous roots occurring at meristematic tissues or growing points.
Field root data helped to explain the results observed during the dry-down period in the greenhouse. No differences in rooting among mowing height treatments equated to no effect in drought stress observed among centipedegrass maintained at the varying
mowing heights, but the rooting pattern measured in the field was unexpected because it has generally been accepted that warm-season turfgrass species increase rooting concurrently with rising summer temperatures. This led to the more pertinent question:
Why did rooting not increase across mowing heights, even though temperatures were sufficient for centipedegrass growth?
Our first thought was that it was possible mowing caused stress to the centipedegrass. Excessive leaf removal, greater than one-third (really above 40% removal (3)), affects turfgrass rooting by ceasing growth for a period that can extend beyond the weekly
mowing frequency followed. However, centipedegrass canopy heights rarely exceeded the 40% threshold that was measured prior to each mowing for any of the mowing heights, and greater shoot growth from increased fertility at 3- and 4-inch mowing heights
did not increase rooting relative to their unfertilized 3- and 4-inch mowing height counterpart treatments. Therefore, mowing or the combination of mowing and fertility treatments did not appear to enhance or negatively affect centipedegrass rooting.
The second thought was that soil temperature may be a factor in centipedegrass rooting because swards exhibited acceptable visual quality during the field trials prior to initiation of the drought simulation. Impacts of soil temperature on cool-season
turfgrass root growth has been characterized in studies such as one conducted on two creeping bentgrass cultivars under climate-control conditions that evaluated cool and warm air and soil temperature combinations of 68 and 95 F (28 and 35 C). Researchers
reported the warmer soil temperature significantly reduced root growth and nutrient uptake even when air temperatures were optimal for shoot growth (7). Periods of warm soil temperatures can lead to root growth stagnation or dieback depending on temperature
severity and duration. This pattern of cool-season turfgrass summer root decline is well documented in the scientific literature and taught as a basic concept in beginning turfgrass courses. Although cool- and warm-season turfgrasses differ in photosynthetic
systems (C3 versus C4), accelerating root respiration from increasing soil temperatures would be expected to occur among species.
Table 2. Environmental conditions for the two field studies conducted in Louisiana
Limited research on soil temperature and warm-season turfgrass rooting have indicated soil temperatures between 61 and 79 F (16 and 26 C) at a 4-inch (10.2-centimeter) soil depth are optimal (1) (Table 2). The upper end of the temperature range was reached
in June and exceeded in July in the field study when soil temperature was recorded to a 6-inch depth. It is important to note that this study is only able to suggest soil temperature as a possible factor as to why increasing mowing height and nitrogen
fertility did not impact centipedegrass rooting. While inter- and intra-day fluctuations in soil temperature are greatly dampened relative to changes in air temperatures, soil temperatures from this study were single-point weekly measurements and
not recorded continuously over the study duration. Further research is needed to better delineate the effects of changing soil temperatures and other soil factors on warm-season turfgrass rooting and drought avoidance.
Even though implementing the cultural practices of raising the mowing height and following proper fertility prior to drought conditions did not increase rooting and greater drought avoidance, it is important to state the treatments did not have a deleterious
effect, either. Therefore, it is still recommended that once the onset of drought occurs, raising the mowing height and curtailing fertilization remain the best practices to increase performance and survivability. The greatest takeaways from the study
is that soil temperatures may have a greater effect on warm-season turfgrass rooting than previously acknowledged, and future research is needed to understand the effect soil temperature has on warm-season turfgrass rooting and stress tolerances.
The research says
- Once the onset of drought occurs, raising the mowing height and curtailing fertilization remain the best practices to increase performance and survivability.
- Soil temperatures may be having a greater effect on warm-season turfgrass rooting than previously acknowledged.
- Future research is needed to understand the effect soil temperature has on warm-season turfgrass rooting and stress tolerances.
Literature cited
- Beard, J.B. 2001. Temperature optimums and lethal thresholds. Turfax Vol. 9. Ann Arbor Press.
- Bonos, S.A., and J.A. Murphy. 1999. Growth responses and performance of Kentucky bluegrass under summer stress. Crop Science 39(3):770-774 (https://doi.org/10.2135/cropsci1999.0011183X003900030026x).
- Crider, F.J. 1955. Root-growth stoppage resulting from defoliation of grass. No. 1102. USDA.
- Fry, J.D., and B. Huang. 2004. Applied turfgrass science and physiology. Wiley, New York.
- Richardson, M.D., D.E. Karcher, K. Hignight and D. Rush. 2008. Drought tolerance and rooting capacity of Kentucky bluegrass cultivars. Crop Science 48(6):2429-2436 (https://doi.org/10.2135/cropsci2008.01.0034).
- Trenholm, L.E., J.B. Unruh and J.L. Cisar. 1991. Mowing your Florida lawn. UF/IFAS Extension.
- Xu, Q., and B. Huang. 2000. Growth and physiological responses of creeping bentgrass to changes in air and soil temperatures. Crop Science 40(5):1363-1368 (https://doi.org/10.2135/cropsci2000.4051363x).
Jeffrey Beasley (jeffrey.beasley@uncp.edu) is a professor at the University of North Carolina at Pembroke, Mike Adams is a former graduate student at Louisiana State University, Dylan Watson is a graduate student at Louisiana State University, and Jeff Kuehny is a professor and director of the Burden Museum and Gardens located in Baton Rouge, La., for the LSU Agricultural Center.