Figure 1. Representative digital images of several cultivars taken at zero and 49 days of heat stress and after a 28-day recovery period. BF, buffalograss; KB, Kentucky bluegrass; SL, slender creeping red fescue; ST, strong creeping red fescue; CF, Chewings fescue; SF, sheep fescue; HF, hard fescue. (Click on image to enlarge.)
In the northern climates of the United States, cool-season turfgrasses established in urban areas are subjected to harsh conditions in winter (ice and salt) and to high temperatures during summer, particularly if they are established close to pavement and asphalt. Seasonal heat stress leads to a general decline of turf quality and growth during the summer months (2).
Overall, heat stress is characterized by reduced vegetation growth (percentage of green tissue) and turf quality (tissue browning). Climate change will likely intensify these seasonal heat-stress periods, potentially leading to even more reduction of turf quality and growth, especially in urban environments. The presence of a well-established and active turf stand results in numerous beneficial ecosystem services. It is therefore imperative to screen for heat tolerance among various turfgrass species and cultivars so that turf managers will be able to maintain high turf quality during seasonal heat periods and keep these ecosystem services functional.
Extensive heat-stress studies, but limited species
Numerous researchers have studied the effect of high temperature (heat stress) on cool-season turfgrass species. Tall fescue (Schedonorus arundinaceus (Schreb.) Dumort.) is known as the most heat-tolerant cool-season grass (4), and Kentucky bluegrass (Poa pratensis L.) is also considered tolerant to heat stress (7). Perennial ryegrass (Lolium perenne L. spp. perenne), a close relative of tall fescue, has generally been thought to be heat-sensitive (7). The fine fescues include five species: hard fescue (Festuca trachyphylla), sheep fescue (Festuca ovina L.), Chewings fescue (Festuca rubra spp. commutata), slender creeping red fescue (Festuca rubra spp. littoralis) and strong creeping red fescue (Festuca rubra spp. rubra). Past studies of these species found large variation in heat tolerance among species and among cultivars of the same species (6), with hard fescue being heat-tolerant; slender creeping red fescue, strong creeping red fescue and sheep fescue being moderately heat-tolerant; and Chewings fescue considered heat-sensitive.
Most of the previous heat-stress research studies were performed with few species or cultivars (maximum of four species) over a small period of heat stress (around 35 days). Additionally, few of these studies included a recovery period in normal conditions. Therefore, we decided to assess the heat response of 34 cultivars and accessions (14 species).
In our experimental design, we added cool-season turfgrass species that had not been tested for their heat tolerance, such as alkaligrass (Puccinellia Parl.), smooth brome (Bromus inermis Leyss.), Canada bluegrass (Poa compressa L.) and a tetraploid perennial ryegrass. In addition, we included two warm-season species, buffalograss (Bouteloua dactyloides (Nutt.) J.T. Columbus) and blue grama (Bouteloua gracilis (Willd. ex Kunth) Lag. ex Griffiths).
Turf heat-stress research: A different approach
Thirty-four cultivars and accessions (entries) representing 14 perennial grass species (see Table 1, Turfgrass cultivars and accessions tested) were evaluated for tolerance to heat stress. Four replications of 34 entries were seeded using a total sowing density fixed at 2 pure live seed (PLS) per square centimeter in jumbo pots containing a mixture of sand and topsoil. The alkaligrass cultivars tested did not germinate well, and an additional 2 pure live seed per square centimeter were seeded in their pots.
After complete establishment — 15 weeks for Run No. 1 and 20 weeks for Run No. 2 — the heat-stress and recovery experiments were started in controlled environmental conditions (growth chambers). Growth chamber temperature was raised to 95 F/77 F (35 C/25 C) (day/night), and the humidity was increased to 70%. Pots were watered to avoid drought stress.
After 49 days of heat treatment, the chamber conditions were returned to normal — a temperature regime of 77 F/59 F (35 C/15 C) (day/night) and a relative humidity of 40% — for a recovery period of 28 days. Data were taken at time zero before the experiment started, after 49 days of heat stress, and at the end of the 28-day recovery period.
Plant performance was evaluated with the normalized difference vegetation index (NDVI), obtained by determining the difference between the near-infrared (NIR) and the red light (RED) (value ranges between 0 and 1). The NIR is reflected by the chlorophyll, whereas RED is absorbed; active leaves will reflect more NIR and absorb more RED. Altogether, healthy and green vegetation will display higher NDVI value (close to 1), and dying/stressed leaves will show lower NDVI value (close to 0). A FieldScout TCM 500 NDVI Turf Color Meter (Spectrum Technologies) was placed on top of each pot, and the NDVI measure was recorded. Digital images of all individual pots were also taken with a customized light box designed to fit over the pots, and images were analyzed for percent green coverage.
NDVI and green cover for turf species and entries
For both runs, at the species level, only hard fescue and sheep fescue displayed consistent, significantly lower NDVI values compared with the highest values after 49 days of heat stress. After 28 days of recovery, hard fescue and sheep fescue again displayed significantly lower NDVI values compared with the highest NDVI values, indicating an incomplete recovery (Table 2, Figure 1, above).
For Run No. 1, NDVI values for Salton Sea alkaligrass, Soil Guard hard fescue, Nanook hard fescue, Gladiator hard fescue, Blue Mesa sheep fescue, Quatro sheep fescue and Premium perennial ryegrass were significantly lower than that of top performer Tirem Kentucky bluegrass after 49 days of heat stress (Table 3). After 28 days of recovery, only Salton Sea alkaligrass, Blue Mesa sheep fescue and Premium perennial ryegrass possessed NDVI values that were not different from the highest value of Xeric strong creeping red fescue. Only these particular entries seemed to have recovered from the heat stress.
Editor’s note: Does syringing effectively cool plants under heat stress, and are there drawbacks to the practice? Researchers investigate in Syringing for turfgrass canopy cooling.
In Run No. 2, after 49 days of heat stress, only hard fescue cultivars (Soil Guard, Nanook and Gladiator) and sheep fescue (J-248, Quatro and Blue Mesa) had NDVI values significantly lower than that of Bad River blue grama, the entry with the highest NDVI. By the 28th day of recovery, all of these entries (with the exception of Gladiator hard fescue) showed an NDVI value still significantly lower than that of the highest performer (Thunderstruck tall fescue), indicating their lack of complete recovery.
The percentage of green was significantly lower for Salton Sea alkaligrass, Nanook hard fescue, Soil Guard hard fescue, Gladiator hard fescue, Blue Mesa sheep fescue, Quatro sheep fescue, Morocco Kentucky bluegrass, Premium perennial ryegrass and Replicator tetraploid perennial ryegrass than for Bison buffalograss, which had the highest value (Table 3). After 28 days of non-stress conditions, hard fescue and sheep fescue entries along with Morocco Kentucky bluegrass and Replicator tetraploid perennial ryegrass were ranked significantly lower than Compass II Chewings fescue, the entry with the most green tissue.
For Run No. 2, after 49 days of heat, Bad River blue grama had the highest percentage of green tissue, and Nanook hard fescue, Soil Guard hard fescue, Blue Mesa sheep fescue, J-248 sheep fescue, Quatro sheep fescue, Premium perennial ryegrass and Replicator tetraploid perennial ryegrass were ranked significantly lower. At the end of the recovery period, only Nanook hard fescue, Soil Guard hard fescue, Blue Mesa sheep fescue and J-248 sheep fescue remained significantly lower than Thunderstruck tall fescue, which had the highest value.
Good performance during prolonged heat
Kentucky bluegrasses and tall fescues are cool-season turfgrasses known to be tolerant to heat (7), and, as expected, the Kentucky bluegrass and tall fescue entries in our study were tolerant to the long period of heat stress.
Interestingly, most of the perennial ryegrass cultivars, except for Premium, tolerated the heat treatment. Traditionally, perennial ryegrass cultivars have been considered sensitive to long periods of heat (7), but decades of development and breeding may have contributed to the higher heat tolerance observed for certain perennial ryegrass entries in our study.
Potential entries for heat tolerance
A 1910 report from Oakley (5) mentioned that Canada bluegrass can withstand heat and drought stress better than Kentucky bluegrass, but this species has been overlooked since then. In our study, Talon Canada bluegrass performed well during the two runs. Canada bluegrass can grow in soil conditions that are unfavorable for Kentucky bluegrass (poor soil fertility) and establish faster. This species could be a great alternative to Kentucky bluegrass in urban green spaces — in particular for lawn or roadside use — but cultivar improvement efforts focused on turfgrass quality are needed.
Alkaligrasses are known to be salt-resistant, and in our study, the alkaligrass entries were slightly affected by the long period of heat. The germination of the cultivars was, however, poor, and previous research in Minnesota has demonstrated that this species is not persistent in low-maintenance environments (3).
Smooth brome is generally used as a forage or for soil conservation. This species is well known for its drought tolerance because, like tall fescue, it can develop long and deep root systems (1). Few studies have been done on the heat tolerance of smooth brome cultivars, and the only smooth brome entry tested in our experiment, BAR BIF 1GRL, displayed relatively good heat tolerance. However, this entry possessed light green leaves, which would not fit well aesthetically in a turfgrass stand.
Fine fescue performance under heat stress
The fine fescue species and cultivars tested in our heat-stress experiment produced different results than those in previously published observations. Overall, the red fescue group (Chewings, slender creeping and strong creeping) showed good tolerance to a long period of heat in both runs. Interestingly, hard fescues and sheep fescues were the most sensitive to heat treatment in both of our runs. Our results are quite different from those of a previous fine fescue study on heat tolerance (6), in which hard fescue displayed high heat tolerance.
Editor’s note: The five fine fescues common in turf systems have key differences. Clarified taxonomic classification can help turf managers better capitalize on each grass’s strengths.
These discrepancies could have arisen from the elevated temperature (~95 F/35 C) measured in the pots. Roots are known to be more sensitive to elevated temperature (8). Also, the soil used in our experiment consisted of a 50/50 field soil and sand ratio with a high amount of clay, whereas the previous studies used autoclaved sand. Hard fescue and sheep fescue are known not to perform well in poorly drained clay soil. We chose this type of growing medium because we wanted to mimic soil that could be present in urban areas. Taken together, this result demonstrates that further research on heat stress should take into account the growth medium used, especially during screening for applied research.
In summary, after subjecting 34 entries to an intense heat stress (95 F for 49 days) followed by a 28-day recovery period, we found variation in heat-stress responses within and among species. These findings will further help turfgrass managers choose appropriate cultivars for sites where heat stress is a concern.
We would like to thank the Minnesota Department of Transportation, through the Local Road Research Board, for funding this research. This research was originally published in Crop Science (https://doi.org/10.1002/csc2.20262).
The research says ...
- This study examined the heat-stress tolerance of 14 species (34 cultivars/accessions) of cool- and warm-season grasses based on NDVI and percent green cover.
- The heat-stress test consisted of 49 days in a growth chamber at 95 F and 70% humidity.
- Kentucky bluegrasses, tall fescues and most of the perennial ryegrass cultivars were tolerant to heat stress, and, overall, Chewings and slender and strong red fescues showed good heat-stress tolerance.
- Future research on heat stress should consider the growth medium used, because hard fescue and sheep fescue do not perform well in poorly drained clay soil.
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Florence Sessoms is a scientist and Eric Watkins is a professor in the Department of Horticultural Science at the University of Minnesota, St. Paul, Minn.