Some like it hot

So you want a grass that can take the summer heat. Who doesn't? That's arguably the toughest time for turf, and if you can find a turfgrass that thrives in the summer sun, you know you've got a winner. But what does it mean to be heat tolerant? What characteristics impart this quality? It's not always easy to define. But understanding what it means is the key to choosing a turfgrass variety that won't wither under the summer sun.

Defining heat tolerance

Turfgrasses that survive summer heat are able to do so for several reasons. One of the most well-understood factors is the physiological difference between cool- and warm-season grasses. The warm-season grasses — also known as C-4 grasses — have evolved a slightly different method of photosynthesizing that functions better in warmer temperatures. That means that even at the peak of summer heat, they can continue to produce the food that allows good growth and vigor.

One of the reasons some cool-season turfgrasses decline in the summer is because they lack this ability. As a result of lower food reserves in the plant, root systems decline at the time of the year when grass plants can least afford it.

Warm-season grasses include bermudagrasses, zoysiagrasses, St. Augustinegrass and several other species. Most warm-season species are so heat-tolerant that, according to Dr. Arden Baltensperger of Seeds West Inc., turf breeders don't really pay too much attention to that attribute. “They've got enough already,” he concludes.

Instead, warm-season turf breeders turn their attentions to other areas of improvement such as greater turf density, finer texture and increased shade and cold tolerance. Breeders are even working on drought tolerance in the warm-season species, explains Baltensperger. That might seem surprising to some, but it's important to understand that heat tolerance, though associated with summer injury from heat, is not the same thing as drought tolerance.

As Baltensperger explains, warm-season species and varieties within species vary greatly in drought tolerance. Seeds West has developed bermudagrass varieties, such as Princess-77, that stay greener under moisture stress. This reduced dormancy is true drought tolerance.

Likewise, studies at the University of Arizona have shown that the variety Yukon (OKS 91-11) can remain green with 25 percent less water than many hybrid bermudagrasses.

The almost opposite attribute in buffalograss and bermudagrass is to go dormant under summer drought conditions and then recover when moisture is available. This is more drought avoidance than drought tolerance. While you might not think of summer dormancy as a desirable trait, it is, according to Baltensperger, now an option in water-restrictive areas such as Southern California. But if you want green bermudagrass through the summer, you'll have to water it, just like you do a cool-season grass.

Cool-season, or C-3, grasses photosynthesize best at lower temperatures. That's why they thrive in spring and fall. Cool-season species include Kentucky bluegrass, ryegrasses, tall fescues and bentgrasses, among others.

Among these cool-season grasses, tall fescue is widely regarded as the most heat-tolerant species. Kentucky bluegrass and perennial ryegrass historically have not been strong performers in the heat, though breeders have made substantial improvements. And creeping bentgrass — definitely a northern species — has been the subject of intense breeding efforts that now are yielding good performance on golf greens virtually everywhere in the United States.

Dr. Melodee Fraser, a turf breeder with Pure Seed Testing Inc., explains that turf's ability to cool itself relies on the cooling of water as it evaporates from the grass blades. If water runs short, turf loses its ability to cool itself and is less able to tolerate hot weather.

As this implies, several factors can affect heat tolerance. For example, much of tall fescue's summer performance is due to its deep root system, which supplies greater amounts of water to the shoots. Thus, heat tolerance in this case is due to the roots, not an ability of the plant's tissue to function at higher temperatures or with less available water.

In fact, some turfgrasses that use more water may actually have better heat tolerance, if adequate water is available.

Going where the action is

Fraser works at Pure Seed Testing's North Carolina facility, where turfgrasses are exposed to the heat and humidity characteristic of the East in an effort to identify the good hot-weather performers. By contrast, Pure Seed's home base, in northern Oregon, experiences relatively mild weather that isn't always enough to separate the strong performers from those that would succumb to the heat.

Fraser screens several turfgrass species in North Carolina, including tall fescue as well as Kentucky bluegrass, perennial rye and creeping bent. However, tall fescue seems to have the best potential for heat tolerance, according to Fraser

The most heat-tolerant tall fescues that Turf Seed Inc. markets — Endeavor, Tarheel and Wolfpack — were developed there. But Fraser notes that she also has seen some Kentucky bluegrasses do well there, as well as the A and G bentgrasses, marketed by Tee-2-Green.

Like Pure Seed, many turf breeders have facilities around the country where they test their varieties for a range of environmental tolerances. Cebeco/International Seed's Dr. Steve Johnson explains that this is important because “field screening” allows you to select for real-world performance involving multiple factors. Johnson also emphasizes that collections of persistent plants in various climates around the country are a key source of breeding material.

Disease is one of the most significant of the real-world factors related to heat tolerance. Often, what brings down turf in sultry weather is disease, which may be favored by summer conditions. If you find a variety with good disease resistance, you may, in effect, have found your heat-tolerant grass.

It's also why a grass selected for heat tolerance in Arizona, for example, might not be a good performer in South Carolina heat, where the humidity is considerably higher.

Dr. Leah Brilman, a breeder with Seed Research, along with Advanta Seeds' Ken Hignight, works with Dr. Ronnie Duncan of the University of Georgia in what Brilman terms “Duncan's torture trials.” Duncan puts turf plots through, well, torture, and this has helped determine some of the tougher grasses for use in the South. Seed Research is introducing two tall fescues — Tulsa II and Regiment II — that came from Advanta and Duncan's program. These varieties endure greater heat and drought before “leaf firing,” or browning. Other varieties are being developed that also have been screened there.

Dr. Jerry Pepin of Pickseed states that “summer stress tolerance” might be a more apt description than simple “heat tolerance.” Drought, heat, disease and humidity all contribute to the conditions that turfgrasses must tolerate in some areas. “Good persistence in miserable conditions is what you're after,” he explains. As an example, Pepin cites some dwarf tall fescues that do fine in the hot, dry Southwest, but won't make it in the Southeast. Humidity and disease are the biggest reasons.

A similar situation exists with perennial ryegrass. A bane of perennial-ryegrass managers lately has been gray leaf spot, which is brought on by heat and humidity. In one sense, then, heat tolerance in perennial ryegrass has become a matter of disease tolerance. Brilman explains that breeders have been pushing perennial ryes farther south, but gray leaf spot has slowed things down somewhat. “Until they get a handle on gray leaf spot, heat tolerance is a moot point,” says Brilman.

The gray leaf-spot problem has gotten the attention of other breeders, too. Dr. Douglas Brede of Jacklin Seed says that Jacklin has developed the perennial rye variety ASAP for improved gray leaf-spot resistance. Though Brede concedes that there's still room for more development, “On a scale of 1 to 9, we're at a 6 or 7, but that's a significant improvement” over what's been available to this point, he explains.

Another issue with perennial ryegrass is the need for less heat tolerance for overseeding types. Perennial rye is an important winter overseeding species, which means that less heat tolerance helps it transition out more easily in the spring. However, varieties that provide the best quality in trials are those that tend to be most persistent. The result is that the available perennial ryes with the best quality are difficult to transition out.

Why do we need more heat tolerance?

The cool-season grasses are being pushed farther and farther south, well into the range of warm-season species. But turf breeders aren't forcing this on anyone. Demand for the appearance and year-round color that cool-season turf offers is strong. Plus, even northern areas have their hot spells. It's hard to see how you could ever have too much heat tolerance. And breeders are working hard to satisfy that demand.

All the turf breeders I interviewed for this article felt there is plenty of genetic potential to increase heat tolerance in cool-season species. Cebeco's Johnson notes that he is working with tall fescue material that even succeeds even in sandy, Central Florida soils.

Pickseed's Pepin agrees, asserting that even though breeding improvements currently are occurring in small steps rather than quantum leaps, there is still significant progress to be made. This includes species not traditionally considered strong hot-weather performers, such as Kentucky bluegrass. For example, Pickseed's Langara is a variety that shows “exceptional” summer performance, according to Pepin. Brilman cites Apollo, Showcase and SR 2284 as other strong summer Kentucky bluegrasses.

Jacklin's Brede has noted an interesting dichotomy in Kentucky bluegrass. He feels that Kentucky bluegrasses “either have [heat tolerance] or they don't.” Even from the same cross, some offspring will have good heat resistance and some won't. As examples, Brede cites Freedom II, good in hot, humid summer weather; and Chicago II, also a good performer, but in cooler areas. Both possess the same parentage.

Brilman feels that Kentucky bluegrass has been more of a breeding challenge than other species, such as perennial rye or tall fescue. “Once we get to a certain point moving south, the Kentucky bluegrasses that do best are what we call ‘Mid-Atlantic’ types. They have very deep rhizomes.” Unfortunately, “It's difficult to get a good seed yield from these types. Do we want better Kentucky bluegrasses? Yes. But do we have them? Sort of,” says Brilman. “We're still trying.”

Breeders are also working on other qualities related to heat tolerance. Brilman explains that Brighton creeping bent is descended from the popular SR 1020 and was developed to combine the heat tolerance of SR 1020 with other qualities that superintendents seek: especially, in this case, color.

Another example Brilman brings up is the need for salinity tolerance. Many golf courses now use effluent for turf irrigation, which exhibits high salt levels. Many of these courses are in arid, hot parts of the United States. If a turfgrass is to thrive in those conditions, it must not only be heat tolerant, but also salt tolerant.

Identifying good varieties (NTEP)

The fact that breeders select the strongest performers from real-world settings is instructive. When you're looking for a strong performer in your location, you should do essentially the same thing. Universities frequently maintain test plots that you can tour, and Cooperative Extension offices often publish turf trial bulletins listing performance of turf varieties in your area.

Another excellent resource is the National Turfgrass Evaluation Program (NTEP). This organization is the primary variety testing service in the United States and makes many of its reports available to the public for no fee (on the internet at

Of great value is the fact that NTEP reports list results from trial locations around the country. Although varieties are often touted as topping the list in overall rankings, the most important criterion is how well a variety does in your location, or at least one similar to yours. And if you need a heat-tolerant variety, look at the summer performance of the variety.

It is also important to focus on trial data where the turf has been maintained in a manner similar to your cultural practices. Brilman notes that heat tolerance at a 3-inch mowing height may be very different than at a 0.5-inch cutting height.

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© 2020 Penton Media Inc.

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