Predicting Turf Performance
Imagine that you had a supernatural gift, one that would allow you to predict how any given turf type or management practice would perform at your location. How much easier would it be to manage turf? To make decisions on new varieties? To explain to managers and golfers the reasons behind unpopular, but necessary practices such as frost delays, aerification, leaching or traffic management?
Fortunately, the ability to make these predictions does not require supernatural powers, a crystal ball or even a pact with the devil. Instead, it depends upon two related factors that you already incorporate into much of your planning and decision-making: the weather and the effect that weather has on turfgrass stress.
Harnessing the weather to work for you, not against you
Several years ago, we developed a concept that we called “turf-growth potential” to explain myriad of ways in which weather impacts turf growth. This model can be used to help educate golfers and grounds managers, to provide a scientific basis for decisions, and to predict the performance of different turf types when exposed to different climates and different overseeding and transition practices.
Warm vs. cool-season turf: Vive la différence!
One of the most important differences between cool- and warm-season turf varieties is that cool-season turf grows best between average air temperatures of 60° and 75°F (with optimum growth at about 68°F), while warm-season turf grows best at average air temperatures between 80° and 95°F (with optimum growth at about 88°F).
The contrasting temperature requirements for cool- vs. warm-season turf are illustrated in Figure 1, at left. Note that for cool-season turf, growth reaches 100 percent of the plant's growth potential (GP) when average air temperatures are 68°F. When it is cooler than 68°F, the GP decreases. Similarly, when it is warmer than 68°F, the GP decreases as well. Warm-season turf growth has a similar shaped curve, but its optimum temperature for growth is 20 degrees higher, at 88°F. When it is either cooler than 88°F or warmer, GP decreases.
According to this model, turf is healthiest and happiest when growth potential is on the rise — during the spring and summer as temperatures are warming up and growth potentials increase to 50 percent and higher. Under these conditions, turf is growing so actively that it is able to stave off stressors such as mild diseases, aggressive aerifications or heavy traffic. For this reason, this is usually the best time to schedule stressful practices such as topdressing, aerifications or even tournaments.
But a strange thing happens as growth potentials start to decline from their high point of 100 percent. This is the time of year when weather grows less and less favorable for turf growth, and when the turf's ability to recover from stress grows weaker and weaker. This is the time of year, then, when stressful conditions should be avoided, if at all possible.
Putting the turf growth model to work
Take a look at the figures at left and on page 15, which show the GP graphs for several locations around the United States. The temperatures used to generate these graphs are the average monthly air temperatures for each location, averaged over the last 30 years.
Brrr!!: In Figure 2 (at left), note that temperatures are warm enough for good growth of cool-season turf (greater than 50 percent GP) from about mid-April through October for Worcester, Mass. Before and after this time period, temperatures are very cool, making it more difficult for cool-season turf to grow. At average air temperatures below 38°F, cool-season turf growth basically stops. Warm-season turf, on the other hand, barely reaches 20 percent GP, even in the warmest month (July). Common sense already probably told you that bermudagrass is a poor choice for a Massachusetts location, but now these GP graphs and the Turf Growth Model back it up.
What happens on the growth potential downslope: In cool climates like those in Massachusetts, temperatures begin to rapidly cool in September. As a result, growth potentials take a sharp drop, causing heavy stress to turf.
One of the practical implications of this drop relates to timing fungicide applications for snow mold prevention, always a tricky decision. Treat too early in the fall, and the fungicidal activity will be gone by the time snow mold fungi hit. Treat too late, and the turf will be dormant and unable to take up systemic fungicides. The climate history in Worcester, Mass., as shown in Figure 2's growth potential graph (page 14), indicates that in most years, you should be ready to treat well before November 1. After that date, cool-season turf growth has ground to a halt, and systemic fungicides will not have the desired effect.
It's too darn hot!: The reverse situation is seen in Figure 3 (page 14) for Guam, where it is too hot year-round for vigorous (greater than 50 percent GP) growth of cool-season turf. Instead, in this tropical location, where the warm temperatures do not fluctuate very much from month to month, warm-season turf dominates the entire year. Can cool-season turf be grown in this location? Most definitely. But compared to the vigorous growth of warm-season turf, it will be stressed by high temperatures throughout the year.
The ideal overseeding environment: does it exist? Yes it does, but in fewer locations than you might expect. In the United States, good overseeding conditions are restricted to the desert southwest: Palm Springs, Calif., or Phoenix, Ariz., for example. These areas are characterized by the following:
There are about six months each year in which warm-season turf clearly dominates cool-season turf (May through October). During these months, it is way too hot for cool-season turf growth, and it usually dies or slows down its growth significantly.
There are about six months each year when cool-season turf clearly dominates (November through April). During these months, it is too cool for warm-season turf (it goes dormant), but it is perfect weather for growth of rye, poa, bent and other cool-season overseeded varieties.
And then, there's the rest of us. Most locations fall somewhere in between the three scenarios described above. Take a look at Figure 4 for Richmond, Va. (below). In this location, there are some golf courses that cultivate only cool-season turf, while others cultivate both warm and cool-season turf. There are also some courses that overseed, and others that do not. Based on the GP graph for Richmond in Figure 4, we can expect the following turf performance:
Cool-season turf will dominate warm-season turf for the entire year and, for this reason, is an appealing choice for many turf managers. However, there is a hidden poison pill, because the cool-season turf is heavily stressed by hot temperatures during the summer. The decline in GP that takes place during the summer months is a dangerous time for cool-season turf. It's here that anthracnose, gray leaf spot, grub damage, weed invasion, heavy golf play or aggressive management practices can really take their toll.
Warm-season turf barely makes it over 50 percent GP, and when it does, it is only for one or two months. This turf will perform adequately during the summer months, but it will be dormant (with all of the problems that dormancy engenders) for the majority of the year. Though overseeding might be tempting in the Richmond climate, it will be difficult to accomplish successful spring or fall transitions, and both the cool- and warm-season turf will struggle as they compete with each other. Mixtures of weakened stands of both turf types are frequently the end result of overseeding in environment such as these.
Where does your location fit in? Do any of the scenarios painted above describe your situation? Or do you fall somewhere in between? To get a better idea of what goes on nationwide, as well as at your own location, go to the PACE Turfgrass Research Institute Web site at www.paceturf.org and click on “Turf Guidelines.”
Practical uses for the growth potential model
Educate golfers and management: Use the information above to educate golfers, management and clients on the following topics.
Why cool- or warm-season turf performs the way it does at your location.
The impact of tournament scheduling on turf performance. Ideally, stressful events such as tournaments should be scheduled when growth potential is on the increase (as it is moving from 50 percent up to 100 percent). And tournaments should be avoided at those times when turf will be most stressed and least likely to recover (that is, when growth potential is decreasing from its high point of near 100 percent downwards).
Why it is a good idea (or not a good idea) to overseed at your location.
Provide decision support for:
Variety selection. If you are considering new plantings, use the growth potential model to see whether warm- or cool-season turf varieties are optimal.
Overseed date. Target the time at your location when warm season turf is <20-percent growth potential and cool-season turf is >50-percent growth potential, for overseeding.
Timing of chemical transition product application. Products such as Revolver and Monument can be useful tools in accelerating the spring transition and improving warm-season turf stands, but timing is critical. We see the best results when you make applications when warm-season turf reaches a growth potential of 50 percent.
Interpret current turf performance: You can also use growth potential to predict how current conditions are going to affect turf performance. To do this, use the “Turf growth potential at different average temperatures” chart (available under “Turf Guidelines” on the PACE Web site at www.paceturf.org). Simply look up the growth potential value that corresponds to the current or forecasted temperatures for your location.
If GP for either turf type is greater than 50 percent, then good growth should be expected.
If the GP is going to decline over the next few days, be prepared for some degree of turf stress — even if the overall GP remains above 50 percent.
If you are maintaining overseeded turf, compare the GP values for warm- and cool-season turf varieties to predict how they are going to compete (or not compete) with one another over the next several days.
Wendy Gelernter, Ph.D., and Larry Stowell, Ph.D., are the research directors for the PACE Turfgrass Research Institute (San Diego, Calif.).
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