The transition zone is normally defined as the climatic zone where the winters are usually too cold for the warm season grasses and the summers are usually too hot for the cool season grasses. That is an area in the United States that roughly corresponds to an area on both sides of a line drawn from about Kansas City to Washington, D.C.
We know that there is very little that can be done to better adapt the warm season grasses such as Bermuda and St. Augustine grass to the colder areas of the country. However, we are finding that it is possible to successfully grow some of the cool season grasses such as Kentucky bluegrass and creeping bentgrass further south than previously thought possible.
From a physiological point of view, one of the main differences between the cool season grasses and the warm season grasses, is that the warm season grasses are better able to carry on photosynthesis at higher temperatures. On the other hand, cool season grasses tend to reduce their photosynthetic rate as the temperatures increase. At the same time their rate of respiration increases. That means they are using more food, but they are producing less. During sustained heat spells the cool season turf can use up more stored energy than it can replace. The plant is literally starving to death. When this happens, the plant becomes weakened and it can easily succumb to one or more of the numerous stress factors that plague it during the long hot summer.
The rule of thumb among turf managers is to avoid feeding cool season turf during sustained periods of high temperatures. Of course, they are trying to avoid such things as salt damage from fertilizer, flush growth that would further weaken the plant, and diseases that may attack any lush turf that might be produced from the flush growth. However, even if the turf managers are successful in their efforts, they still stand a very real chance of losing turf due to what amounts to starvation.
Many turf managers have found that Nature Safe allows them to grow much better cool season turf in areas that suffer from sustained heat. Some of this success is due to such things as the lowered salt index in Nature Safe, the lack of flush growth after applying Nature Safe, superior root systems, and the suppression of pathogens like fungus diseases and nematodes. There is one usually overlooked factor that helps the cool season turf survive when it is fertilized regularly with Nature Safe – the extra carbon dioxide that a healthy population of soil microbes produce as a by-product of their activity. With this extra carbon dioxide the cool season turf can carry on more photosynthesis than would otherwise be possible. This is the same mechanism that helps semi-shaded turf to prosper in areas where it could not otherwise survive. Even though the plant may be unable to carry on photosynthesis at high efficiency levels, it may be able to produce just enough extra food to make it over the hump. The extra food produced from the CO2 can mean the difference between life and death for the plant. That is one of the main reasons that Nature Safe fertilized turf survives some of the hottest summers in the North as well as in the transition zone while neighboring turf that is being maintained under different regimes frequently suffers.