It is important to understand that the differences between warm-season and cool-season grasses does not just relate to the fact that warm-season grasses are best adapted to warmer areas than cool-season grasses. The easiest way to discriminate between the cool and warm-season grasses is to classify them by their photosynthetic efficiency.
Warm-season grasses begin the process of carbohydrate production with a four-carbon compound, whereas the cool-season grasses use a three-carbon compound. This is why we call warm-season grasses C4 (carbon-4) grasses and cool-season grasses C3 (carbon-3) grasses (Jones, 1985).
There are also visible differences in the cell structure of warm and cool-season grasses at a microscopic level (Christians, 2004). This article will focus however on the practical difference between warm and cool-season grasses. These include:
- Rooting: Warm-season grasses usually have a deeper root system (which will survive and grow even in very hot conditions);
- Water-use: Cool-season grasses have a high water requirement than warm-season grasses. In warm weather the cool-season grasses must leave their stomates open longer than warm season grasses to capture CO2 (which results in water being lost more readily);
- Shade-tolerance: Warm-season grasses gene rally require full sunlight in order to photosynthesise, whereas some cool-season grasses require only ¼ to ½ full sunlight to photosynthesise; and
- Cold Tolerance: Warm-season grasses will go dormant or even die in cool conditions. The warm-season grasses vary in their susceptibility to the cold, so cold tolerance is an important selection factor in many situations (particularly transition zones).
Which grasses are cool or warm-season grasses?
Some cool-season grasses perform reasonably well in transition zone areas and can, with careful management, tolerate hot and humid climates. This is why we often see certain cool-season grasses confused with their warm-season counterparts. Probably the most commonly confused grass within this group is creeping bentgrass, which will perform well in warm climates but is actually a cool-season grass. The following table lists important turf species.
Megan Z. Hood B.Appl.Sci(Hons). RIPTA. Turfgrass Agronomist, STRI Ltd