Roughly 7,500 grass species are grown around the world, but only 14 species are adapted as turfgrasses that have been used extensively. Kentucky is situated in the transitional climatic zone of the United States, the middle point between the cool north and the warm south, with warm summers and cool winters. Because of its unusual climate, no single grass is suitable for all situations and locations. The majority of the turfgrasses that are appropriate for use in Kentucky are known as C3 grasses, or cool-season grasses. Cool-season grasses differ from warm-season grasses (C4) in many ways, but most notably in their photosynthetic pathways. Warm-season grasses can tolerate and even thrive during the warm summers while cool-season grasses may become heat-stressed. Conversely, winters in Kentucky may be too cool for warm-season grasses and greenup in the spring may be long and arduous. Warm-season grasses enter a dormancy period during the fall and winter and may stay in this state as long as six or seven months.