Cool-season grasses are grass types that thrive in areas with cold freezing winters and hot summers. This area is roughly defined as New England, the Upper Midwest, the High Plains, and Northern California up to the Pacific Northwest. These grasses grow best when temperatures are between 65-80 degrees F, which is why they do most of their growing in the spring and fall.
The most common grasses in this category are Kentucky bluegrass, perennial ryegrass, tall fescue, and fine fescue. You'll often see these seeds blended together for different needs and uses, such as high-traffic areas or lawns with sun and shade.
The Transition Zone is an overlap area. Some warm-season grasses do well there due to the hot, dry summers, and so do some cool-season grasses because of the cold winters. Tall fescue is more heat- and drought-tolerant than the other cool-season grasses, which makes it a great choice for growing in the Transition Zone. Bermudagrass, centipedegrass, and zoysiagrass are the main warm-season grasses grown in the Transition Zone. They're drought-tolerant, and can withstand cooler temperatures better than the other warm-season grasses. However, they often go dormant and turn brown during the winter months when temperatures are consistently below 60 degrees.