Why Grasses Vary from Green to Brown

Hot and dry conditions occurring throughout Wisconsin this summer have put considerable amounts of stress on plants. Grasses, for example, currently range in appearance from brown and looking dead to green and flourishing, with assorted shades of each color in between. This clearly illustrates various grass species react differently to the heat of summer.

Summer stress aside, diverse kinds of grasses naturally vary in visible appearance, including height, growth habit, leaf color and thickness, and development of flowers and seedheads. However, one fundamental internal difference among grasses relates to temperatures and the ability to efficiently go through photosynthesis, the process of producing food for growth. Every species of grass is either cool-season or warm-season, based on preferred temperatures for photosynthesis during the growing season.

Cool-season grasses, as the name implies, prefer cooler weather in spring and fall for maximum food production and growth. In the heat of summer with temperatures consistently in the mid-eighties and into the ninety degree range, they will naturally slow down to the point they may go dormant. Common Wisconsin lawn grasses are cool-season, including Kentucky bluegrass, perennial ryegrass, and fine fescues.

Within a lawn you may often notice variations in color among these cool-season grasses in summer. This may be due to different cultivars of the same species reacting differently to stress, or growing condition variables. For example, cooler shade areas may look greener than full sun.

Conversely, warm-season grasses find temperatures in the 80 degree range ideal and can produce food very efficiently. The only significant warm-season lawn grass we see in Wisconsin is zoysiagrass, which is extremely aggressive in the heat of summer but dormant for much of spring and fall. As a result, it is not widely used here.

There are, however, two additional significant groups of warm-season grasses that are very commonly found throughout the state, most notably the native ornamental grasses, including popular species such as little bluestem, big bluestem, switchgrass, Indiangrass, prairie dropseed, and side oats grama. They are warm-season yet some of these are native to prairies extending into Canada, which points out the difference between growing season preference and hardiness. Cold hardiness refers to tolerating winter cold, whereas warm-season refers to ideal temperatures for active growth.

Finally, the most notable group on the negative side are warm-season annual grasses that typically become weed issues in lawns and gardens. Crabgrass leads the list, along with foxtails, barnyardgrass, and goosegrass. As warm-season grasses, these species flourish in the heat, outcompeting cool-season lawn grasses and overgrowing vegetable and flower plantings. As annuals, the good news is they will die off once fall arrives.