Grass (Gramineae) forms a living carpet across the earth's treeless surfaces and is vulnerable to all types of weather, depending upon its location. Grass is completely dependent upon sunlight for photosynthesis, which is the way it manufactures its own food, and on rainfall for the moisture it needs. Adequate sunlight and moisture are sometimes not enough, however, to protect grass against weather extremes such as severe cold or drought.
Grass growth depends on a combination of factors, including adequate sunlight and warmth. Under normal conditions, it generally gets enough moisture to combat heat extremes. However, moisture deprivation during times of extreme heat and drought is lethal to grass. Moisture not only transports nutrients from the soil to the grass plants, it also keeps them hydrated. Grass leaves, or blades, release the water they don't use into the atmosphere during a continual cycle of absorption and transpiration. If the water supply is cut off or extreme heat causes the available moisture to evaporate too rapidly, the plants literally dry up, the leaves die back, and photosynthesis, or food production, stops.
Grass subjected to extreme winter weather, including severe freezes, may suffer from a condition known as dessication. Also called winterkill, this condition arises in late winter where grass isn't protected by snow cover and is exposed to high winds. Dessication is most often seen in grass located at higher elevations where there is less snow to act as insulation. The soil surface may thaw, but at deeper levels, roots are trapped and cannot absorb the water they need, killing the grass plants.
Excessive Moisture and Disease
The list of diseases that affect grass is long, and many of them are exacerbated by weather conditions that keep it too wet and cool, which provides the ideal environment for bacteria and fungi to thrive in. Fusarium blight, which affects some types of Kentucky bluegrass, is most often seen during hot weather in grass plants deprived of moisture. The disease manifests itself as patches in the lawn that go from light green to reddish-brown to straw-colored before dying. Too much shade may promote powdery mildew, which is characterized by a white powder on grass blades. Snowmold develops under the snow and forms a crust of leaves covered in white or gray mold with small sunken black spots, called sclerotia, in the leaves.
Maintaining healthy grass means being vigilant about all factors, including the weather, that can affect it. Many extreme weather effects aren't immediately noticeable, but there are steps you can take to minimize their occurrence. Regular fertilization keeps grass plants strong and robust and increases their resistance to weather effects. Planting grass suited to the area also helps, as some varieties are more tolerant of heat, cold, shade or drought than others. Being ready to supply additional moisture during dry periods is crucial, as is knowing when grass needs help beyond regular maintenance, such as applying fungicides to combat diseases. Proper mowing, which includes removing no more than one-fourth to one-third of the plants' height, is also crucial to its health and ability to withstand weather stress.