In the Northern Great Plains area, prescribed burns and wildfires usually occur in late winter, spring, late summer, or fall. In the winter, due to the ice and snow, the moisture content in the vegetation is too high and the temperatures are too low. After "green-up" in early May, the grasses and other herbaceous plants sprout. These live plants have a very high moisture content and inhibit the spread of fire, making prescribed fires in the spring and early summer difficult or impossible. If late summer brings hot, dry weather, prescribed fires could be possible in prairie areas in August and September. Fall burns can be done after the grasses are frost-killed and dried. This fall "window" usually begins in late September or early October and lasts 2-8 weeks until the onset of winter precludes any fires until the weather warms up again in the late winter or early spring.