Collards grow best when temperatures are between 60 and 65 degrees Fahrenheit, and will bolt or become rough in hot weather. Summer planted collards tolerate light to medium frosts, allowing gardeners to continue to grow them well into fall in most regions. Young collard plants grown in the spring are less tolerant of frosts.
Fall collard greens allow gardeners to extend the growing season for two or three months in all but the coldest climates, and collards actually taste sweeter after a few light frosts. Fall-planted collards also suffer fewer diseases and pests and generally don't have problems with bolting due to hot weather.
Although collards are naturally hardy vegetables that tolerate all but the heaviest frosts, you can extend the season by providing some frost protection. The simplest device is floating row covers -- lightweight agricultural fabric that lays directly over collards and provides protection from medium frosts. Floating row covers do not protect collards from the weight of heavy snows or heavy frosts. Hoop tunnels or cold frames offer more substantial frost protection and in most climates gardeners can grow collards through most of the winter.
Plant collard greens in the spring as soon as the soil is soft enough to work and mid-summer -- six weeks before the first expected frost. Mulch plants with straw or untreated grass clippings to reduce weed growth, conserve water and provide some protection from cold temperatures. Provide more significant cold protection if you wish to prolong the growing season past late fall.