Calamagrostis 'Karl Foerster' is a "cool-season grower." It sometimes starts growing very early in the season (even with snow still on the ground) but is conversely unhappy in a very hot, dry summer, when it may go semi-dormant and straw-colored to ride out a drought. In such years, however, as soon as cooler temperatures return in the fall it will throw out a new burst of green at its base, which will all become vertical stems the next season.