Karl Foerster is cool-season grass. Cool-season grasses put on most of their growth in spring before temperatures begin exceeding 75 degrees Fahrenheit and in the fall when temperatures cool down. They generally maintain good color through the summer but won't grow much when it is hot.
Cut back cool season grasses in very early spring.

Cool season grasses tend to look good even as the weather cools. Leave their foliage in place until spring and then as soon as the snow is gone cut them back. Leave about 1/3 of the plant in place. Trimming cool season grasses too harshly can irreparably harm the plant.

Divide cool season grasses in spring or early fall. Cool season grasses are actively growing in spring and fall. These grasses can be transplanted at either time of the year but early spring is probably the best time to divide. If you do divide them in the fall, be careful that the freeze/thaw cycles of winter don't heave the plants out of the ground, this happened to a couple of my coral bells last winter.

Cut clumps of grass to the ground in late winter before new shoots appear.