Prune geraniums that overwinter at the end of their winter dormancy and before the new growing season begins in early spring. Plants which grow year-round are best pruned immediately after their growing season in late fall or early winter. Remove dead and unhealthy branches first. The health of a branch can be determined by its color and its hardiness.
To encourage optimal growth, prune back anywhere from one-third to one-half of the plant, focusing on wild and unsightly growths and branches which interfere with sunlight or the air circulation within the plant. Remove all diseased branches immediately and sterilize shears afterwards to prevent spreading the sickness. Sharpen shears as necessary to make clean cuts which heal quickly.
In addition to pruning, use pinching on geraniums to best grow a neat and compact plant with lots of flowers. After a branch has grown a few inches, pinch off a small amount at the end of the branch. Pinching encourages the plant to grow new stems off of the pinched branch rather than a longer branch. Many short branches better create a full but compact bush. Pinching for shorter branches also encourages the growth of more flowers, leaving less bare space on branches.