Answer: Forsythia should be pruned in the spring right after it blooms. The plants are naturally very large and a bit ungangly and unfortunately pruning can't really control that. The best you can do is to remove some of the biggest, oldest stems by cutting them off at the base and then tip back some of the wildest branches.
An old, overgrown bush can be cut off at the ground all at once or can be renewed over a period of about three years. To do this, remove about a third of the oldest and largest branches at the base in year one, in year two take away half of the remaining old stems, and in year three remove the rest of the oldest stems.
Be sure to do your pruning in the spring right after it blooms. The shrub blooms only on old wood, so pruning later in the season will remove next spring's blooms.
Q&A Library Searching Tips