When should I prune forsythia?
Forsythias are shrubs that flower on old wood, i.e.on shoots produced in the previous growing season. They should be pruned immediately after they have bloomed so that there will be time for new wood and buds to develop before the winter.
Forsythias will produce more flowers if this pruning is done every year. A portion of the shoots can also be pruned to within a few inches of the ground to encourage new growth. These procedures will result in a compact and shapely bush.
Courtesy of NYBG Plant Information Service