By mid to late August most summer blooming shrubs like hydrangeas are finished blooming. Now is a great time to cut back the spent blooms. Any other flowering shrubs that have slowed down, like spirea, weigela, butterfly bush, and crepe myrtle can also be trimmed back about one third of their size.
Trimming to a third of the original plant size encourages healthy new growth. If you’re wanting to control height, you can prune as much as one half the size of your plant. You can also do any major pruning in mid to late winter when the plants are dormant, without leaves and it’s easier to work on them.
Early spring bloomers like forsythia and lilac should be pruned after they bloom. So do not trim these back now as you might remove their spring flowers.
Knock-Out roses can be pruned almost any time of the year. Sometimes it’s easier to tackle these thorny big boys during the winter months when their foliage is gone. Cooler weather also makes this big job more pleasant.
Stop feeding any summer flowering shrubs by late August. Plant growth needs to slow down preparing the shrubs for winter dormancy. Be sure to keep your plants watered through the fall, especially if they are newer plantings.