Knowing when your shrubs bloom is guide to when they should be pruned.
Proper pruning can encourage more blooms, fuller plants, and healthier plants, said a University of Illinois Extension horticulture educator.
"Spring blooming shrubs such as forsythia, lilac, and spireas should be pruned within two weeks after they have finished blooming," said James Schuster. "Avoid pruning spring blooming shrubs in the summer, fall, or winter if you want maximum flowers the following year.
"Spring blooming shrubs produce their flower buds for next year on the new shoots that are grown this year. The shrubs need most of the summer and fall to produce mature flower buds that are capable of blooming."
Summer flowering shrubs such as hibiscus and most hydrangeas produce flower buds on the new growth produced each spring. Therefore, summer flowering shrubs are pruned during the winter or very early spring before new growth appears and not in the summer or fall.
"Multi-stem shrubs can be renewed which is the thinning/removal of the largest stems in the plant," said Schuster. "The standard thinning is one-fourth of the oldest and largest diameter stems. They should be cut off between one and three inches above the ground. Cutting the stems this close to the ground helps minimize disease and insect problems that could be fatal to the shrub.
"In addition, the new growth is grown off the roots and not the stumps. This helps maintain a healthier and fuller shrub."
Some of the shrubs that respond well to renewal pruning are lilacs, mock orange, weigela, forsythia, red and yellow twig dogwoods, arrow wood viburnum, and St. Johnswort.
"A lot of the horticulture literature suggests you cut the stems off at four to six inches," he noted. "This reduces the plants' chances of sending up shoots from the roots. As a result, the plants stay narrower but then they are also more disease and insect prone."
Another pruning method is rejuvenation. This pruning should only be done every five to eight years because it is very stressful to the plant. This method removes all of the shrub's stems to a one to three-inch height.
"Because of the severe pruning it should only be done on plants while they are still dormant," he said. "Plants that respond best to this pruning method are the small, multi-stem shrubs such as an Anthony Waterer spirea.
"There is one exception--never prune a potentilla with this method unless you modify the pruning height. Potentillas need two buds per stem for survival of that stem. Go upward from the ground and cut the stem off about one-fourth of an inch above the second bud."