If you have overgrown plants that are smothering the house or taking over the landscape, now is the time to strike. Hollies, yews, viburnums, forsythia, azaleas, rhododendrons and many more take well to hard pruning. Butterfly bush should be pruned very hard, to within inches of the ground, every year.
The only plants you can’t prune severely are conifers such as junipers, cedar, pine, spruce and fir. These species do not form adventitious buds, nor do they have latent buds capable of sprouting after all other buds have been removed.
Brutal pruning to lower the height and spread of plants is best timed when the plants are dormant, meaning several weeks before the soil begins to warm. Well-established vigorous plants have extensive root systems in the ground with an abundance of reserved energy. Early pruning directs that reserved energy to the most viable vegetative buds in the stems. Thus the earlier plants are pruned hard before growth starts, the more new growth they will generate.
If you are cutting azalea stems the size of your index finger, as soon as temperatures rise you will see hundreds of green buds emerging from under the bark up and down that stem. Each of those buds is capable of producing branches. While the buds are still soft and green, wipe away at least half of them with your fingers. If you allow all of those buds to produce branches, the stems will look like a bottlebrush.
When pruning forsythia and weigela, always remove branches that have gray bark near the base of the plant. Prune as close to the ground as possible to promote new vigorous stems to emerge from buds at the soil line. Remove all stems smaller than a pencil in diameter. These weak, flimsy shoots will generally not flower and will only droop with the ends of the stems touching the ground and rooting in.
When pruning lilacs, inspect the larger stems for borer holes. Lilac borers generally attack stems one-and-a-half to two inches in diameter. Cut infected stems near the ground, and either burn or send them away with the trash. Allowing those infested stems to remain will only result in younger stems becoming infested before they approach maturity. You don’t want that because lilac flowers are produced on second-year wood.
Do not try to rejuvenate any plant whose stump is larger than two inches in diameter by cutting it back to the ground. Stumps are capable of sprouting, but the sprouts will topple when the center of the old stump, which is mostly dead tissue, begins to rot, two to three years after it has been cut.
To maintain generally well-behaved plants, prune after flowering when the petals are dropping to the ground.
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