April is a good time for pruning shrubs, but you need to do a little reconnaissance before getting out the loppers because when you prune can affect flowering.
Homegrown New Hampshire host Emma Erler, lead horticulturist with Kirkwood Gardens, says some plants produce flower buds on current season’s growth, while others bloom in the spring from buds formed the previous year. And that makes all the difference.
If you want to maximize the flower show, prune spring flowering trees and shrubs shortly after they finish flowering.
Spring flowering shrubs, those that bloom on last season’s growth, include flowering quince, forsythia, bigleaf hydrangea, star magnolia, ninebark, lilac, viburnum, and weigela.
Lilacs are the star of spring in New Hampshire as well as the state flower, and as such they have diva-level pruning needs. Lilacs must be pruned within two weeks of their flowers fading in late spring to avoid cutting off next year’s blooms.
Lilacs form new flower buds for the following spring on the current season's growth shortly after blooming ends. This immediate post-bloom 2-week period for pruning ensures you are only removing old, dead, or diseased wood before new buds set.
If you prune lilacs in summer, fall, or winter, you remove the buds for next year, which means no lilac flowers!
Plants that bloom in the summer should be pruned before growth begins in the spring because these plants develop their flower buds on the current season’s growth.
These include butterfly bush, summersweet, rose of Sharon, smooth hydrangea, panicle hydrangea and spirea.
“Although it may seem contradictory, pruning stimulates and directs new growth,” according to Erler. If you’re trying to control the size of a tree or shrub with heavy pruning, you may actually be making the problem worse, since the plant will produce lots of new, vigorous branches.
Less pruning can actually equate to slower growth. Old plants that have lost vigor may benefit from severe pruning for rejuvenation, but younger, livelier plants may become unruly.
A good rule of thumb is to remove no more than one third of a plant each year. Removing more than that amount can cause excessive stress and an undesirable amount of regrowth, ultimately weakening the tree or shrub.
The timing of pruning may also affect how a plant responds. When woody plants are pruned in the dormant season, buds are removed that would have grown in the spring. Reserves that are in the plant are then redistributed amongst the fewer remaining buds, and the developing shoots are much more vigorous.
Late summer pruning is the time to reduce growth without stimulating bud growth. When plants are pruned in the summer after the spring growth has ceased, plants are much less likely to produce new shoots.
Occasionally, late summer pruning may stimulate an additional growth flush that may be susceptible to an early frost or freeze, so be careful on plants that are marginally hardy.
If you have a gardening question for Homegrown NH, email or send a voice memo to HomegrownNH@ NHPR.org.
See you in the garden!
Homegrown New Hampshire is a collaboration between Squam Lakes Natural Science Center and NHPR.
