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Garden Life: Advent of autumn means it’s time for pruning

By Robb Rosser

Published: September 17, 2015, 6:03am

A month ago, I talked with you about deadheading, pinching out and cutting back perennial plants. Now that we can feel a bit of autumn in the air, my gardening friends are asking for the next step in pruning tips. By now, even late-flowering shrubs, trees and perennials are showing signs of fading blooms. Without flowers to catch our attention, the size and shape of a plant becomes more noticeable.

When giving advice on pruning, I think it is important to go back to the basics. First, know that you do not have to prune every tree and shrub in your garden. If you focus on planting the right plant for the right place in your garden, most woody and evergreen plants will rarely need to be pruned, and then only to remove dead or diseased plant material or for light, general shaping. If a plant is healthy and looks good, you may not need to do any pruning at all.

The key to understanding how to prune a plant is to know why a plant needs to be pruned in the first place. The question to ask is this. “What is the purpose for pruning this particular plant?” The most effective reasons for pruning are to help establish the shape of a plant along its natural lines, to improve flower or fruit production, to control the time of bloom as in pinching back chrysanthemums, and for espalier and hedge shaping. These are all methods of seasonal pruning that enhance a plant’s best qualities.

Don’t make maintaining your garden more difficult than it needs to be by pruning to reduce the size of a plant every year. Weak growth can be stimulated to grow vigorously by hard cutting back, and vigorous growth is best checked by light pruning. This is important to know if you are fighting to keep a large shrub within the boundaries of a small space. The best pruning advice I can give you regarding a plant that is too large for its location is to move it where it has room to grow to full maturity without pruning.

Spring-blooming plants bloom on wood that formed last year. Prune these only after they have finished flowering. A prime example in Washington is the rhododendron. Only after the plant has finished blooming would you deadhead, or snap off the spent flower heads. This is pruning to encourage next year’s bloom. You can also reach into the shrub and cut out crossing or overly long branches to enhance the natural shape of the plant. Some other spring bloomers are azaleas, forsythia, mockorange and flowering quince.

Heathers need to be sheared after flowering. Use hedge trimmers and cut the entire shrub back just far enough to take off the spent flower heads. This will give the heather a neat, compact shape, eliminate the messy look of dead blooms and prevent the heather from dying out in the middle. Most pruning mistakes will cover themselves up after a season of good growth.

Plants that should be pruned from late winter into spring are plants that will bloom the following growing season, usually in summer, on wood produced in the previous growing season. These include abelia, Rose of Sharon (hibiscus syriacus), butterfly bush (buddleia davidii), caryopteris spp., shrub roses and Russian sage (Perovskia). If you do not cut back a butterfly bush, it will grow from where it left off last season. When it flowers, it will be a weak-kneed, top-heavy plant, 12 to 15 feet tall or more. Hard pruning every year will produce a well-shaped shrub that flowers vigorously in late summer or fall.

Prune shrubs grown for colored winter stems shortly before new spring growth begins. These include the shrubby dogwoods (cornus alba and cornus stolonifera), the bright, salmon-red salix britzensis and the ghostly white, stemmed bramble rubus cockburnianus. The key to successful pruning of all of these shrubs is to keep them eternally young; vigorous new shoots are longer and more brightly colored or bloom more freely than older, unpruned growths.

It is only natural to need to refresh your memory before tackling a job that you do so infrequently. That is why I recommend buying a soft-cover, basic pruning manual with plenty of clear pictures and taking it into the garden with you. Each time you are going to prune, do a quick review of that specific plant. Make it a part of your pruning habit on each outing, especially if you have a large yard with a variety of trees and shrubs or are particularly interested in the art of pruning.

Robb Rosser is a WSU-certified master gardener. Reach him at Write2Robb@aol.com.

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