Garden Variety: Figuring out the best time to prune specific shrubs
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Shrubs respond best to pruning when the act is performed at certain times of year, but when that time occurs varies with plant species and locale. How do you know when to prune what you have, and what is the best method to use? Follow these general rules for landscapes in the Midwest.
For all shrubs, keep in mind that pruning stimulates growth. If you shear the plant to get a certain shape or otherwise make all the cuts at the same level, all new growth will emerge from just below those cuts. If you want a more natural shape, making cuts in different levels or planes within the plant is important.
Despite the good that pruning can do, pruning cuts are wounds. The more wounds you make, the more the plant must work to seal over the wounds and protect itself from insects and diseases. Prune only when plants need it, and stick to the rules for the right time of year for the species to minimize stress to plants.
Avoid pruning from about mid-August to frost if possible. The new growth that is stimulated to grow from the pruning process will still be too tender for winter if shrubs are pruned during this time. Shrubs pruned in late summer to early fall are likely to suffer more winter injury than shrubs pruned at other times of year.
Always make sure blades of shears, pruners, loppers and saws are sharp before you begin. Make clean cuts rather than tearing bark or tissue around the cut. If the pruners you are using are too small to clip a branch that needs to be removed, use loppers or a saw instead.
With pruners, use models with curved blades (bypass type) instead of straight blades (anvil type) as the straight ones tend to pinch and damage remaining tissue.
If using shears to shape plants into hedges or shapes, try to keep the bottom wider than the top, like the letter A. If the top is wider than the bottom, the top of the plant can shade out the lower parts and cause dieback in those portions.
If using pruners to selectively prune, make cuts at an angle just above a bud — think of it as creating a triangle at the tip of each cut branch with the bud at the corner and the pruning cut as the diagonal.
When to prune gets more complicated. The big divisions among plants in this regard are subshrubs, deciduous (leaves drop in the fall) shrubs and evergreen shrubs. Then there are few different groups within those big classifications.
Subshrubs are plants with woody stems that are on the delicate side and tend to suffer a lot of dieback each winter. Examples of subshrubs are Blue mist spirea (Caryopteris), Russian sage, butterfly bush, lavender, artemisia, and dwarf Japanese spirea including Goldflame, Little Princess, and Anthony Waterer.
Prune subshrubs after new growth flushes out in the spring by removing dead branches and branch tips. Dwarf Japanese spirea can be sheared to remove branch tips. If desired, subshrubs can be pruned/shaped again after blooming.
If you missed pruning your subshrubs when they leafed out in April or May, go ahead and clean them up now, or with plants like lavender, wait until after bloom. The important thing to remember with subshrubs is to avoid pruning too early in the spring. Pruning stimulates growth and early spring cold nights can damage the new growth.
Deciduous shrubs are usually broken into two categories: spring bloomers and summer bloomers.
Spring bloomers like forsythia, lilac, some viburnums, old-fashioned spirea (Bridal wreath) and others should be pruned after they finish blooming, anytime from post bloom to early summer.
Summer and fall bloomers like roses, sweetspire, hibiscus, hydrangeas, abelia, beautyberry and crape myrtle are best pruned in the spring before they begin blooming. They can still be pruned now, but doing so may reduce the number of blooms, except with roses that may need deadheading to stimulate additional blossoms.
Very early spring is the best time to prune and shape evergreens, especially if making substantial size reductions.
For light pruning, arborvitae, boxwood, junipers and yews can be pruned through the growing season (stop in August as with all species). This makes them prime candidates for shearing into hedges, smooth shapes, and topiary for gardeners who prefer a manicured look.
— Jennifer Smith is a former horticulture extension agent for K-State Research and Extension and horticulturist for Lawrence Parks and Recreation.