Correctly pruning your plants can add years to their lives and increase yields of healthy, disease-free fruits and flowers. Improper pruning methods can conversely stress out your plants and shorten their life expectancies.
While some plants can be hacked down to the ground, only to grow back just as strong and as pretty as ever, most require a bit more trimming finesse. To simplify the when’s and how’s of pruning, the guidelines below will get you started.
When to Prune Your Plants
Even the cleanest cuts by the most skilled hands cannot undo the damage caused by pruning at the wrong time, but some pruning can be done anytime. Dead, diseased or broken limbs and branches can be pruned whenever you spot these issues on your plants.
The same holds true for branches that are crossed or rubbing the bark off one another. Sucker growth—the growth you see coming up from the roots of some trees and shrubs—can safely be pruned off any time, as well as shoots that appear randomly along the main trunks of trees.
Timing for the pruning of ornamental and shade trees depends on the purpose of the pruning.
Shaping and thinning on trees should be performed during dormancy, before new buds are forming and starting to swell in late winter or early spring. If you can’t bear the thought of missing out on the magnificent show of cherry or plum blossoms, some light shape pruning can be done after flowering, but save major tree surgery for the dormancy period.
For evergreen trees and shrubs, which have needles like a pine tree or leaves like a holly bush, the best time of year for shaping and pruning to control size is mid-summer, once that year’s new growth has been hardened off.
For evergreen trees specifically, cutting any lower branches that are lying on the ground (basal pruning) can be done any time of year. Cutting or training leader branches at the top can be done any time of year as well.
For deciduous shrubs—those that lose their leaves in the fall and re-bud in the spring—the general rule of thumb is if it flowers in the spring, prune after flowering (old wood); if it blooms in the summer, prune in late winter or early spring.
For deciduous perennials and grasses, even though most herbaceous perennials die back completely above ground, it is better to wait to cut them back until late winter, before the following year’s new growth forms.
Some horticulturists may disagree with me on this one, as the previous year’s branches or foliage may cause the development of molds or diseases, which is a risk. The benefit to leaving the growth, even though it may be brown and unattractive, is that in colder climates, it acts as insulation for the root system, and in areas that receive snowfall, the branch structure can trap some of that snow, resulting in further insulation.
Grasses should not be cut completely to the ground, but rather an 1-2 in. above, or 6-12 in. for larger grasses.
Pruning annual flowers can be done whenever necessary. Many common flowering annuals such as petunias respond favorably to a light pruning when they are looking a little worn out by sending up another flush of flowers. Dead blossoms should be removed as they appear; the same goes for dead and diseased leaves and branches.
Of course, what would any gardening endeavor be if there were no exceptions to the rules? Fruit trees, shrubs and vines should be pruned for shape and to improve air circulation during dormancy. For raspberries and blackberries, cut out any branches larger than pencil-sized in diameter and cut back all others to a height of half to one-third of their current height.
For grapes, the best results are obtained by pruning them to grow on a trellis with one central trunk that branches out in either direction about 1-2 ft. off the ground, then another section of trunk with another set of branches about 2 ft. above the other. Grape cultivation requires some practice and a separate resource devoted to that should be consulted (or better yet, keep reading Maximum Yield for a future article on just that).
Other notable exceptions include roses, rhododendrons and hydrangeas. Roses can withstand a heavier cutting back in late winter than many other shrubs. Up to two-thirds of most rose types can be pruned back. Climbing roses should be pruned more selectively to maintain their height and climbing effect.
Rhododendrons and azaleas (and all plants in the Ericaceae family, including blueberries) should not be heavily pruned. Simply take out any deadwood and crossed branches, and make minor height adjustments. Rhododendron flower trusses should be removed by hand after they are spent. This will encourage better budding and flowering for the following year.
Many people have trouble getting their hydrangeas to bloom, especially in climates that experience late frosts. In some cases, this is due to the type of hydrangea you have. “New” varieties can be pruned any time of year, as they bloom on both old and new wood.
Old varieties should be pruned in late winter or early spring. Even though they do not bloom in early spring, hydrangeas set their flower buds then. A late freeze can kill new flower buds on any shrub or tree that sets buds early.
Check with your local garden center to see if you have an older or newer variety if you are unsure. If you have a hydrangea that flowers blue and you wish for it to be pink, lower the pH of your soil. For those who wish the opposite, lower the pH. Hydrangeas that flower white will remain white.
How to Prune Plants without Killing Them
Now that you know when to prune your beloved trees, shrubs and perennials, the next step is learning how to do so. It is important to know and identify the nodes on your trees and shrubs. These are the points where the branches originate from the trunk or where they branch out from other branches.
In general, it is not good to make inter-nodal cuts—to cut a branch between two branches, leaving a stub. The result is many weak branches quickly taking the place of the cut branch. These shoots are more prone to breakage and diseases than the original branch.
To allow for better wound healing and for the nutrients and water flowing up to be diverted properly to the branch just below your cut, cuts should be made just above a node.
When cutting or pruning branches from an ornamental or shade tree, the cut should be made at the branch collar, not flush with the trunk. This is the slightly raised portion that interfaces between the trunk and the branch.
When cutting large limbs, cut them in sections. This is safer for you and it helps the tree absorb the stress of the weight of the branch as it is being removed.
When pruning evergreen trees and shrubs, shear about half of that year’s new growth. The new growth on pines, called candles, should be cut individually instead of being sheared wholesale. When pruning deciduous shrubs, no more than a third to half should be cut off at any given time.
Hedge shears, either hand-held, gas or electric-powered, are usually sufficient, but loppers or hand-held pruners should be used for younger plants and sparse plants.
When pruning annuals, thinning shears or bypass pruners should be used, or pruning by hand. On geraniums, studies have shown that hand-plucking the spent blossoms and discolored leaves is better for the plant than cutting them off.
Now that you know how and when to prune your plants, check out Proper Pruning Techniques: Part 2, for information about pruning for propagation and tips on selecting and maintaining your pruning equipment.