When to Prune

• By Cleve Campbell
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• March 2017-Vol.3 No.3
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• 7 Comments

I remember the joy of unwrapping my new pruners; I just couldn’t wait to get outside to check out how sharp they were and to attack a few shrubs that were swallowing up the deck. I just had to slow those suckers down before they engulfed the house and started growing though the windows. It was time to declare war on those overgrown bushes. I was jacked; with pruners in hand, I headed out the door, craving some pruning action. Before I could even give the doorknob a half turn, I heard that all-too-familiar voice, “Where are you going with those pruners?”  I responded quickly, “I’m just heading out to try out my new pruners.”  My response was quickly squashed with a stern frown and voice.  “Just because you have sharp pruners doesn’t mean it’s time to prune!”  Well, that certainty put a damper on that old garden myth, “You prune when the pruners are sharp.”  Now that we’ve got that myth out of the way, let’s take a more scientific look at the when-to-prune question, along with some other pruning basics.

What is Pruning?

Pruning is the selective removal branches while maintaining the plant’s natural form.  Pruning is NOT the same as shearing, which is clipping of all new shoots to create an artificial form. Based on aesthetics and science, pruning can also be considered preventive maintenance. Pruning correctly during the formative years may prevent many problems.

Why we prune

Pruning is good for the overall health of a plant. We prune for a number of reasons, including: improving structure, controlling plant size, to define garden space (hedges), to create special effects such as a topiary or espalier, to reveal colorful bark or enhance winter silhouette, to affect flowering and fruiting and for safety  — to prevent personal injury or property damage. Pruning improves the health of the plant by removing dead wood and branches that are dying from disease or severe insect infestation, as well as branches that have been broken by animals, storms, or other adverse conditions.

On the left, an unpruned 15-year-old shade tree. On the right, a 15-year-old shade tree that was properly pruned when young. Illustration adapted from “How to Prune Young Trees,” Tree City USA Bulletin No.1, National Arbor Day Foundation.

Now back to my new pruners, and that proverbial question of when is the right time to attack those overgrown bushes swallowing up the deck.  After a lot of research I came to the realization that correct pruning depends on two things: what plant is being pruned and why.

When to prune

Pruning requirements for trees and shrubs will not only vary according to to species, but will also depend on the purpose of pruning. If pruning is necessary because branches are dead and the tree or shrub causes a safety hazard, pruning can be performed at any time. However, the overall health of the plant should always be taken into consideration before addressing pruning issues. It is important to know that detrimental diseases can easily be spread if trees and shrubs are pruned at the wrong time of the year. For example, oak trees (Quercus spp.) should only be pruned in the winter months when the trees are dormant to prevent the spread of a common fungal disease called oak wilt.

Each tree or shrub has its special pruning time, depending on the time of year that it produces flowers. Pruning your plant at the proper time is critical for success. If you prune a flowering or fruit tree at the wrong time of the year, you’ll probably miss out on that plant’s blooms or fruit for that season. While it isn’t the end of the world and the plant will recover and flower again the following year, it’s definitely disappointing. The risk of missing out on a season’s worth of flowering is probably the main reason most gardeners fear pruning their shrubs or trees.

To determine when to prune a plant without interrupting its bloom cycle, you need to know your plant. One of the keys to not interfering with the plant’s bloom cycle is to know if the plant flowers on “old wood” or “new wood.” These terms get thrown around a lot, but are often confusing. How old is “old wood”? —  6 months?  one year? two years or longer? And what qualifies as  “new wood”? Here’s what that means:

Plants flowering on “new wood” do NOT develop flower buds until AFTER growth begins in the spring.  Plants that flower on new wood develop the flowering buds in the spring and generally flower later in the season. Some examples of plants that that flower on new wood include roses, butterfly bush, and crape myrtle.

Plants that flower on new wood can be pruned in early spring, just as the new growth begins. This leaves plenty of time for the plant to recover from pruning and still create flower buds for the bloom season. The ideal time to do this is after the buds have emerged on the stems but before they expand. Early spring is the busy pruning season for plants that flower on new wood.  In early spring you can see where the healthy new growth is located, and pruning before the buds leaf out means that the plant doesn’t waste energy on buds you’ll just be cutting off anyway.

Plants that flower on old wood form the flower buds for next year’s blooms during the current  growing season.  Thus, the buds are carried over through winter on last year’s growth —“the old wood.” After these plants bloom, they begin to form the flower buds for the following year. Plants that flower on old wood generally flower early in the growing season. Some examples of shrubs that flower on old wood include fringe tree, forsythia, azaleas, rhododendrons, and lilacs.

Plants that flower on old wood can be pruned immediately after they finish blooming. If you prune before they bloom (late winter to early spring) you’ll remove the flower buds. If you wait too long after they’ve finished blooming, they may not have time to create flower buds for the next year.

Still not sure when to head out to the yard with those sharp pruners?  A pruning calendar can be found at the end of this article.

Deciduous Trees

Trees that shed their leaves annually are classified as deciduous. In general,  most deciduous trees  are pruned when they are dormant, which simply means that period that begins in the fall when the tree loses its leaves and which ends in spring when the buds start to swell.  The ideal time to prune deciduous trees  is late winter to early spring. At this time, wound healing is rapid. Also, dormant pruning will have less of an effect on the tree’s growth than pruning during other times of the year.  Summer pruning tends to suppress the growth of both suckers and foliage. Late summer or early fall pruning causes vigorous regrowth, which in some species may not harden off by winter, leading to possible cold damage.  Another advantage of dormant pruning with deciduous trees is that with the leaves gone it is easier to see the structure of the tree and select branches.

Young tree pruning is often preventive, eliminating structural problems.  And remember: whenever unexpected damage from vandalism or bad weather occurs, prune immediately, no matter what the season.

Evergreen trees

Evergreen trees have their leaves year round and include most conifers such as pine and cedar, and some broad-leaved trees such as the holly. Evergreen trees in general require less pruning than deciduous trees. Certain conifers —  spruces, firs, and white pines —  have whorled branches that form a circular pattern around the growth tip and should not be pruned into the interactive center (where no needles or leaves are attached) because new branches won’t form to conceal the stubs.

Most evergreen pruning is done for corrective reasons, so seasonal timing is usually not as important as it is for deciduous species. Pruning doing dormancy is the most common practice and will result in a vigorous burst of spring growth. Pines and other whorl-branched conifers become denser if new growing tips (candles) are pinched in half as they expand in the spring. Whenever unexpected damage from vandalism or bad weather occurs, prune immediately.

When NOT to Prune

Pruning plants does a number of things to their growth systems.   Pruning can stimulate new growth, and new growth is tender. If a cold snap hits, which can happen unexpectedly and fast anytime in fall, any tender new shoots can be damaged, weakening your plant.  Fall pruning  is not recomended for another reason, too:   because decay fungi spread their spores profusely in the fall, and wounds are slower to heal on fall cuts. Fall is a great time in the garden with the changing seasons. But for most plants, it’s best to to leave the pruners, saws, loppers, and clippers in the storage until late winter.  But as I mentioned earlier, dead, broken or hazardous limbs can be removed any time of the year.

Pruning is the removal of selected living or dead parts of a plant to maintain appearance, health, and to regulate growth. It is used to increase flowering, rejuvenate, shape and improve structural strength of shrubs. One of the keys to successful pruning is timing. Knowing your plant and knowing the response you’re trying to elicit from your plant are critical factors in determining when to prune.

Thanks for stopping by The Garden Shed. We look forward to your visit next month.

PRUNING CALDENARS

Shrub Pruning Calendar 

Legend:

* = Best time to prune
x = Do not prune except to correct damage, hazards, or structural defects
– = Timing is not critical

Note:

1. Flowers are produced on new (current season) wood
2. Flowers are produced on wood from past season, pruning while dormant will reduce flowers
3. Make pruning cuts well below diseased wood (fire blight) – disinfect shears between cuts
4. Remove old stems to ground yearly to renew
5. Midseason shear if a formal hedge is desired
6. Do not cut into old wood that has no leaves or needles
7. Spring/summer prune to remove azalia caterpillars and galls
8. Fall/early winter pruning can reduce winter hardiness
9. Snap candles (new growth) in half when needles are 1/2 to 2/3 their normal mature length

Calendar Source: A Guide to Successful Pruning, Shrub Pruning Calendar, Virginia Cooperative Extension Publication 430-462, http://pubs.ext.vt.edu/430/430-462/430-462.html

Deciduous Tree  Pruning Calendar 

Legend:
* = Best time to prune
x = Do not prune except to correct damage, hazards, or structural defects
– = Timing is not critical

Note

1. Avoid pruning in late winter/early spring due to sap flow (more cosmetic than detrimental)

2. Avoid pruning from spring through summer due to insect or disease problems

3. Avoid pruning from October – December due to reduced cold hardiness

4. Avoid pruning after July because flower buds have set

Calendar Source: A Guide to Successful Pruning Decidous Tree Calendar, Virginia Cooper Extension, Publication 430-460, http://pubs.ext.vt.edu/430/430-460/430-460.html

Evergreen Tree Pruning Calendar

Legend:
* = Best time to prune
x = Do not prune except to correct damage, hazards, or structural defects
– = Timing is not critical

Note

1. Seldom needs pruning – remove multiple leaders, dead and broken branches
2. Don’t prune into old wood having no leaves or needles
3. Prune during growing season to make more compact or dense
4. To avoid reducing berry production; don’t prune during bloom period
5. Prune to prevent oak wilt infection
6. Prune to remove cankers
7. Flower buds set on previous season (old) wood; winter pruning will reduce spring flowering

Calendar Source: “A Guide to Successful Pruning: Evergreen Tree Pruning Calendar,” Virginia Cooperative Extension Publication 430-461  http://pubs.ext.vt.edu/430/430-461/430-461.html

Sources:

“A Guide to Successful Pruning: Pruning Basics and Tools,” Virginia Cooperative Extension Publication 426-455, https://pubs.ext.vt.edu/430/430-455/430-455.html

“A Guide to Successful Pruning: Pruning Shrubs,” Virginia Cooperative Extension Publication 430-459, https://pubs.ext.vt.edu/430/430-459/430-459.html

“A Guide to Successful Pruning: Pruning Deciduous Trees,” Virginia Cooperative Extension Publication 430-456 http://pubs.ext.vt.edu/430/430-456/430-456.html

“A Guide to Successful Pruning: Pruning Evergreen Trees,” Virginia Cooperative Extension Publication 430-457 http://pubs.ext.vt.edu/430/430-457/430-457.html

“When to Prune-Tree Care Tips & Techniques,” The Arbor Day Foundation,                 https://www.arborday.org/trees/tips/when-to-prune.cfm