Science of Pruning Q&A

In a recent webinar from UNH Extension and UMaine Cooperative Extension, UNH Extension field specialist Emma Erler delved into the science of pruning, helping viewers take the guesswork out of pruning trees and shrubs in the home garden and landscape.

We didn't get to all the questions viewers had for us, so we are sharing them here as a written Q&A. Enjoy!

Madea asks, "What are water sprouts?", and Launa asks, "Should water sprouts that appear after a tree pruning also be removed?". Christine also asks, "Are there times to strategically keep water sprouts?".

Water sprouts are vigorous upright shoot that develop in the canopy of a tree. They are very common in apples and crabapples. They should be removed because they are weakly attached to branches and are very prone to storm damage. They can also compete with the central leader of a tree, ultimately negatively impacting its overall structure.

Water sprouts (sprouts in the canopy) typically only need to be removed once a year in the late dormant season. And excessive number of water sprouts can be an indication of stress, like over-pruning, root damage, or storm damage. Removing a few water sprouts over the summer is okay, but if too many are removed they will likely grow back in short order. 

On old or declining trees water sprouts could potentially be trained into a new leader or lateral branches. 

Rebecca asks, "Is it heading cuts that result in pollarding?".

Exactly, pollarding is a process that involves topping young trees with heading cuts, and then removing new sprouts each season back to the point of the original cuts. Pollarding requires annual pruning to be successful and is more appropriate for some tree species than others. 

BJ asks, "What is the best type of cut to prune large camellia bushes/trees (over 8 ft)?".

The answer to this question depends on what your objectives are for the camellias. In general, thinning cuts are best because they provide for a more natural growth form. Thinning cuts can be used to shorten branches back to a main or lateral branch, leaving some shoot tips intact and maintaining apical dominance. Thinning cuts can be used to shorten limbs, open up the canopy and direct growth.

Mike asks, "What percentage of the tree/plant should you look to prune each year?".

Although it may seem contradictory, pruning stimulates new growth. In general, the more a plant is pruned, the more extensive its regrowth will be. Those that try to control the size of a tree or shrub with heavy pruning may actually be making the problem worse, as the plant produces lots of new, vigorous branches. Less pruning can actually equate to slower growth. A good rule of thumb is to remove no more than one third of a healthy plant each year. Removing more than that amount can cause excessive regrowth and ultimately weaken the tree or shrub.

Allison asks, "Can I shorten main branches of my peach tree without the heading effects?".

Thinning cuts can be used to shorten limbs by cutting back to a main or lateral branch. The only time heading cuts should be used on fruit trees is when they are very young to develop branches. Thinning cuts are usually more appropriate on mature trees. 

Trisha asks, "When pruning a larger branch should anything be applied to the wound?".

Rather than seal out infection, wound dressings often seal in moisture and decay. In most cases, it is best to simply let wounds seal on their own. Over millennia, trees have developed effective mechanisms for this. Unlike people or animals, woody plants are unable to heal damaged tissues. Instead, they compartmentalize wounds with layers of cells that prevent damage from spreading any further. A properly pruned tree or shrub will seal off wounds and prevent decay organisms from entering the trunk. Naturally, small wounds seal much faster than large ones, making a clear argument for pruning and training plants when they are young.

Christine asks, "Is it worth belatedly cutting the residual stub protruding from the partially healed branch collar?".

Pruning young apple trees is essential to making them grow structurally strong. An apple that is just a year or so old may require some thinning of lateral branches to avoid crowding. Start by removing oversized branches that are more than half the diameter of the trunk, and excessively low branches that are less than 20 to 22 inches above the ground. You should be left with a clear leader and no more than 5 to 7 lateral branches. Check out this fact sheet for more guidance.

Gabriela asks, "Are there any special considerations for pruning very old trees?".

Very old trees have often lost much of their vigor and can only tolerate light pruning. Removing large limbs will often lead to decline. Old trees should only be pruned as necessary, removing as little live tissue as possible. Structural concerns may be better addressed with cabling as opposed to branch removal.

Tammy asks, "At what age/stage should you begin to prune young fruit trees? I have a cherry tree - 4 seasons post transplant, that has two stems. How do I determine which one to take out?". Theresa asks, "We have some young pear trees that have multiple branches very close to the base of the tree. Should we remove them all?". Susan asks, "Should you prune a new apple tree after planting?".

Fruit tree pruning usually starts the year after planting. Cherries can be pruned into one of two systems: leader or open center. If pruning to the leader system you'll need to select one stem as the leader and remove the other. I would be inclined to keep the more robust of the two stems. If they are nearly identical, then I would keep the one that is the straightest and most upright. With a young tree you can't really go wrong with the leader you pick.

Excessively low branches (those less than 20 to 22 inches above the ground) should be removed. Additionally, you should also remove oversized branches that are more than half the diameter of the trunk. The remaining limbs should be spaced evenly around the trunk and there should be a clear central leader. 

Apple trees shouldn't be pruned much the first season after planting. Just remove dead or broken branches as necessary. The following spring you can start pruning and training your young tree. Learn more here.

Carol asks, "Any advise on pruning a very overgrown forsythia? I want to keep the natural branching shape.".

An overgrown forsythia can be pruned in a couple of ways. One option is to remove a third of the oldest branches shortly after flowering in the first year, followed by half in the second year and the remainder in the third year. Cut branches as close to the ground as possible to encourage new growth to emerge from the base. Use this approach if the forsythia serves as a screen or an important backdrop in the garden. A more severe approach is to cut all of the branches to the ground and let the shrub resprout. This type of total rejuvenation can be done every three to four years in order to keep growth in check and achieve the tidiest possible look to the shrub. 

An anonymous viewer asks, "If you have branches that cross over each other, should one of the crossing branches be removed?".

Yes, if two branches are crossing and rubbing against each other one of them should be removed. Keep the branch that has the strongest crotch angle, does not cross through the canopy, and does not mirror another branch on the stem. 

Theresa asks, "Does pruning off branches on plums with black knot actually prevent future black knot?".

Pruning out infected stems will help control black knot disease, but will rarely eliminate it entirely. Prune out all infected branches in late winter (late February through the end of March) and destroy them by burning, burying, or throwing them away. Remove at least 3-4 inches of healthy tissue below each knot to ensure elimination of the fungus. Heavily infected trees should be removed completely, including wild host trees such as black cherry (Prunus serotina). However, the most effective way to avoid issues with black knot in the future is to plant resistant varieties of plum.

Meg asks, "When does the late dormant season start in New Hampshire in Maine?".

I consider the late dormant season to be the end of February or beginning of March. Trees should still be fully dormant at this time and the worst of the winter weather conditions past. 

Betsy asks, "I have numerous aged apples that haven’t been pruned for decades. Many overgrown water sprouts, and large, long weighty branches pulling trunk over. How aggressive can I get without damage, and should all happen in one year or over time?".

Ideally, overgrown apple trees should be pruned over the course of several years, removing no more than 1/3 of a tree's canopy in a single year. I would recommend starting by removing some of the large water sprouts and shortening long limbs using thinning cuts. 

Tess asks, "I have a winterberry that didn't do too well this summer (pest damage) and looks spindly now. Would it help to cut some of the apical stems?", and Brenda asks, "When should you prune winterberry?".

Shrubs like winterberry can be rejuvenated by removing some of the oldest stems at the base. This will trigger new shoots to grow from the root system. In the following season remove the shoots that are small and weak, typically those that are less than a pencil-width in diameter. You can completely rejuvenate an old shrub by removing 1/3 of the oldest stems in the first year, half of those that remain the following year, and the final 1/3 in the third year. 

Winterberry flowers and fruits on new growth, so it should be pruned in early spring before the buds break. 

Christine asks, "Is climate change a consideration with pruning, such as impacts on fungal disease, early freeze and thaw cycles, etc?".

Climate change can have major impacts tree phenology, but shouldn't influence pruning overly much. The ideal time to prune will remain late winter/early spring before bud break. The exact timing of the late dormant season may change over time, but that will still be the best time to prune. As the climate in New England gets wetter, fungal diseases may become even more problematic, reinforcing the need for proper pruning to open up the canopies of susceptible species. 

Jeff asks, "My espalier apple tree has lateral branches - when and how should I prune?" and Marcel asks, "Any special advice for old overgrown apple trees?".

The best time to prune apples is from late February through mid-April, before the flower buds break in the spring. Dig into the specifics of how to prune through this UMaine resource.

Reclaiming an overgrown apple tree can take several years. I would start by removing all dead or damaged branches and then determine which stem should be the central leader (trunk). The height of the trees can then be reduced by cutting back upright branches to strong horizontal lateral branches. You'll also want to prune out water sprouts and those that are crossing or rubbing. Aim to remove no more than 1/3 of the tree's canopy in a single season in order to avoid stimulating the tree to produce excessive water sprouts. Learn more about caring for mature apple trees in this UNH Extension resource.

An anonymous viewer asks, "I didn't prune and winterize my rose bush early December, when should I?".

Roses should be pruned at different times depending on their species and variety. Repeat blooming roses should be pruned in the spring just before the buds break dormancy. Old-fashioned roses that only bloom once a year should be pruned just after they finish flowering. These types should not be pruned in the spring or else you will remove flower buds that developed on the previous year's growth. 

Marzie asks, "Can apricots be trimmed like apple trees? (Style of pruning, season of pruning?)".

Apricots can be pruned to either a leader system (like apples) or an open center. Whichever system you choose, pruning should begin when trees are young and performed in early spring before the buds break. This fact sheet details both pruning systems.

Rachel asks, "We pruned a juniper last summer and unfortunately left a few stubby branches on the lower trunk. Should I wait for those to try and heal before I go back in and try to make a proper cut?".

Branch stubs should be removed because they are both unsightly and impede plants from sealing over wounds. I would look at doing this sometime this spring once the snow is gone and new growth is just beginning. 

Charlie asks, "When should a hydrangea be pruned?".

To simplify things I recommend pruning all hydrangea species in the spring. Panicle (Hydrangea paniculata) and smooth (Hydrangea arborescens) hydrangeas should be pruned in March or April before their buds begin to break. These species bloom on new growth of the current season. Bigleaf hydrangeas (Hydrangea macrophylla) bloom primarily on old growth from the previous season, so I suggest waiting to prune shoots until the buds swell and begin to break. This makes it easier to assess which buds survived the winter and should prevent you from accidentally removing viable flower buds. Learn more in this UNH Extension fact sheet.

Meredith asks, "If I'm trying to keep my fruit trees tiny, how should I prune them?".

Pruning will not alter the natural habit of a fruit tree. The only way to keep a fruit tree small is to grow it on a dwarf rootstock. The root system of a fruit tree is responsible for controlling its ultimate size. Reputable nursery catalogs will often offer varieties grafted to dwarf, semi-dwarf, or standard rootstocks. A tree with a dwarf root stock will remain the smallest regardless of pruning. You can ensure your trees are productive by using the best pruning system for the species (leader system or open center system). These methods are described for plums, cherries and apricots here and apples and pears here.

Bramsey asks, "What do I do if I have an apple tree with branches equal sized to the trunk. It sounds like I should just prune it back not remove. Is this correct?".

Yes. If you remove a large upright branch that is acting as a second trunk, there will not be a branch protection zone the develop callous tissue over the wound, and disease is likely to develop. It is usually better to reduce the size of competing branches by cutting them back to strong horizontal lateral branches. 

Charlie asks, "When should a holly be pruned?".

When hollies require pruning it should be done just before spring growth begins. In northern New England this would typically be in early to mid-April. 

Tom asks, "I have several older apple trees with 3 or 4 primary trunk stems being held together with ratchet straps to prevent breaking under heavy snow. Is it too late to prune or how should a prune each primary stem/trunk?".

It's hard to say without seeing exactly what your trees look like. Usually co-dominant stems that are about the same diameter lack a branch protection zone that will prevent decay organisms from invading the trunk. Older stems may have also developed heartwood, darker colored wood that is very prone to rot when exposed to the open environment. Removing one or more of the co-dominant stems may be necessary in order to create good structure, but it is likely that there will be some decay at the site of the pruning wounds. 

R. Edmonds asks, "Can you talk about the best time and the difference between maintenance pruning and rejuvenation pruning of mature woody perennials like azaleas/rhododendron/pieris?".

In many cases azaleas, rhododendrons, and pieris don't require much pruning. Occasional thinning cuts can be used to encourage new growth within the interior portions of shrubs, reduce plant size, and create a fuller, more attractive shrub. Rejuvenation pruning shouldn't be necessary unless a mistake was made and large-growing shrub was planted too close to a building. Rejuvenation means cutting the entire shrub to within a foot of the ground in the early spring and letting it regrow from dormant buds. This type of pruning should be used as a last resort because shrubs don't always respond well to severe pruning and may decline. 

An anonymous viewer asks, "Are the pruning principles you talked about apply to grape vines which require heavy pruning each year?".

The principles I discussed also apply to woody vines to a certain extent. However, grapes require specific pruning for maximum production and to keep them under control. Learn more here.

Linda asks, "What about pruning to help support heavy fruit? I have a peach tree that has very heavy peaches that stress the branches."

Pruning will reduce the amount of fruit a peach tree bears and can also improve the structural strength of limbs. Thinning fruit is also an important practice that can't be overlooked. When conditions are just right peach trees can bear an excessive amount of fruit that puts strain on branches. For peaches, fruit should be removed in mid to late June, so that there is no more than one fruit for every 6 to 8 inches of shoot length. 

Barbara asks, "Should suckers be removed in dormant season only?".

Suckers (sprouts from the base of the plant) can be removed at any time during the growing season, and may need to be pruned off more than once. Water sprouts (sprouts in the canopy) typically only need to be removed once a year in the late dormant season. And excessive number of water sprouts can be an indication of stress, like over-pruning, root damage, or storm damage. Removing a few water sprouts over the summer is okay, but if too many are removed they will likely grow back. 

An anonymous viewer asks, "We had several oak trees taken down on our lot to allow for some sunshine.  They are in our back yard and we have not grinded the stumps.  How do we prevent growth/suckers on these stumps?". They also ask, "Our yard has many, many mature oak trees that I would like to prune to allow for more sunlight.  Since they are so tall, I thought we would trim the lower larger branches but would like your recommendation."

Treating the suckers that emerge from the trunks with a systemic herbicide will kill the roots and prevent re-sprouting. A product labelled for brush or woody plants would be an appropriate choice and should be effective if all instructions on the label are followed. If you don't want to use chemicals, repeatedly cutting suckers multiple times throughout the growing season will eventually exhaust and kill the root system too. 

Removing some of the lower branches might help a little, but the best results would come from having an arborist thin the overall canopy. Another option of course is to plant shade-loving perennials beneath the oaks. 

Bonnie asks, "Isn't there a direction of how to prune the tree? for example start pruning by taking all the dead wood?".

1. Remove all Dead, Damaged, Diseased branches.
2. Remove all water sprouts (fast-growing, upright branches) and branches that cross and rub each other. 
3. Remove branches that hang down or grow upright to compete with the main leader. Additionally, reduce the length of or remove branches that are greater than 1/2 the diameter of the main trunk.
4. Eliminate branches that have narrow crotch angles (less than 45 degrees) to reduce the change of splitting or breaking. 
5. Remember to prune moderately and only cut off branches if you have a good reason to. Pruning can be spread over several years so that you aren't removing more than 1/3 of the canopy in a single year. 

Laura asks, "How to decide what to prune when a <6” caliper shade tree has 2 codominant leaders with a very narrow crotch between the leaders?".

I would be inclined to prune one of the leaders back to a strong horizontal branch with a thinning cut. This should slow down the growth of the competing leader and reduce the stress on the narrow crotch. In a few years, once the remaining leader has assumed dominance and is out-growing its competitor it may be appropriate to entirely remove the branch with the narrow crotch angle.

Rick asks, "I have a redbud that I trained as a single stem but has branched asymmetrically. It has since sprouted a vigorous vertical shoot at ground level against the trunk on the side with minimal branching. Would it be ok to allow this to develop into a second trunk to create a more balanced looking multistem tree?".

Redbuds are actually grown as multi-stemmed specimen trees fairly often, so it could be perfectly fine to allow this sprout to grow. Something to be aware of though is that many redbuds are grafted. If the sprout is developing below a graft then it may not be the same variety as the rest of the tree. Additionally, suckers are often weakly attached to the lower trunks of trees, so this sprout may be more prone to storm damage. 

Charlie asks, "Can you cut a knockout rose way back, and if so, when?".

Knockout roses fall into the category of modern ever-blooming roses. They bloom best on new or current season's wood, and respond well to hard pruning in early spring. This means removing about 1/2 to 2/3rds of the shrub's height and thinning the canes. I would recommend starting by removing all of the dead canes, followed by all of the small or weak canes. Leave 3 to 5 large, healthy canes, and cut them back so that they have 3 to 5 outward-facing buds each. 

Bllu asks, "Is there a web site or book where we can research how to prune each type of tree and shrub?".

The following books are excellent guides:

• An Illustrated Guide to Pruning. 2011. Edward. F. Gilman
• Essential Pruning Techniques. 2017. George E. Brown and Tony Kirkham
• RHS Pruning Plant by Plant. 2012. DK
• The Pruning Book. 1997. Lee Reich
• American Horticultural Society Pruning and Training. 2011. Christopher Brickell and David Joyce
• and many others...

Shannon asks, "Do you have a resource (video or specific article or website) on pruning a mature pear tree with a variety of branching issues.... ?".

The same principles of pruning mature apple trees apply to pears. I recommend following the instructions in this fact sheet: https://extension.unh.edu/resource/growing-fruits-care-mature-backyard-apple-trees-fact-sheet 

Jon asks, "I have apple trees (very established ones!!) - what are the best tools to use to prune? I have a pole saw that also has snippers at the end. Are there any tools to avoid or is it more about just being careful and keeping the cut in the right spot?".

Good quality bypass hand pruners, a pair of loppers (also bypass), and a good pruning saw. These tools will serve you well as long as you are careful to make your cuts outside of the branch collar. Keeping your tools sharp is also helpful - clean cuts seal over much better than ragged ones. 

Shelly asks, "I have an older birch tree with a stump at the base along with the main trunk. There are smaller 'trees' coming off the old stump. Should I cut this new growth and when would I try this to avoid sap leakage? It is near a main road in proximity to plowing as well."

It sounds like you have suckers growing from the cut stump of the birch. These should probably be removed , and if you want to avoid excessive sap leakage, it should be done in late May or early June. However, sap leakage isn't usually very harmful to trees, it just looks unsightly. You could make your cuts in the late dormant season with the understanding that there would be sap leakage in the spring. 

David asks, "Is it preferred to remove the witches broom growth on blueberry bushes, or is it best to leave that growth?".

Witches brooms are caused by a rust fungus that grows into the bark of blueberry plants. The disease typically spreads to infect entire plants. Pruning out witches' brooms will reduce symptoms of the disease, but it is likely that new witches' brooms will form. Infected plants usually need to be dug up and burned, buried, or brought to the landfill. Unfortunately there is no way to cure infected plants. 

Russell asks, "I pruned the interior vertical shoots on of a mature crab apple in November. I believe I left the collars in tact. There are black linear spotches on the exterior bark in the trunk and branches. Should I spray bark with dormant oil? If so when?".

I would have to see a picture of these splotches to determine their cause. Regardless, it doesn't sound like there is any need to spray a dormant oil. Oils are typically used for insect control, which I don't have any reason to suspect.

D. Evans asks, "Wouldn’t water sprouts be kept on the pleached trunks in a hedgerow?"

In a pleached hedge branches would be bent down and interwoven to form a living wall. When first establishing the hedge you might train water spouts into a horizontal position. Once the hedge was mature though, you would likely shear off many of the water sprouts. 

Allen asks, "I have a 5-in-1 cherry tree that I would like to keep at a reasonable size. How do I prune this to a) keep the height under control and b) not affect any of the varieties."

You should be able to control the height of this tree to a certain extent by cutting the main upright growing branches back to well-developed horizontal lateral branches using thinning cuts. If the horizontal branches are at least half the diameter of the cut stem, they should assume apical control. This fact sheet should serve as a good pruning guide.

Peter asks, "Is there any harm in leaving the stumps behind if I am pruning and creating a Young Red maple sugar bush on 2 acres with 40 or so maples too young but have about 13 tappable trees now. I don’t know if the decay of 40 plus smaller tree roots hurt trees stressed by canopy work?".

Cutting the smaller trees shouldn't impact the larger ones. What you should expect though is lots of sprouts to emerge from the cut stumps that will need to be cut back several times in order to exhaust their root systems. 

Cynthia asks, "Any reason not to use a chainsaw for pruning?".

Chainsaws are really important pruning tools for larger cuts. You'd be hard-pressed to find an arborist without one. Just keep in mind that smaller cuts are much better than large ones. Young trees that are well-trained rarely need major interventions with a chainsaw. 

Meg asks, "It seems like suckers are very common with lilacs, just a kind of natural thing. Is that the case? or is there something you can do to avoid that?".

Like many shrubs, lilacs can sucker profusely. There really isn't anything that can be done to avoid them. You'll just need to thin the suckers periodically to remove thin, weak stems to promote the growth of a smaller number of robust suckers. These shoots will eventually replace old stems that no longer flower.

Audrey asks, "I have a fig tree (growing more like a fig bush) that has several comparable main branches. It is 2.5 years old. Last year had one fig fruit. I am trying to winter it over in zone 5b by heavily covering it in winter. Any suggestions on pruning?".

Figs don't usually require much pruning. The main stems should just be spaced enough so that that they are not rubbing against each other. Lateral branches should only need to be removed if they are crossing or rubbing. Learn about caring for figs in our region here.

Did you miss this webinar? Email Pamela Hargest at UMaine Cooperative Extension for a link to the recording.

Explore the rest of the Winter Webinar Series on the UMaine Cooperative Extension website, and register to be able to tune in live or get a link to the recording and resources afterwards.

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