Pruning trees and shrubs: Keeping them small

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Introduction
Excerpt overview
Technique widely applicable and practical

Set limits for your plants
Be aware of the plant's growth rate
3 good times for restriction pruning

Start clipping at the top
Choosing main limbs
Step back to assess the shape
Direct the new growth
When to have confidence in leafless wood

What about flower
Be considerate of the plant's effort/expense

Diagram, deciduous tree
Diagram, pyramidal evergreen
Hands-on training

This space is too small for a full sized blue spruce, 50 feet tall and 25 wide. Yet at its present size its blue form is attractive there. So we prune this spruce to keep it this size; it was pruned the year before this photo was taken. Starting at the tip, Janet removed over three feet of the leader, selected and tied in a new leader. Then she descended each side and shortened branches there by 1 to 3 feet to keep the narrow pyramidal form. The tree can grow for two years before being pruned again.

This space is too small for a full sized blue spruce, 50 feet tall and 25 wide. Yet at its present size its blue form is attractive there. So we prune this spruce to keep it this size; it was pruned the year before this photo was taken. Starting at the tip, Janet removed over three feet of the leader, selected and tied in a new leader. Then she descended each side and shortened branches there by 1 to 3 feet to keep the narrow pyramidal form. The tree can grow for two years before being pruned again.

Most gardeners overplant. One property may be too thick with trees, planted by a gardener who had no idea that those s/he planted would grow so quickly. Another may be a shrub jungle belonging to someone who didn't double-check the garden center tag descriptions, unaware that shrub size on such tags is often grossly understated. These gardeners may be overwhelmed and even embarrassed by plantings grown far out of control.

A third property may be planted with even more trees and shrubs than either of the first two, yet its gardener is in control and pleased with the attractive result. The difference is this third overplanting was done with an awareness of each plant's potential size and growth rate, and the intention of pruning each one to restrict its size.

You can be that third gardener, able to have a diverse collection of trees and shrubs yet keep them in line indefinitely. All the basics are here in this article, and August, November or February are good times to apply them in your garden.

 

A technique widely applicable and practical

 

Any tree or shrub can be kept smaller than its potential, if you're willing to do the necessary pruning at appropriate intervals. Doing this is not difficult and does not take much time -- 30 to 60 minutes per plant is about average. It can be done just once a year or once every two years, depending on the plant's growth rate and how crisp or loose you want the plant's outline to be in the interim. The more crisp the desired look, the more often a plant must be pruned.

So why don't more people keep their plants in line? Because too few realize it's possible or necessary. Very few have done what we have in seeking and studying trees and shrubs kept attractively small and healthy for decades or centuries. ( Such places as this...) Only a handful buck standard American pruning convention by committing the technique to paper or conducting classes.

So what you read here or learn in our hands on sessions may contradict what's in other pruning guides. We aren't here to defend the differences. ( Check here!) Plants we have pruned this way for decades and others that have been pruned this way for generations make a better argument. There are no absolutes in pruning. How and when we prune depends on our objectives, which vary as much as our personalities. This technique is just one more way to prune, as applicable, safe and practical as any other in the right situation.

 

First, set limits for your plants

 

The first step in restriction pruning is to draw some lines. Decide how tall and how wide you can allow the plant to become. Often, the tallest we allow is "gardener plus a ladder" because we do not want to work from special lifts, erect scaffolds or bring in heavy equipment.

Recently we planted something that some authorities report as growing just 10 feet tall and wide. Yet it is from a species with the potential to be 20 to 30 feet tall and wide. It's a variegated angelica tree. (Aralia elata 'Variegata' -- thanks to Goldner Walsh Nursery manager Joel Miller, who tracked down this beautiful, hardy, rare tree so customers like us can finally have one of our own. We hope you also ask at your local garden centers about special plants, because many garden centers are willing to seek special plants for you.) The place where we have planted it can only accommodate a tree that's ten by ten, so we will watch it and be ready to cut if it oversteps those bounds.

If that happens, we will begin pruning it in the year it becomes too tall, too wide, or both. And we'll plan to prune it every year or two from then on.

Be aware of the plant's growth rate

 

Whether we prune a plant annually or every two years depends on its growth rate. A tree like redbud (Cercis canadensis) can produce shoots three or four feet long in one year, which means it may jump that far past its bounds each growing season. We're likely to put that plant on an annual pruning list. On the every-other-year list are slower plants like dwarf false cypress (Chamaecyparis obtusa 'Aurea') that grows just 6 inches per year. (But slow doesn't mean small, as you can see where Steven cut two "slow" dwarf falsecypresses!)

To gauge the growth rate, look at how much new growth there is on the end of a branch at or near the top of the plant. There is a change in bark color, and often a branch-encircling ridge called the terminal bud scar, between old and new growth.

Don't judge a plant by the growth rate of its side branches, branches near the bottom or in the interior. Look at the tip buds and tallest limbs to decide how much the plant grows in one year.

The first step in keeping trees and shrubs in bounds is to set firm limits. We are willing to let our own red horsechestnut (Aesculus x briotti 'Carnea') grow to its full potential, 30 to 40 feet tall and round. In smaller quarters in a client's collection, we might set its maximum height at "Janet plus a ladder" so we could prune as needed without special equipment.

The first step in keeping trees and shrubs in bounds is to set firm limits. We are willing to let our own red horsechestnut (Aesculus x briotti 'Carnea') grow to its full potential, 30 to 40 feet tall and round. In smaller quarters in a client's collection, we might set its maximum height at "Janet plus a ladder" so we could prune as needed without special equipment.

You decide how tall and wide you can allow a tree or shrub to become. These limits also describe a shape. For simplest pruning, the imposed shape should conform to the plant's natural habit, as it does here for this naturally round red horsechestnut.

You decide how tall and wide you can allow a tree or shrub to become. These limits also describe a shape. For simplest pruning, the imposed shape should conform to the plant's natural habit, as it does here for this naturally round red horsechestnut.

The bud at the tip of each branch will grow more than those behind it along its limb. Also, the bud at the very highest point of the plant will grow more than all others. Sometimes that tip bud of the highest limb or "leader" produces a shoot twice as long as those below it on the same branch, and three times longer than those on the lowest limbs.

This red horsechestnut branch grew from one fingertip to the other this year. Its dominant bud grew a bit more than twelve inches, while its twin grew only half that. The new growth has lighter bark color, and begins at a twig-encircling ridge -- the terminal bud scar.  (What may look like a dead, bud-less tip above the new tip buds is a spent flower stalk. It will be shed in time.) The buds now set at the tip of the branch will repeat the process next spring, one of them exhibiting its dominance by outgrowing its twin.

This red horsechestnut branch grew from one fingertip to the other this year. Its dominant bud grew a bit more than twelve inches, while its twin grew only half that. The new growth has lighter bark color, and begins at a twig-encircling ridge -- the terminal bud scar. (What may look like a dead, bud-less tip above the new tip buds is a spent flower stalk. It will be shed in time.) The buds now set at the tip of the branch will repeat the process next spring, one of them exhibiting its dominance by outgrowing its twin.

We prune this redbud every two years to keep it the size you see here. Next year we'll cut each main limb back by five or six feet. The tree will quickly cover its truncated form in new foliage.

We prune this redbud every two years to keep it the size you see here. Next year we'll cut each main limb back by five or six feet. The tree will quickly cover its truncated form in new foliage.

When we prune the redbud, we cut each main branch at about the spot where it's been cut before. One such old cut is here in the "Y" above the saw tip.

When we prune the redbud, we cut each main branch at about the spot where it's been cut before. One such old cut is here in the "Y" above the saw tip.

Three good times for restriction pruning

 

It's important to recognize that hardy plants grow in this orderly fashion, following an annual schedule including a time when they stop extending new growth as they set themselves up for winter. We like to do our restriction pruning during periods after a plant has set its next year's buds but before the next spring's growth begins. In Michigan that's between mid-August and early- or mid-April; we most often do this pruning in August, in November after leaf-fall or in February during a thaw.

When we cut after mid-August, the plant doesn't have time to make much or any new growth. The form we see when we finish pruning will not change until new growth begins. That can be as much as six months, a long time for something to remain "done" in a garden.

So it's a good thing to prune a plant in August if that plant will be attractive immediately after being pruned. A yew or spruce that looks shaggy at the end of its period of unrestricted growth takes on a more neat outline right after pruning. We can enjoy that neat shape all winter.

On the other hand, we're more likely to prune in late winter or very early  spring if the tree or shrub has to be pruned quite hard to keep it within bounds. An example would be that redbud which must have its limbs shortened by 3 or more feet. It can look a bit stark until new growth begins.

By the time it quits growing to harden itself for winter, the tree or shrub has already set its growth hierarchy for the coming spring -- its tip buds have been programmed to grow faster than side buds, highest bud faster than all. If we remove tip buds after this growth hierarchy has been set for the coming spring, there is no time left for new growth to form, mature and change the pecking order. We clip off the tip bud on a branch, leaving in place a batch of side buds programmed to grow at relatively equal pace for much of the season, a rate that is more sedate than what the tip bud would have achieved.

So doing this pruning between August and April has the effect of slowing the plant's outward and upward reach during the next year. Its energy is divided among more limbs, all of which will remain for a longer time within our set outline.

Start clipping at the top

 

Even so, the plant will tend to grow longer shoots from its top than from the sides and bottom. So the top takes the biggest cuts. Thus we always start at the top of the plant when we prune to restrict size.

We'll cut that main limb back far enough that its leader can grow without crossing our line until we plan to prune again. We cut it back by two years' growth if we're on a two year schedule. If the plant is to be cut annually, we cut each of those limbs back just one year's growth inside our line.

After cutting back the top of a limb, we shorten its side branches as necessary. We can be less harsh with these, since as side branches they will not grow so much as the tip would have. We simply shorten or remove side branches as necessary to keep growth within our limit and eliminate clutter.

Then, if it's a tree with multiple main limbs as in this sketch, we move on to cut the other main limbs the same way: Shorten the top and then trim side branches.

To make shortening cuts to the end of each branch, see the Diagrams, pyramidal evergreen and deciduous tree.

 

Main limbs are your choice

 

Selecting the framework for pruning is where some art comes into play. It's up to you to decide that the tree represented by the sketch above will be most beautiful with just five main limbs. Choose for limbs that separately and together draw graceful lines, and for branches that don't fight each other but can each occupy their own parts of the sky.

If the plant has more than one main limb, we start with the tallest. That would be limb 2 or 3 in this sketch.

If the plant has more than one main limb, we start with the tallest. That would be limb 2 or 3 in this sketch.

"Fight for the sky" is a concept requires considering the tree as it appears from above.  Here's that same cartoon tree with its "keeper" limbs and their foliage outlined from above. Although there is some overlap, there are no big duplications. Whenever you prune, you remove limbs that "cross" from one main limb into other limbs' territory, such as any branch that sprouts from main limb 3, crosses the center of the canopy and mixes with main limb 5's twigs.

"Fight for the sky" is a concept requires considering the tree as it appears from above. Here's that same cartoon tree with its "keeper" limbs and their foliage outlined from above. Although there is some overlap, there are no big duplications. Whenever you prune, you remove limbs that "cross" from one main limb into other limbs' territory, such as any branch that sprouts from main limb 3, crosses the center of the canopy and mixes with main limb 5's twigs.

Once those five are designated as "keepers," that guides decisions about which side branches are cut and how much, after the main limb's shortened.  In this example, limb A will come off because it's duplicating limb 1's foliage -- trying to fill the same air space. Likewise, limb B is cramping limb 3's style, and limb C must go because it makes the tree look bushy and its leaves are vying for the same air space as those of limbs 3 and 4.

Once those five are designated as "keepers," that guides decisions about which side branches are cut and how much, after the main limb's shortened. In this example, limb A will come off because it's duplicating limb 1's foliage -- trying to fill the same air space. Likewise, limb B is cramping limb 3's style, and limb C must go because it makes the tree look bushy and its leaves are vying for the same air space as those of limbs 3 and 4.

You can see photos of main limb selection and examples of overall effect such as these below, in Cut to control blue willow and  Crabapple shaped and reduced:

Step back to assess the shape

 

Natural shape's easiest!

As we cut side- and lower branches, decisions have to be made about shape as well as size. Step back from the plant from time to time as you prune. Take in the pruned top as well as the side you're currently working. From a distance you will see the whole picture and make better choices.

This begs the question of what shape a plant should be. That's totally up to you, but it can be a good idea to take the plant's natural form as a guide.

Some plants grow about as tall as wide, forming a round crown. Others are naturally pyramidal, columnar, horizontally spreading or vase shaped. If you prune to enhance the natural shape rather than to create a form that's contrary to that plant's nature, your annual or bi-annual pruning will be simpler and there will be less touch-up pruning to do on "errant" shoots in between.

Direct the new growth

 

As you trim each limb (see diagrams pyramidal evergreen and deciduous tree; and see Afraid to cut Japanese maple for photos of reducing and thinning limbs) imagine the remaining tip buds' growth. Whichever are highest above ground and furthest along the branch will eventually become dominant. The energy of the whole branch will flow first to the tip and top buds and the limb's growth will go in the direction they "point."

Sometimes it's necessary to shorten limbs that passed first muster, in order to leave one chosen bud in the lead. What you're doing is shortening branches that threaten the dominance of your chosen lead bud. In the case of the very highest bud remaining on a plant, the one you expect to take over as primary leader, you may want to take the extra step of using a soft, biodegradable tie to hold it in an upright, superior position until spring. Nature will take it from there.

Some plants sprout from bare wood, some don't!

 

On most needled evergreens including juniper, arborvitae, pine and spruce, new growth will begin only from a branch that has both needles and a tip bud. If you cut a limb on one of these evergreens to leave only a needle-less or tip-less stub, it will remain or become bare, then die. So make important cuts to just above side branches that have needles and tip buds. Take out any needless wood. (Photo examples of cuts to just above needled side branches in Shag cut for low juniper in Shrink or shape birdsnest spruce, and in Prune a mugo.)

Try not to worry about leaving gaps. Step back and look from a few paces away. Rest assured the remaining buds will grow to fill what may seem terrible gaps from close up. (Photo example in Shape a dwarf white pine.)

All deciduous plants -- those that drop all their leaves each year -- as well as evergreen rhododendron, azalea, euonymus, boxwood and yew, can sprout from leafless wood. So cut away!

 

What about flower?

 

We worry too much about flower. Pruning in late summer can certainly remove flower buds, but if you prune as described here flower buds will also remain on the plant. You only lose the bloom on the part of the plant you don't want -- the part that would be too tall or wide. (More about all kinds of pruning, and adjustments sometimes made for flowering species, in What's Coming Up 86.)

If you can't stand to lose a single bloom, then change your pruning schedule to do everything I've described here but do it within a few weeks after the plant blooms. That means you'll prune a cherry in late May, redbud and crabapple in June, and mophead hydrangea and weigela sometime in July.

In these cases you will find that you have to do touch-up pruning in addition to an annual or bi-annual clipping. Now and then during the year you'll have to thin overly bushy limbs and remove tips that break away from the outline. That's because pruning post-bloom, during a season of active growth, will stimulate branching and allow the plant time to set up a new tip dominance after the cut.

We prune this redbud every year right after it flowers. It blooms well but also needs touch up pruning throughout the year to keep it from overstepping its space in this small garden.

We prune this redbud every year right after it flowers. It blooms well but also needs touch up pruning throughout the year to keep it from overstepping its space in this small garden.

Be considerate of the plant's efforts and expense

 

When you finish your restriction pruning the plant may have lost a third or half of its buds or leaves, and a good portion of wood. The plant expended energy and used resources from the soil to make that material. It's time to pay for this.

In restriction pruning we do want to reduce a big plant's energy to some extent. That helps in the effort to keep it small. Yet we can't overlook the fact that plant parts which formed and were removed from the site before their time represent a drain on the soil's nutrient bank.

So spread an organic, slow release fertilizer over the root zone of the plant. Scratch it in or cover it with mulch.

Now you're done for a year or two. Enjoy the shapes you've made and the realization that you can plant even more so long as you keep pruning.

Here's another good reason to do restriction pruning of evergreens in fall. The top of this hemlock will make a beautiful small Christmas tree, and the clippings from side branches can make a wreath for the door.

Here's another good reason to do restriction pruning of evergreens in fall. The top of this hemlock will make a beautiful small Christmas tree, and the clippings from side branches can make a wreath for the door.

Restriction pruning, step by step

 

Deciduous tree or shrub:

1 - Set height and width limits as you desire.

2 - Check top growth to determine the plant's growth rate.

3 - Reduce the top growth to within your desired outline. Reduce it by one or two years' growth, to match your pruning schedule. This tree will be pruned annually, so its leader will be cut as shown. Remaining tips will be able to grow for one year before breaking the height limit.

4 - Some branches may remain that are within one year's growth of a height or width boundary. This is acceptable. Since they formed as interior and side branches, their growth will be slower than a tip branch for much of the next year.

5 - Tie in a new leader, if desired for shaping. Use soft, wide, biodegradable material to tie a flexible side branch into upright position. Prune all other branches to preserve the status of the tip of this new leader. It must be the highest bud on the plant.

6 - Shorten all other main branches so they can complete one year's growth but still be within the desired outline. Clip each one back to a side branch that will become a well-placed, graceful new tip. It may be necessary to prune branches that may compete with this new tip.

Note - This drawing is two dimensional, a "slice" through a tree which has main limbs radiating in all directions from the trunk. In most cases you will be pruning numerous sets of side branches, which are not shown here in order to more clearly illustrate the process.

 

Step by step: Pyramidal evergreen

1 - Height and width limits you set. Pruning is simpler if it enhances the plant's natural shape, rather than imposing an unnatural shape. This spruce is easy to maintain as a pyramid. It would require more frequent touch-up if pruned into a column or rounded form.

2 - Check top growth to determine the plant's growth rate.

3 - Reduce the top growth to within your desired outline. This tree will be pruned every two years, so its leader will be cut as shown. The new tip will be able to grow for two years before breaking the height limit. The cut leaves a stub which will serve as a brace when the new leader is tied in.

4 - Tie in a new leader if necessary. (Unnecessary in this example, where only a very young shoot remains to become the new leader.) Use soft, wide, biodegradable material to tie a flexible side branch into more upright position. Prune all other branches to preserve the status of the tip of this new leader. It must be the highest bud on the plant.

5 - Shorten all other main branches to two year's growth within the desired outline.

6 - Some branches may remain that are within two years' growth of a height or width boundary. This is acceptable. Since they formed as interior and side branches, their growth will be slower than a tip branch for a year or two.

Note - This drawing is two dimensional and shows only the uppermost part of the plant in order to more clearly illustrate the process. In most cases you will be pruning numerous sets of side branches, radiating in all directions from the trunk. In Reduce a Spruce you can see this process in photos.

This spruce tip came from a spruce I prune every two years to keep it 10 to 12 feet tall and 6 feet wide. Its growth rate is about 18  to 24 inches per year so I remove at least 3 feet of leader each time I prune it, and tie in a side branch as a new leader.

This spruce tip came from a spruce I prune every two years to keep it 10 to 12 feet tall and 6 feet wide. Its growth rate is about 18 to 24 inches per year so I remove at least 3 feet of leader each time I prune it, and tie in a side branch as a new leader.

For hands-on practice in this technique:

 

Attend  Garden by Janet & Steven pruning sessions and classes (we post them as they are scheduled, in Where we're appearing.) There you can complete that step that is so important to many gardeners, to try the process hands-on.

 

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Related links

  • Prune to keep a tree small
  • Crabapple shaped and reduced
  • Cut to control blue willow
  • Afraid to cut Japanese maple