Boxwood Pruning

What better place than our Nation’s Capital to have a bipartisan discussion about shrub pruning. We are going to discuss three different boxwood and how to prune them.  Never mind the Latin; it’s not going to help.  These three boxwood are:

  • English boxwood
  • American boxwood
  • Korean boxwood

No one in particular is my favorite and all three do different things.  American box is the tallest, English boxwood is the fattest and Korean box has a little of both.  I have an acquired affinity for these Asian immigrants, and I believe they have earned their keep.  They are quite adaptable to most surroundings and have made themselves comfortable where others have not. 
There are two techniques for pruning boxwood, especially English box.

  • Pruning with hand pruners
  • Pruning by plucking or breaking with your fingers.

Both are acceptable, and have been done for hundreds of years.  I use bypass handpruners and love my Felco 8’s and 6’s!  If you have problems with your hands as I have, I suggest you try the Felco 6’s.  They are a comfortable pair of smaller hand pruners.

How does boxwood want to be pruned so that it can reach its genetic potential and live in the site we have chosen?  Arguably, pruning a plant incorrectly is not the worst thing we can do.  It may look like it but it’s not nearly as cruel as forcing a square plant into a round planting hole and then wondering why it looks the way it does.  If we logically and knowledgeably put each of our plants in the perfect location according to the demands of that plant we could solve a great many plant problems.  Dr. Bonnie Appleton says, plant according to environmental requirements first, size second, maintenance requirements third and finally according to our esthetic desires.  So, let’s assume for pruning purposes our boxwood is in the perfect location and see what pruning delights we can come up with. 

English boxwood:  I believe pruning English boxwood properly is the best step towards good health and should be the first step in any plant health care program.  English boxwood becomes dense, too dense for their own well being.  They seem to outgrow themselves, becoming so tight that everything on the plant’s interior dies from lack of sunlight.  Boxwood, as any other woody plant, must have leaves in order to survive.  They feed themselves through their leaves.  We don’t feed boxwood or any other woody plants for that matter.  It is critical that we understand this tid bit of information clearly; we do not feed plants.  Plants feed themselves.  We simply supply elements to the soil through the application of various materials that I am afraid are often applied incorrectly.  We can discuss that later as well.  Plants need leaves to feed themselves so the more leaves we can help them produce the healthier the plant becomes, right?  As is the case with any other broadleaf evergreen, opening the plant just the right amount to allow for the infiltration of sunlight will spark interior growth of leaves creating a larger food source within the plant.  We want to prune our English boxwood in an open fashion allowing the plant to produce leaves along its interior surfaces, a phenomenon that begins soon after pruning, and is a great reason to prune boxwood just before bud break.  It is critical that we thin our English boxwood.  Thinning English boxwood fulfills two important requirements.

  • It opens the plant to light and air so the boxwood can produce leaves along its interior surfaces.
  • By producing leaves and twigs along its interior we now have new areas to which we can reduce the plant should the boxwood ever need to reduced in size.  Pruning boxwood correctly is an important investment in the plant’s future quality of life.

This type of thinning is critical with English boxwood but does not happen overnight.  The process begins soon after pruning but takes years of continual pruning to get the box where it needs to be.  But you say I will have holes in my boxwood.  You are correct, but your holes will be uniform and possess continuity.  The openings in a shrub are just as important as the limbs that remain.  Have you ever seen a polka dot dress with one great big polka dot in the middle of the dress?  Of course not, and the shrub with one great big hole in the middle would look just as ridiculous.  Thinning English box takes some biological prowess coupled with an artful eye.  Openings in the plant must be thought out and uniform or the hole we have created will look like a mistake rather than a plan. 

The first year you open your boxwood it will be difficult to reduce without making it look like that is what you had in mind; so, concentrate on this being a long term project with the first step being the thinning phase with some reduction two to four years into your plan.  Remember pruning should be subliminal and somewhat mysterious.  You want someone to look at your work and say something looks different rather than,”boy, someone sure gave that plant a haircut”!  If we try to reduce the plant the first year, the above will likely be the response. 

How much do we thin is usually the next question.  Lynn Batdorf put it best.  Spread the boxwood with your hands and look inside.  Measuring from the top of the plant to the inside, towards the bottom, notice how the leaves stop growing along the stems the further you get into the plant.  This is due to lack of light.  You may have six-inches or maybe a foot of green leafy growth on the stems from top to the inside.  In general, when you thin, only take out as much of the stem as you have length of green growth.  Did that make sense?  If you have nine-inches of growth, only prune out a section of stem up to nine-inches long.  You can take out more but as a rule this seems to work well.  If you follow this green layer all around the top and sides of the plant, your boxwood will be well thinned.  Try to choose the longest sprigs throughout the top of the plant and you will be able to reduce it a bit while thinning it at the same time.  Remember to prune the whole plant not just the tops that are easily accessible.  Your plants have sides that must be thinned as well.

When should I prune my boxwood?  Good question.  I have pruned them during all seasons, even during their growth period in spring, but if I were to choose, I would pick the dormant season as the best time to prune, after the threat of sub-zero weather has passed.  So, I try to prune all of my boxwood between February 15th and May 1st.  This begs the common question, “what about Holiday décor?  I don’t think Pan and Santa ever argued about this so, simply take the garland you need in November-December and leave any hard or heavy pruning for after February 15th.  I get arguments about this every year and here is my response.  I was once asked to prune a row of stunning English boxwood in January.  I pleaded with the homeowner not to make me prune them during that time of year.  They insisted it be done right away.  I begged again, pleaded my case and lost.  In short, I pruned the plants during icy weather, the temperature went to 5 degrees that night and the next week every boxwood was straw brown.  The homeowner demanded the plants be replaced and I’ll let you guess who replaced them.  The moral of the story is; boxwood that have been recently thinned can be affected by great temperature fluctuations, so I recommend any heavy thinning be done after February 15th in the Mid-Atlantic.  If I may beg your patience, here is one more bit of experience I will share with you.  In my career, every time, and I stress every time I have done something to a client or with a plant that I knew was the wrong thing to do, it came back to bite me, whereas doing the right thing as a professional is always fulfilling.  Our clients have chosen us out of trust and respect and we can return that respect by doing what is right for them and their plants.  This is one part of business that I use as a guide.  It not only helps me work, it helps me sleep. 

Korean boxwood can be pruned just as English boxwood; in fact they mimic each other quite well.  Many properties are beginning to use Korean rather than English boxwood due to the Korean’s sturdiness and resilience.  When English boxwood becomes tight as we mentioned, they begin to lose their leaves, internally.  Korean boxwood, on the other hand, has the innate characteristic of holding their inner leaves even when tight.  Korean boxwood is a tough, lovely plant and I recommend them hardily.  They do not have the exact look of the English and in general lack the blue green foliage of English and American.  I believe some of this lack of color is due to how they are pruned.  I have used my own experimental group of Koreans and have opened them drastically every other winter.  They are now just as blue as they can be.  Proper, genetically respectful pruning leads to results that can be measured not only botanically but ethereally as well.  When I thin Korean boxwood I know that I can make larger cuts and not be worried about how much I am removing.  They respond well to hard pruning and can be thinned just as the English.  Korean boxwood is a great plant and the timing for pruning them is the same as English.  Many of the Korean I had to prune and repair after the snows of 2010 and 2011were all but crushed and lost.  Many I had to cut back to sprouts.  This year they once again fill their spaces and to my surprise need pruning again this winter. 

Now, for the big guys, American boxwood.  Years ago, Ed, Craig, Ron and I had to climb a stand of American Box with rope and saddle.   American box like those are quite common in the Mid-Atlantic.  This boxwood is completely different from English or Korean and comes in smaller varieties as well as the common larger variety.  I am in particular love with one called Dee Runk, an upright fastigous variety.  Again, Lynn Batdorf says generally, the only thing American boxwood need if given enough room is the continual removal of deadwood and branches that are in someone’s or something’s way.  This boxwood grows in a more open fashion and can actually be pruned as you would a small to medium size ornamental tree.  Shorten them only if necessary, but they will withstand a significant reduction pruning.  Just as any woody plant it is best not to reduce them so give them the room they need and let them live for hundreds of years.  Species and site selection is the pioneer decision when you begin your planting project.  I think that’s what I will put on my headstone, “In this well dug hole lays the correct, well buried, human being”. 

Let’s review:

  • Thin English boxwood to allow the passage of light and air to the plant’s interior.
  • Thin English boxwood annually until the interior branches are covered with leaves.  At this time you will be able to reduce your plant if that is the goal.
  • Prune your boxwood lightly for Holiday décor in November-December, and save any heavy thinning of boxwood until the threat of sub-zero weather has passed.  Try to complete your pruning before bud break in May.
  • You can thin Korean a bit more than English during one pruning; otherwise they prune very much the same.
  • American boxwood tends to only require the removal of deadwood and some light thinning. (But, if you need a rope and saddle, call a professional!)
  • Allow American boxwood to grow unrestricted, giving them all the room they require, which is a good amount.
  • I promised we would touch on it and this seems like a good place.  Even the old garden books admonish us not to shear boxwood and I have seen too many wonderful old gardens with boxwood that are in a spiraling decline due to this pruning practice.  There is a place for shearing in the garden but boxwood is not one of those places.  They already grow so tight that they lose their interior growth and shearing simply exacerbates the problem.  Shearing is a practice that can lead to other maladies suffered by boxwood.

Bugs and diseases, (as long as we are at it)!

  • Boxwood need to be planted in a well-drained site.
  • One to two inches of mulch is all they require.
  • Keep boxwood branch crotches litter free and clean them of old leaves each year.  This is not only a healthful practice but a cosmetic one as well.  A blower works well.
  • If your boxwood will not grow in a particular site, choose another type of plant.
  • The insect, psyllid, will bother all three boxwood but is not a serious threat to their health.  This little leaf-cupping pest is more of a cosmetic concern than one of health.
  • Mites will affect all three boxwood and should be treated.  Mites can have eight generations annually, so it is best to meet with your arborist for effective timing and control.
  • Leaf miner, (As a rule) only affects American boxwood and will destroy the plant if left unchecked.  Always, always, always treat to aid in the control of leaf miner on your American boxwood.  A hefty, runaway herd of leaf miner on your American boxwood will begin to defoliate your plant during the second year of infestation.  There are few bugs over which I fret but this is one of them.  Do not let leaf miners go unchecked on any American boxwood.  Lynn Batdorf stressed this point as well.  By the way Lynn, thanks for all your help.  You really got me started down the right road to boxwood culture, as you have so many others.  Everyone, be sure to buy Lynn’s’ book on boxwood.  The name of the book is, Boxwood Handbook, A Practical Guide to Knowing and Growing Boxwood by Lynn R, Batdorf.  I use it all the time, thanks Lynn!

That should do it.  Hope it all makes sense.  I believe thinning is the most important issue concerning boxwood and for that matter is applicable to all evergreen plants.  Here’s a funny truism:  Removing leaves and branches in a botanically correct, uniform way will allow the same plant to become thicker and denser.  That’s right, the more we remove correctly, the more the plant will put back.  I won’t say that for large shade trees or many deciduous ornamentals but with most broad-leaved evergreens, it works.  Give it a try and watch what happens.  Don’t just let your boxwood stand there bored to tears, prune them! 

Happy Pruning, Peter!

Copyright 2007 by Peter Deahl. All rights reserved.

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