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trees and low shrubs in the museum landscape

Pruning with a Purpose: A Gardener’s Guide to Thinking Like an Arborist

Don’t just cut branches, prune with a purpose! Learn from Smithsonian Gardens arborist Jake Hendee how to add another dimension to your tree and shrub pruning skillset. 

 Resource Pages: 

 Knowing and Identifying your Trees 

Stem Girdling Roots 

Introduction to Pruning 

Structural Pruning 

Other Pruning-Related Resources 

Q&A: 

Q–Please recommend resources for pruning conifers. 

A–I can’t suggest a great book-length resource for pruning conifers other than general pruning books such as Dr. Edward F. Gilman’s Illustrated Guide to Pruning, but there are a number of very helpful online resources and fact sheets available from public gardens and university extension services. A quick search reveals a nice synopsis of the topic from the Morton Arboretum here: https://www.mortonarb.org/trees-plants/tree-and-plant-advice/horticulture-care/pruning-evergreens. The primary challenge for pruning conifers is understanding where the lateral and latent buds on that particular tree species are. It is an all too common mistake to see gardeners unintentionally kill a large branch or entire tree or shrub by expecting lateral buds and latent buds where they do not exist. 

Q–Can you give advice on what to do with a small or “volunteer” tree that has started to develop two leaders?   

A–Assuming your objective is to improve structure to grow a strong, long-lived shade tree and that the tree species and its site are compatible with that objective, your ‘volunteer’ sounds like a great candidate for structural pruning! I’d suggest starting with the University of Florida’s structural pruning page (https://hort.ifas.ufl.edu/woody/structural-pruning-flash.shtml); an online search should yield a huge number of additional great resources. 

As a very general statement, one of the first steps for structural pruning a tree is to reduce or remove a second, competing leader. 

Q–I have a Nandina that has turned into a tree.  How should I cut it back?  It is really wide and tall. 

A–I really wanted to get into shrub pruning techniques a bit more with this presentation, but time was too limited. I’d suggest looking at shrub pruning resources such as the Morton Arboretum’s Pruning Deciduous Shrubs page (https://www.mortonarb.org/trees-plants/tree-and-plant-advice/horticulture-care/pruning-deciduous-shrubs). (I know, Nandina isn’t technically deciduous!) The section about renewal pruning sounds most helpful to you, but the shrub’s response is of course dependent on its current health, site factors, and all of the other factors discussed in the talk. 

Q–Is there a better season to prune trees – late fall or early spring? 

A–I’d suggest doing some research and knowing your tree before picking up the pruners. Considerations such as disease transmission season (oak wilt, Dutch elm disease) and whether flowers bloom on the current season’s growth or the previous season’s growth can be important to deciding when to prune, but most trees will respond well to good pruning throughout the year. I especially like pruning during the fall, winter, and early spring because you can see the structure of the tree the best, there is less debris, other gardening tasks are on hold, and it isn’t too hot!  

I’ve seen a lot more trees harmed by the lack of pruning than by ill-timed pruning. I’ve also seen a lot of trees harmed by bad pruning, so do your research and move slowly and methodically until you gain confidence. 

Q–Regarding mulching, I live where it is five degrees cooler than Hades in the summer.  We are over 100˚F for 3-4 months (no rain) and often over 110˚ for a week or two or three or more.  In addition, I am under water restrictions and can only water 2–3 days a week during the summer (the rest of the time it is once per week).  I mulch heavily to keep the soil cool (BTW we cool off to about 80˚F at night).  Should I not mulch around the trunk of the tree?  I use natural mulch so it breaks down (the soil is terrible here) and to keep the roots cool. My yard is all on drip including all of my trees. 

A–The problem with over mulching against the base of the tree is that all of that moisture conserved by the mulch can’t make it to the leaves of the tree because the tree’s vascular system becomes quickly constricted by stem girdling roots. Remember that tree roots often extend far out beyond the tree’s dripline. Rake that mulch wide out, not up! A wide mulch ring 2-4” in depth (as wide as you want!) and, most importantly, not touching the pronounced flare at the bottom of the tree are nearly always the best ways to conserve moisture and provide great (highly-organic!) soil without killing the tree. Here’s a great, easy fact sheet on mulching from the International Society of Arboriculture: https://www.treesaregood.org/portals/0/docs/treecare/propermulching.pdf  

Q–How do you prune a circular root?  

A–I’d suggest looking at A Practitioner’s Guide to Stem Girdling Roots of Trees. The tricky part about circling and stem girdling roots is that the root that may be killing your tree may also be the root keeping it alive. If the tree is important to you, proceed cautiously and don’t be afraid to consult a professional. 

Q–People in my neighborhood are starting to treat mature trees like a person’s head and giving it a complete and damaging “haircut.” Is there any resource you know of in the D.C. area that provides hang tags I could distribute in my neighborhood to help end this disturbing practice? 

A–I agree – I see the topiary pruning system misused a lot. Fortunately, the abuse of topiary pruning is usually just ugly and expensive to maintain, but it generally doesn’t create other really significant issues. Another pruning abuse, tree topping (see https://www.treesaregood.org/portals/0/docs/treecare/WhyToppingHurts.pdf), does a lot of damage to our urban tree canopy and creates many dangerous situations. 

I am not aware of any resources that can alert people to the many well-intentioned but destructive tree practices we see some doing in their home gardens. 

Q–Should latent buds be removed? 

A–No, latent buds can be quite helpful, and removing them would be quite damaging. These buds are often hidden in thick bark. The annoying adventitious, succulent interior branches known as “watersprouts” are sometimes messy and numerous, and unfortunately a result of latent buds. Watersprouts are usually best removed; a stem exposed to excessive light from overpruning or outer crown dieback due to tree health issues is going to have many more problems with watersprouts than another tree without overpruning or outer crown dieback. You might find the presentation slides and further reading about eco-dormancy helpful in managing these branches. 

Q–How often in a year can an Eastern Redbud be pruned? I have an 8-year-old Eastern Redbud in a rowhouse backyard. The crown is very, very dense and grows quickly. I have it pruned once a year at this point, with the goal of thinning the crown. Can this very prolific tree be pruned twice a year? 

A–An eastern redbud sounds like a great tree for a rowhouse backyard! It should be no problem to prune it twice a year, as long as other considerations discussed in the presentation are appropriately addressed. For trees at home that I start from seeds or cuttings, I prune them multiple times each year with very light doses, in perpetuity. I can’t help but read this question as framing your objective as “controlling the size of the tree” which can be a very challenging goal to accomplish because of the havoc that plant hormones can wreak when trying to control the size of a healthy tree. Maybe taking a second look at your objective would help. Can you re-frame your objective from controlling the size and prolific nature of the tree to one of minimizing and managing the conflicts its size and vigor create? Is it possible that plant hormone confusion caused by frequent heavy pruning is contributing to the almost unmanageably prolific growth response? 

Q–Nurseries cut back terminal buds to remove suppressive auxins and encourage lateral branches. But then how do they restore having a single lead trunk if the whip has been “topped”? 

A–I love this question! I think the answer lies in the difference of objectives behind heading off the terminal bud of a young nursery tree to encourage branching and canopy development (a good practice!) versus topping a mature tree to control its size (not a best practice!). In the nursery, the top lateral branch that fills in behind the headed–off terminal bud will be the new central leader, and good nurseries will do proper structural pruning to favor the continued development of that central leader. In the landscape when a tree is topped, decay almost always sets in and the removal of the suppressive auxin sources results in a flush of growth attached to wood that is now decaying—not a good situation.  

See this resource for more about the topping of landscape trees: https://www.treesaregood.org/portals/0/docs/treecare/WhyToppingHurts.pdf 

Q–Please describe exactly where Cut #3 (cutting a branch at the trunk) should be made. 

A–This is such an important point to avoid damaging a tree. Cut #3 should be made just outside the visible swelling known as the branch collar. For branches that don’t show a clear branch collar, most pruning how-to’s detail how to find this line by looking at the angle and position of the “branch bark ridge” relative to the branch you are removing: 

Q–Can you speak to pruning a fruit tree…apple or peach? 

A–That’s a huge topic, but I’d emphasize the necessary objectives of fruit tree pruning systems. Branches need to be close enough to ground so fruit can be picked and strong to hold heavy loading from fruiting. Thinning out can reduce flower buds and ensure the tree has the resources to get a full crop to maturity. Better air flow through the tree can help discourage—but not prevent—some pest and disease issues. I think you’ll find the many fruit pruning systems combine these and other objectives to maximize the success of getting a crop, whether it be to market or for a homemade peach cobbler. 

I think many new fruit growers depend on pruning to fix pest and disease issues that are not necessarily pruning related. Remember, fruit trees also require a successful integrated pest management plan. 

Q—I just lost a Swamp Maple–very sad. Please provide a recommendation for a fast-growing replacement. 

A–I’d suggest consulting a regionally-specific tree selection guide such as the Casey Trees Tree Selection Guide to find good options. When searching for a fast-growing replacement, I’d encourage you to make your best common–sense assessment of the moisture, soil, light, and climate conditions the replacement needs to have. There are plenty of resources out there that are able to help with options such as your local extension service. 

Q–If a Steeds holly loses all of the leaves in one area due to lack of sunlight (shade from an overgrown neighboring bush), is there any way to revive growth where the leaves have fallen off?  (The neighboring bush has been removed.) 

A–Latent buds to the rescue! There’s a good chance that when latent buds on the holly receive that extra light, they’ll break eco-dormancy and fill that side of the holly back in. It may take a while, though, to fill all the way back in. 

Q–Can you address crape murder? 

A–Avoid it! I generally recommend reading the crape myrtle pruning publication from Knox and Gilman at the University of Florida. Here at Smithsonian Gardens we’ve used a specialty pruning technique called pollarding in one specific context to avoid the ugly and indiscriminate topping of crape myrtles popularly known as “crape murder,” but we prefer letting crape myrtles grow to their natural forms and heights. 

Q–When is the best time to prune crepe myrtles? What is the best strategy to keep them from getting too tall? 

A–Depending upon your objective, crape myrtles can be pruned any time of the year. They flower on the current year’s growth, so you’ll see people tend to favor good crape myrtle pruning and not-so-good crape myrtle pruning (“crape murder”) during the dormant late fall, winter, and early spring months. 

Managing height can be done by: 1) planting an appropriately size cultivated selection of crape myrtle, 2) pollarding, and/or 3) tip pruning. I strongly discourage “crape murder”—the ugly, damaging, indiscriminate, and haphazard “topping” of crape myrtles each year. I see a lot of crape myrtles managed for height that don’t need to be, so make sure your pruning objective of size management is consistent with your tree, site, and intended landscape design. 

Q–Three years ago (before I knew better) I planted a Chinese dogwood with organic amendments in the hole.  The tree has started to sink a bit.  I have cleared the dirt around the base, but this has resulted in a bit of a bowl shape around the base of the trunk.  Should I remove more soil to expose the root flare?  Will the root flare raise a bit as the tree grows larger?  The tree is currently about 10-12 ft. tall and looks ok health-wise. 

A–I’m very familiar with this situation when trying to address trees that have been planted too deeply in the past. It’s not ideal, but I very much prioritize having a clearly exposed root flare in most cases. A lot of existing trees in our landscapes have major issues related to planting depth, so we end up addressing many of these root flare issues with a compromise that is the “least worst scenario.” The root flare will grow up slightly as the tree ages, if it is not constricted by stem girdling roots. 

Q–My Meyer Lemon tree looks healthy and produces lots of lemons; however, most of the branches have lost leaves towards the trunk of the tree. Pruning recommendations? 

A–A Meyer lemon tree is a bit outside my comfort range, but I think you’ll find many trees retain foliage where conditions are sunniest and best for growth, and drop leaves in heavily shaded parts of the canopy. If having interior leaves is important to accomplishing your objectives for the tree, progressively thinning the outer canopy (more about thinning here: https://hort.ifas.ufl.edu/woody/thinning.shtml) would likely introduce enough light to help break the eco-dormancy of the shaded interior latent buds. 

Q—What are your thoughts on how early in a tree’s life cycle one should prune? 

A–I prefer to start pruning trees on the Smithsonian campus about one year after transplanting to our landscape, but there’s a very good case to be made to prune the tree on the day you plant it. The important thing is to start pruning for your identified objectives as early in the tree’s life as possible and continue to do so in frequent, light pruning doses. For trees at home that I start from seeds or cuttings, I prune them multiple times each year with very light doses, in perpetuity. 

Q—How do you sharpen a long pole pruner? 

A–The designs and blade types vary a bit, but I’m able to use the same sharpening tool for my pole pruner that I do for my hand pruners and loppers.