Join Smithsonian Gardens arborist Jake Hendee for a thought-provoking Let’s Talk Gardens examining the epidemic of tree and shrub health problems caused by our most basic garden maintenance practices.  Healthy trees, shrubs, and urban forests require a paradigm shift about how we manage the trees in our gardens, yards, parks, and streetscapes. You simply won’t ever see trees the same way again.

Webinar Video

 

 

Q & A 

About Stem Girdling Roots

What is it about the deep planting and over-mulching that causes the tree’s roots to circle? Are they just not able to grow in the correct direction through all the soil and mulch?
Trees and shrub roots will grow in almost any rootable soil. Unfortunately when soil and/or mulch is piled over the root flare, it introduces roots around the tree’s most vulnerable part—the relatively thin part of the trunk above the widened part. In contrast to the root flare with large horizontal roots that block circling roots, there is no obstruction on this thin part of the trunk to prevent roots from circling and eventually girdling the tree. 

Can you tell us once again why roots grow around the trunk?
Roots grow anywhere that there is rootable soil. When we pile mulch and soil up around the base of the tree, the obstruction that the root flare naturally poses to circling roots is negated by the soil/mulch that is up around the narrower part of the stem. 

At Smithsonian campus, how many trees have you taken down due to unfixable root issues?
It’s a lot more trees than we lost to pests and diseases combined! 

Can SGR happen on mature trees?
Yes, it’s very common. And slowly losing a large shade tree to preventable and foreseeable issues is devasting. 

Are there varieties of trees that are less susceptible to SGR?
Yes, but I can’t point you to a good resource. Most trees will develop stem girdling roots to some extent if exposed to some combination of overmulching, mismulching, deep planting, and poor nursery practices. 

Root Flare 

Can you explain the flare on top of a rootball?
The definition of root flare is the widening at the base of the trunk where roots begin to form. For purposes of planting and mulching correctly, I’d suggest using the following rule as discussed in the presentation: If you can clearly discern the location & direction of at least 3 primary structural roots as they run radially from the tree, then you are looking at the root flare. 

How far should the exposed root flare extend around the tree?
The literature and industry standards are frustratingly vague on this topic, but  it’s my opinion that the root flare should be exposed enough that you can clearly discern the location and direction of at 3 primary structural roots at the flare. I’ve never seen a tree damaged from having too much root flare exposed, but I’ve seen ten or hundreds of thousands in decline/dying/dead from root flare issues. 

Selecting and Purchasing Trees 

So is the best practice to buy bare roots and plant them instead of from a nursery where they are already potted?
Containerized or potted trees generally have poorer root systems but are really convenient because they store well, are easy to transport, and short-term establishment is quite easy. On the the other hand, bare root trees generally result in better root systems but require very specific handling, are more difficult to establish, and transplanting seasons is limited. Field grown or B&B trees fall somewhere between these two extremes. Know the costs and benefits of each approach, and adjust accordingly! 

In a new nursery tree when all roots are under the top soil of the pot how do you find the flare roots?
Very carefully with a soil knife, trowel, pruners, probe and/or other tool. Ideally, you would hope to find the tree in the nursery or garden center with the root flare already at the right height, but that’s currently pretty difficult to find. 

How can you tell if a nursery tree has circling roots as they may be buried deep in the pot?
You can’t always tell 100% without doing a destructive autopsy of the root ball. I look in two areas: what’s above the root flare and what’s circling the outside of the root ball where it meets the container. If I can confirm that roots aren’t circling/girdling and/or can be easily corrected because the circling/girdling roots are small, the tree is good enough for me. 

How do you inspect in the nursery?
Ask kindly for permission or even ask for help finding the root flare of the tree. Until we as consumers make our preference politely but clearly known, nurseries and garden centers will continue to provide us with trees that may not have the best long-term prospects. 

Is washing all soil off of roots before planting, helpful?
Some people do this, and we’ve experimented with it. Bare rooting a tree before planting certainly helps correct root defects, but it’s certainly a lot of work and increases chances of short-term tree failure if the tree isn’t well cared for. 

How would I identify an “incorrectable circling root” at the nursery / garden center?
Uncorrectable = Big. The bigger the circling or stem girdling roots, the more difficult it is to correct without damaging the trees. Some “rules of thumb” would indicate that any root more than about 25% of the cross-sectional area of the main trunk on an otherwise healthy tree is “uncorrectable”. 

Planting 

How do you best handle girdling roots at time of planting?
At the time of planting, it is best to remove any circling or girdling roots above the root flare. Removing large or numerous roots at planting of course adds some short-term stress for the tree, so your best bet is trying to find trees free of roots above the flare that have been grown properly in the nursery field or in the container nursery. 

Any guidelines for width and depth of planting holes for trees and woody shrubs?
The planting hole should be no deeper than necessary to plant your tree with the root flare clearly above soil grade, about even with final grade. (See question below about how to prevent subsidence.) The width is ideally 3x the width of the root ball. 

 We can plant flare 2-3” above grade but how do we prevent subsidence of soil over time?
I have this problem too! Remember that the bottom of your planting hole (and only the bottom!) should be undisturbed and/or re-compacted so that it doesn’t subside. (Soil on each side of the tree should not be compacted). If you’re not sure that alone is going to work, then just plant a little higher!  Not many trees have been killed by planting too high, but scores of trees have been killed by planting too low. 

What do I do if the tree that was planted by my village in December was planted too deeply? Replant it or something else?
If it’s your tree on your property, you could certainly try to handle cautiously and replant it a little bit higher. If it’s on the city’s property or public right-of-way, I might kindly reach out to my city arborist or public works department and let them you know you care about their trees succeeding on your street!  Be kind!  This is a tough issue to fix, and they may already be working with their employees and landscaping contractors to fix these issues already. 

Many trees are sold with a year guarantee,  If they are already planted, what recourse does the consumer have if now we know they have been poorly chose and/or poorly planted?
It really depends upon the agreement, but I’m not familiar with situations where this has been corrected other than very immediately after installation. I’d encourage focusing on getting the next tree right and doing what you can to correct existing issues (such as discussed in the last third of the presentation). 

Mulch 

What is bad mulch?
Anytime mulch is covering the root flare of your tree or shrubs, that’s bad mulch. 

Do we need to plant trees with any mulch?
Mulch properly applied (not over the flare!) will help the short-term survival of your new tree without compromising the long-term prospects of the tree. 

Why do we mulch – is it better to just not mulch?  
Mulch is great for trees in terms of conserving moisture and improving soil, but only if it’s not killing the tree too! A school of thought has developed that maybe we should not mulch our trees because we just end up damaging the trees most of the time; the more optimistic answer would be that mulch is amazing for our trees is we can correct our old habits and keep the mulch off of the root flares. 

Is it best to only mulch between the radical roots then?
Mulch between the large radial roots that are slightly raised above ground is pretty safe to do, but be cautious that continued mulching doesn’t end up on top of the main root flare! 

I work in a public library garden.  The big mowers from the “landscaping company” ding the tree bark or mow the tops of the root flare.  So we have put a 5 foot bark mulch of 2-4”, away from the trunk.  Is this good practice?
This is a great practice as long as that mulch isn’t piled over the top of the root flare. In my experience, a lot of the mulch that is applied to “protect” the trees from lawnmowers ends up causing damage that looks like true lawnmower damage. 

 Do deep leaves in a forested area have the same effect as mulch volcanoes? Should I not move excess leaves to the woods?
Leaves in a forested area do not accumulate as quickly as mulch volcanoes tend to be built up. All-the-while, the top of the root flare slowly rises as the tree grows. Stem girdling roots do occasionally happen in nature, but not at the rate that they do in our landscapes. And, if a tree falls in the woods and nobody hears it, does it make a sound? 

What about trees that do not have mulch but have years of fallen leaves (from same tree) around its base?
See the answer above. Naturally fallen leaves will generally not build up enough to damage the tree, but a pile of fall leaves swept up from around the yard for several years certainly could cause some problems. 

What about LeafGro or compost?  How close to the trunk?
Compost is also generally great for tree root zones, but only if not applied over the top of the root flare. I would generally use the same rules as mulch. 

Has there been a discussion about mulch previous to this presentation?  I see many types available at big box stores …especially dyed mulch.
I don’t think we’ve done a Let’s Talk Gardens about mulch itself, but thanks for the idea! 

Should ground covers be a minimal distance from trunks?
Sometimes ground covers like English ivy can trap enough soil and mulch around the flare of the tree to cause problems, but generally ground covers or perennial plants aren’t going to cause stem girdling root issues. Depending on the type of plant, it may aggressively compete with the tree for limited moisture and nutrients. 

Pine bark mulch vs hardwood. Is one better than the other? What about wood chips?
There are costs and benefits to each, but I love all three products for tree care and gardening in general. 

 Fixing Stem Girdling Roots 

So what do I do about a tree that has girdling roots?
Check out the last third of this presentation. There are plenty of options, but it’s often much easier to prevent these than it is to try to undo 10-20-30-100 years of unchecked stem girdling root growth.

Can anything be done to help a tree with visible girdling roots?
Check out the last third of this presentation. There are plenty of options, but it’s often much easier to prevent these than it is to try to undo 10-20-30-100 years of unchecked stem girdling root growth.

Can a tree or shrub with SGR be rehabilitated?
Check out the last third of this presentation. There are plenty of options, but it’s often much easier to prevent these than it is to try to undo 10-20-30-100 years of unchecked stem girdling root growth. 

If you cut the stem girdling root can the tree survive?
It depends upon the size of the root and how much of the root system it makes up. A lot of the literature indicates you can remove stem girdling roots up to 25% of the cross-sectional area of the trunk on an otherwise healthy tree, but I’d encourage you to consult a certified arborist if the tree is important to you and/or if you’re unsure.  

What is the best time of year to remove girdling roots, especially the larger sized ones? when dormant?
Right on – the dormant season to minimize stress to the tree. But many small circling and stem girdling roots can and should be removed most anytime you have the opportunity. I save the big ones for fall and winter. 

If the tree is planted deeply and it’s below grade, how do you keep it from just filling back in with soil if you remove soil to find the flare?
This is one of those find the least worst case scenario situations. I generally look for an opportunity to leave a slight depression or slowly adjust the grade over time. Oftentimes, this situation is a matter of accepting the tree wasn’t planted well, and that I will do better next time. 

I have a tulip poplar the is about 1 1/2 foot across it has one foot bout 1 1/2 in in diameter across the top of one of the root flare roots.  Can I just cut it?.  This tree grew wild in my lot and has never been mulched.
As a general rule of thumb, removing a 1 ½” diameter root circling the base of a healthy 18” diameter tulip-poplar with hand pruners or loppers should not cause any health problems at all. If in doubt and/or if the tree is really important, consult a certified arborist.  

Other 

Can you talk about SGR as it relates to shrubs grown in large containers (ie: like those in areas like the Haupt Garden)?  Should shrubs  in containers be regularly repotted and if so, on what schedule?
Trees and shrubs in containers such as patio containers will eventually become quite rootbound with circling roots. The schedule of correcting the roots and repotting is quite dependent upon the species and situation. If your goal is to keep the trees or shrub forever, frequent removal of small circling roots around the exterior of the container and re-potting or up-potting will presumably help you out a lot. 

Does this also apply to conifers?  ie root flare present?   Yes, 100%. 

Does this apply to small ornamental trees as well?
Yes. But it’s my belief it probably occurs at a slower pace than a larger trees. We also often tend not to value our smaller shrubs and ornamental trees as much as our large shade trees that we want to thrive to 100+ years old. Losing a large shade tree to a stem girdling root is devastating. I’ve been the

Does this include palm trees?
I’m not a palm expert, but as far as I know, this is not a problem with palms. This is primarily a problem for the majority of woody plants that add secondary growth and new vascular tissue in concentric rings around older tissue. 

Does sunscald occur for other, non-girdling reasons?
Yes, sunscald can occur for other reasons, but I rarely see it in DC without a pretty obvious explanation related to moisture stress. In moisture stress situations such as a new tree planting and/or where a stem girdling root is constraining the vascular tissue, sunscald is very common on certain thin-barked species such as red maple. 

Can you expand a bit on sunscald? What causes it?
In my experience, sunscald can occur because of unlucky weather condition. In D.C., I really see it without an obvious explanation related to moisture stress. It’s also more common on thin-barked trees. 

What causes over suckering?
There is an incredibly complex answer to this question that involves plant hormones, types of and causes of dormancy and lack thereof, bud types and locations, and plant genetics. Related to the topic of root flare issues, I have noticed that suckering from the roots can be correlated with the other root flare issues discussed in this talk—particularly on basswoods and lindens.