Is there an elm that doesn’t work for bonsai? There may be, but I never heard of it.

Elms are strong and relatively fast growers, not too badly bothered by insects or disease, they adapt with no apparent difficulty to container culture and can take pruning all day long. Their leaves readily diminish in size under bonsai culture, just as their roots easily become fibrous from regular pruning. Chinese elm (Ulmus parvifolia) is a traditionally favored species for bonsai use and may be the best of the bunch, but other kinds of elms make good candidates, as well. Other usable Asian species include David elm (Ulmus davidiana) and Siberian elm (Ulmus pumila). European species include field elm (Ulmus minor) and wych elm (Ulmus glabra). In North America we have American elm (Ulmus americana), cedar elm (Ulmus crassifolia), winged elm (Ulmus alata), rock elm (Ulmus thomasii) and slippery elm — also known as red elm (Ulmus rubra). The subject of this entry is a slippery elm.

The North Carolina Arboretum bonsai collection has just one slippery elm, and it came to us as a donation in 2013. A bonsai friend from South Carolina by the name of Ken Duncan gave me the little tree when I was visiting with his study group. The group’s meeting was at Ken’s place, in his garage, which also served as his bonsai studio. After the meeting Ken was showing me around his expansive collection of bonsai and pre-bonsai plant material and a number of similar young trees in small pots caught my attention. I didn’t recognize the species of tree so asked what they were and Ken identified them as slippery elms he had collected as seedlings. I told him I’d never worked with slippery elms, whereupon he took one of the pots off the shelf and handed it to me, saying, “Here, give them a try.”

Fortunately, I made a picture of the small clump of elms shortly after bringing them back to the Arboretum:

Ken sometimes liked to make very small bonsai — miniatures — and perhaps that’s what he would have done with these elms if he had kept them for himself. Perhaps he might have planted them as they were into a group planting, because they already had enough branching detail to serve that purpose. My own inclination is toward larger-scale bonsai, so I planted the little clump in the ground and let them grow. Here is what happened after just two growing seasons:

The clump of elms had originally consisted of five trunks, all growing off the same root system. In the above image it can be seen that the number of trunks had been reduced to three. The reduction was necessary because once the elm was freed from the pot and had its roots in the earth, it grew with impressive vigor and five trunks became too unwieldy to manage. If I had known how the tree was going to behave I might have dug it up at this point and continued training it in a pot. I didn’t know, however, so the slippery elm stayed in the grow bed.

My standard procedure when growing a woody plant in the ground in preparation for making it into a bonsai, is to prune the plant just once a year, typically in the very early spring. The only reason for having the plant in the ground is to accelerate trunk girth, and that happens most readily when the plant is allowed to grow with minimal restraint. The annual pruning is a means of keeping the plant from getting completely out of hand.

It turned out that keeping this elm under control while giving it the freedom of growing in the ground was not practicably possible. When I cut it back in the spring the tree would simply explode with new growth, as if it would just as soon be a big bushy shrub if I wasn’t going to let it climb up to the sky. The following two images show the elm from opposite perspectives, before the annual pruning in the spring of 2018:

Good luck trying to find a tree in that tangled mess! I finally decided the best I could hope for would be a stout but very short trunk, with an acceptably flaring base and a few good surface roots:

The elm stump was left undisturbed in the grow bed because I was concerned that such severe cutting back, in combination with being dug up and replanted in a container, might be too much for the poor little tree. I needn’t have worried. Here is what the stump produced in one growing season:

It was now plainly evident that the only hope for this elm to become a bonsai was to get it out of the ground and back into a pot. This was accomplished the following spring:

Six years in the grow bed had wrought a tremendous transformation in this elm. Whereas it had started out as a clump of five slender stems, the largest not thicker than a pencil, the tree now had one very hefty trunk about five inches in diameter and nine inches in height. At the time of collection, many of the stems that had grown out of the stump were removed entirely. Those that remained were ones I thought might be useful as branches, and were distributed around the upper end of the stump in a manner that allowed room for each to grow and develop. In the above picture, note the new growth popping out from latent buds all over the truncated trunk and branches. This trait is one reason why elms work so well for bonsai.

In bonsai there is a tree form referred to as a “broom”. It is typified by a single trunk that diverges at a given point into numerous limbs that reach upwards and outward, carrying secondary and tertiary branches and forming a full and rounded canopy. The trunk is seen as the handle of the figurative broom while the spreading canopy is the brush, with the many branches playing the part of bristles. In nature this form of tree is usually found growing in open ground where it has room to spread out, and certain species are more inclined to grow this way than others. The broom form can also occur when a mature tree loses it’s top due to some sort of severe damage, and grows a new top out of the remaining trunk. This particular elm is following that natural example. The difference is that when a tree in nature regrows following the loss of its top, the new growth competes for available space and the outcome is random. But when a broom bonsai is made from a cutback tree stump, the resulting new growth is managed and directed by the grower.

In the next two images, made two months after the previous photo, the first attempt at organizing the rampant regrowth can be seen:

A picture made after two more years shows further refinement in the basic structure of the tree:

There are two primary methods of developing the shape of bonsai branches. One method is referred to as cut and grow, and involves achieving branch movement by a series of pruning cuts made over a prolonged period of time. This elm was shaped exclusively by the cut and grow method for the first nine years of its time at the Arboretum. The other branch training technique involves the use of wire wrapped around the branches, which allows the grower to bend the branch into a certain configuration and have it held in that position by the wire. The wire is left on the branch so long as it doesn’t start constricting it and biting in. When the wire is removed the branch will usually hold the shape it was given. I’ve always preferred the cut and grow approach because I think it produces a more authentic looking result. Cut and grow takes a long time, however, and using wire as a training device can speed up the process. There is no reason both methods can’t be utilized on the same tree, and that’s what I chose to do with this elm in the spring of 2022:

There is risk involved with using wire to shape an elm. Because the tree grows so strongly, it does not take long for branches to swell to a point where the wire becomes very tight and needs removal. Failure to remove the wire in time can lead to disfigurement of the branch in the form of a telltale spiraling scar. Wiring scars are particularly egregious on deciduous trees, which have to stand naked for half the year. Given the growth rate of this specimen, wire applied during the growing season needed removal after only four to six weeks.

The next image from June of 2023 shows the elm up to its usual tricks:

And once again the response is to go through all the new growth and choose what gets removed and what gets to stay:

Cut and grow, cut and grow, cut and grow — the game goes on and on. With each succeeding round the shape of the crown is more fully described and the character of the tree begins to emerge:

This elm is currently planted in a large plastic pot with the soil level about two inches below the top of the rim. When a picture is made at the right eye level for bonsai viewing, the rim of the pot blocks the view of the tree’s base. The following image looks down on the tree, distorting its silhouette but allowing a view of the base, which is a pretty respectable feature of this specimen:

After being cut back strongly in June of 2024, as seen above, the elm was allowed to grow freely until just recently. Here is how it looked prior to the most recent pruning session, viewed from the same downward perspective:

The gallery below gives an in-the-round view of the tree before pruning (click on any image for full view):

The slippery elm is beginning to get the rough textured bark characteristic of mature trees. That’s a desirable development, but it creates a maintenance issue. Moss and mold make themselves at home in the furrows and fissures of the bark and need to be removed in order to keep the bonsai clean and healthy. Big trees in nature often have moss growing on them without any damage occurring, but it’s different with little trees. Moss buildup on a bonsai trunk can keep the affected area too wet and potentially lead to fungal problems.

Cleaning around the miniature bark plates without dislodging them is a delicate operation:

There were numerous substantial wounds made on the trunk when large branches were removed during the tree’s time in the ground and some of them are still in the process of being covered by callous material. Two such wounds occurred in close proximity, virtually on top of each other, resulting in a large and ugly scar. I was hopeful these twin wounds would cover over and be less noticeable, but that wasn’t happening:

As part of this most recent styling session on the elm, I used a rotary carving tool to connect the two wounds and make them into one larger opening. My hope is still that callous will eventually cover the entire opening, but that could take ten years to happen. In the meantime, this look is more acceptable to me:

In order to get a better image of the elm, I removed it from its container so the base can be more clearly seen. This is the current state of affairs for this specimen (click on any image for full view):

The root ball is well-developed enough to hold together when removed from the pot, but there is still room enough for the tree to grow another year without being repotted. This elm might have three or four rounds of new growth in the coming season, leading to further branch elongation and ramification. That might mean it can finally go into a bonsai pot in 2026. Even if that’s the case, however, it likely won’t be a showable piece for several years after that. The wounds need more time to cover over, or at least to age and weather to a more agreeable appearance. More importantly, I still haven’t found the right look for this bonsai. I have a feeling that the true character of this tree — its personality — is yet to be discovered and brought out more fully, and I’m not sure what it’s going to take to accomplish that. I’m not worried about it, though. Experience tells me to be patient, to keep observing, to keep working on development while waiting for the tree to reveal itself, and sooner or later it will.

An interesting and unanticipated outcome of digging the elm out of the ground five years ago was the resulting forest of elms that grew from root fragments that were left behind in the grow bed: