Two years ago, I grew a bunch of Zelkova serrata (Japanese elm) from seed. The goal was to develop them into formal broom style trees, or 'Hokidachi' in Japanese.
Source: https://servimg.com/view/15577549/749
In the picture above, you can see a good example of what I'm trying to achieve. All the branches emerge from one point in the trunk and fan out, creating a broom-shape. You can find this shape in nature, with trees that grow alone in fields, where they can maximize their light-exposure. Since they don't have any trees surrounding them, competing for light, they can grow uninhibited in every direction. Their trunks also grow straight as an arrow!
The elms I grew from seed two years ago, still have a very long way to go! In order to create a 'perfect' broom shape, you best start with very young seedlings. In the pictures above you can see I pruned a couple of twigs. The general rule is that you prune the branches so that they bifurcate, or split into two branches. If you leave more twigs or branches coming out of one point, it'll thicken over time, creating ugly bulges.
Notice that the trunk also splits into two branches from one point. There are examples of broom style trees that have more than 2 branches emerging from the trunk. To make things simple, and to reduce the risk of creating bulges, I chose to only leave 2 branches to form the canopy.
After pruning the twigs, I wrapped the 'canopy' with some soft thread or string, which I will remove in spring when the leaves emerge. I tried to tie the branches together, without snapping them. The goal is to make the branches grow as straight up as possible. This proces is repeated every year and over time the tree will grow into a broom-shape, with branches fanning out evenly.
Common flaws or mistakes
In the pictures above, you can see that this seedling didn't have a straight trunk. In order to correct this flaw, I wired the trunk with aluminum wire and straightened it. This wire will be left until spring. The branches were also pruned to one pair, at the point where the trunk splits.
Another flaw to correct in these seedlings, is the angle at which the first two branches grow. On the left, you can see that the gap between the two first branches is very wide and almost U-shaped. This gap needs to be as acute as possible.
Notice that the gap in the seedling on the right, is V-shaped. Over time, this will create a much more realistic broom shape. To correct the U-shape, you need to tie the first branches together much tighter. You could even use aluminum wire, if the branches are already so stiff that soft thread doesn't do the job anymore.
In Spring, I will write an update about these seedlings and explain how I care for them during the growing season. Stay tuned!
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