Pruning common hornbeam in spring

Published on 25 May 2022 at 11:50

Hornbeams (Carpinus betulus) are becoming one of my favorite species for bonsai cultivation. They grow fast, respond well to different pruning techniques and you can easily reduce their leaf size. I also think they make great bonsai for beginners! Only repotting them can be a bit tricky sometimes, as I learned the hard way.

The group planting I will use as an example for this post, was originally put together in 2018. Back then, I dug up a couple of small saplings from a forest that was being cleared. They were aprox. 5 to 7 years old trees. I had planted them in a large wooden box to let them recuperate. Sadly, when I transplanted them into this shallow bonsai container, 2 of the 7 trees died.

I tried to replace them with 2 bare-root saplings I bought from a regular plant nursery. It was quite difficult to plant them into this pot, since their tap root was very long. One of them didn't make it through spring last year. So now I'm still left with 6 trees, which is considered a 'faux-pas'. In forest plantings, you should try to use an uneven number of trees, to avoid symmetry. But that's something for a future article...

In this article I will talk about some of the basic pruning techniques I use for maintaining and developing hornbeams.

spring growth

Hornbeams produce very long shoots from mid to late spring, as temperatures rise. The tree wants to extend its foliage as wide and high as possible to catch the most sunlight. More sunlight means more sugars, which increases next years' reserves.

For a tree in nature, this is great, but we're trying to make miniature trees here. So we'll have to control this growth.

In essence, you have two choices: cut the extending growth, or leave it. The new growth can be used for a couple of things:

 

  • As a new branch for the future design
  • As a so called 'sacrifice branch' for thickening certain parts of the tree. After the desired girth is achieved, the branch is then cut.

  • If the growth emerges from a weak part of the tree, leaving it can 'strengthen' that area and promote vigour.

 

If you don't want to achieve any of these things, you can decide to cut the new growth, once it 'hardens off'. This means, cut it when it starts to lignify, or become rigid and woody.

Cutting back new growth

Take a look at the following three pictures. I'm holding an extension that has grown from the beginning of spring till now, late May. The red arrow points towards the first leaf that emerged from the bud that opened at the end of last winter. Many species don't develop new growth from this point. In Japanese, this is often referred to as 'susoba', or the first budless leaf. If you were to cut back to this leaf, the tree wouldn't respond by sprouting a new branch there.

So you'll have to cut further along the branch. But how do you know how much you can cut? Well, there isn't a sure fire way of knowing, but there are guidelines.
Imagine you divide the tree into 3 zones, with different 'energy levels', or growth potential:

  • The top (or 'apex)' = strong growth
  • The middle = medium growth strength
  • The bottom = weak growth

You can decide how much of the new growth you want to cut, according to the zone it's growing in. For example:

  • The top:
    • Leave 1 to 2 leaves and cut the rest
  • The middle:
    • Leave 2 to 3 leaves
  • The bottom:
    • Leave 3 to 4 leaves

 

The important thing here, is that you don't count the first budless leaf (susoba). This makes sense, because if you leave only the susoba, there will be no new growth and the branch may die off.

In the pictures, you can see I left the susoba + one leaf, since this branch was growing in a very strong area in the apex. I expect the new growth to emerge from the base of the petiole of the first leaf, indicated by the blue arrow.

The purpose of pruning

I talked about the reasons why you would leave a new shoot to grow. There are of course a couple of reasons why you can decide to remove the new growth:

  • It's growing in an undesirable place
  • It's growing outside the silhouette of the tree
  • You want to promote ramification, and smaller leaves
  • Shorter internodes
    • The area between the leaves and buds on a shoot or branch is called an internode. In bonsai you want every growth to be as small as possible, including the length of the internodes. By cutting back the first flush of spring growth, you stimulate a second flush, which will have much shorter internodes. In the end, this will create a more compact tree.
  • More movement
    • Note that the new growth will come out in the direction the bud is pointing.
  • Taper
    • This aspect is often overlooked! By cutting back a branch, you ensure that the base is thicker than the tip of the branch. This creates the illusion that the branch has been growing for years on end, like in a full grown tree.
  • Reduce vigour
  • Promote backbudding
    • When you cut the tip of a branch, dormant buds closer to the trunk will start growing. This is important, because it allows you to maintain a small size.

 

In this picture you can see another example of a shoot that was pruned. The cut point is indicated by a black line. I'm hoping that by cutting the shoot, two new branches will emerge from the points where the two remaining leaves are attached to the stem (blue arrows). The red arrow, again, points toward the susoba. I almost know for sure that nothing will grow from the bud of this leaf.

Basically, I'm hoping that by cutting one branch, two branches will emerge. Often this will be the case, but it's not entirely reliable. It depends on the vigour of the branch.

So by cutting back, this branch will become more ramified, the new leaves will be smaller and the internodal length of the new growth will be shorter.

In the next and example, you can see a difference in internode length. When this shoot first started growing, it had very short internodes, i.e. the distance between the three lower arrows. After that first period of growth, the shoot started extending a lot farther, resulting in longer internodal length. I cut this shoot, leaving two leaves (and the susoba). The resulting cut made sure that I'm left with the shortest internodal length possible, as you can see in the second picture. This will result in a densely ramified branch.

Dormant buds, light and leaf cutting

One last thing I want to talk about, are dormant buds. These are buds on older wood, from which new shoots can emerge. In the next picture, the dormant buds are indicated by blue arrows. The red circle shows new growth emerging from a dormant bud.

These buds are very important for bonsai, as they allow you to cut back to a point from which the tree can start growing again. By doing this, you can maintain a compact size.

In order to make sure that these buds stay healthy, you have to allow light and air to penetrate the interior of the canopy, where the dormant buds are located. This is accomplished by regular pruning.

Another technique you can use, is 'leaf cutting'. Basically, you cut large leaves in half so that light can reach the buds closer to the trunk of the tree.

An added benefit of leaf cutting is that you balance energy over the whole tree, by taking away part of the 'food source' (the leaf) of a particual part of the tree. When you do this, try to aim for an average leaf size on the whole tree.

If a part of the tree is very dense, you can also choose to remove the whole leaf, especially ones that are more on the outside of the canopy.

This techique is mostly used for trees that are in the refinement stage of develpment. If you want strong growth, don't cut the leaves!

Reduce the size of large leaves to let more light into the interior of the canopy. This will make sure that dormant buds stay healthy until spring of next year.

After cutting back, the tree is allowed to grow freely for a couple of weeks. Make sure you feed with a low nitrogen fertilizer if you don't want long internodes or strong growth. Since the tree has less leaves than before, the water intake will also be less. So take that into account when you water your bonsai.

That's it for this article! If you have any questions, feel free to comment or get in touch. And don't forget to subscribe to our mailing list for updates on new blog posts!


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