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Botanical Name – Carpinus betulus
Botanical Name – Carpinus betulus
Common Name – European hornbeam
Native to – Europe to portions of the northern Middle East
Botanical Information –First described by the father of Botany Carl Linnaeus in the mid 18th Century. A tree generally 45 feet but can attain 60 feet in height. Alternating leaves are an elongated oval shape with toothed edges up to 5 inches long and 2 inches wide with distinctive veins so typical of the genus. Fall coloration is usually brown to yellow. Bark is a lovely smooth grey and the wood is dense and strong. Flowers form in male and female catkins after the tree has leafed out in the May timeframe depending on your location. Often used as a hedge or topiary subject.
General – Carpinus betulus is an excellent bonsai subject with many admirable attributes. This tree is used more in Europe where it is native but many fine examples can be found in the United States as well. Usually used as an informal upright or natural style, but for the more ambitious enthusiast it could be trained as a formal upright, root-over-rock or multi-tree group or forest.
It is recommend that trunk chopping of this tree be kept to a minimum and when it is done it should be on young thinner trunked trees in training as wounds heal very slowly. The highest quality bonsai are trees that are trained from seedlings and never chopped at all. While we are used to displaying our trees in the spring and summer months when we can enjoy the leaves (and this tree with its distinctively veined leaves is no exception) the best time to display and view this tree is during the dead of winter. The bark is a bright silver grey and with proper pruning the branching structure will become fine and delicate.
The root system of this tree in its native habitat is rather flat and close to the surface. With attention, one can make a fantastic “nebari” system on trees in training if you use a low flat container for training. In nature these trees are fairly wide spread and inhabit a number of different soil types, even dry rocky soil where they will become small in stature and naturally dwarfed. They do prefer however a soil more generous in humus and clay and tend to occupy those areas that are naturally more moist on a consistent basis
Cultivars – Columnaris Fastigiata Globosa
Carpinus betulus should be given good quality, direct sun to keep the leaves from getting too large but protection from that strong hot light in the middle summer months would be appreciated.
Rated for a U.S. growing zone 5 makes it hardy to -5 degrees F when grown in the open ground. As a container plant it should be given winter protection in zones 7 and lower. This is not a good tree for those living in tropical or sub-tropical areas where there is not enough winter cold to satisfy its chilling requirements.
All of the species of Carpinus need a good amount of water and should never be allowed to dry out. For older trees nearing their scheduled repotting will consume more water or at least it will seem like it because of the dense tight root system. Plants in this condition should be soaked on a regular basis in a bucket of water to ensure the whole root mass is moist.
Any general purpose “complete” fertilizer applied during the growing season will be more than adequate. Remember to never fertilize a tree when the soil is completely dry or when they are sick. It is recommended that occasional “flushing” of the container is done to eliminate fertilizer salt buildup. Flushing is the process of using plain water and applying it to the plant 2 or 3 times so that a good soil flushing is accomplished.
Effective and diligent pruning is one of the keys to this genus. As mention earlier, the highest quality trees of this genus are those that are started young and trained for multiple years. That means that you must have an idea of the eventual design that you have in mind from the very beginning and prune accordingly. Large cuts should be avoided at all cost as the ability of this tree to generate scar tissue over the wound is extremely slow. It’s best to have a plan and work to it with maybe some minor changes along the way just because of the way your particular plant is growing. The major branch structure elements should be clearly set out, which will make your job all that much easier along the way. Once you have the major elements in place then it’s just a matter of refinement of the pads and delicate small branch structure that is one of the highlights of this tree in the winter months. The growth of this tree is moderate but the early spring branches are quick to grow out so diligent pinching during the growing season is essential. Take your time and minimize large scars and you’ll be on our way. How you prune early spring growth is relative to what you’re trying to accomplish. If you’re working on the major branch scaffolding then you would let that spring growth grow out more to gain girth before you pinch it back. If on the other hand you are developing the fine branches for the canopy you would pinch more often to get small delicate twiggy branches. Take your time and be clear on your objective and let that guide your pinching technique. Because the wounds take so long to heal, this is not a forgiving tree in that respect. Large scars, 1 inch across or more can take up to 10 years or more to totally heal on a tree that is in a container.
Wiring should be done in the fall just after the leaves have dropped off and before the first killing frost. At this time of the year the branches are more pliable and will respond to your wiring without much difficulty. You must still be careful when you wire though as the wood on the larger diameter branches is hard and stiff and will break if you’re not careful. This is particularly true at branch junctions so some finger support at the junctions is recommended when you’re making the bend. Another recommendation for this tree is to use “covered wire” or raffia on the branches to eliminate staining and scaring. Most trees with thin smooth bark can be damaged easily and this tree falls into that category. Just like wound healing, if you do get bark scars or girdling they will take many years to go away.
The root system of Carpinus betulus is vigorous and should be repotted every 2-3 years.
Trees in shallow containers will require repotting more often than those in deeper containers as the root system on this tree is so vigorous it will heave in the pot which should be your signal to repot. The roots will encircle the container and build up on the bottom in a mat which should be carefully unwound before you start to cut them. Don’t be afraid to cut back the feeder roots pretty severely as they will grow back quite rapidly. You do need to take care and not do too much pruning on the main structural roots and if they need pruning you can do them one section at a time and not more than 1/3rd of them and any single time over multiple repottings. The structural roots or nebari should be trained while the tree is young.
Propagation of the species is by seed and the cultivars by either cuttings or grafting. Seed should be collected in the early fall when it is ripe and planted immediately and kept moist over the winter allowing it to experience freezing temperatures or, stratified at refrigerator temperatures for 3-4 months prior to planting. The seed should not be allowed to dry out as it will thicken and require double dormancy stratification treatment of 1-2 months warm stratification at greenhouse temperatures followed by 3-4 months cold stratification prior to planting.
Cuttings may be taken in mid summer when the wood is half hard with good results. Grafting is not recommended for trees destined to become bonsai.
Pests and Diseases
Watch for scale during the summer months and spray for them with dormant spray prior to putting your tree away for the winter. One or two inspections for scale during the winter months are recommended.