Chapter 82 Carpinus caroliniana, C. japonica, C. orientalis, C. turczaninowii
82.1 American, Japanese, Oriental, and Korean Hornbeams
This airy American hornbeam does not have the tight, highly ramified branch structure that is typical of its Asian cousins, but it is a hardy native that is easy to grow in our area. Link to original image.
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82.1.1 Overview
Most of my experience is with American hornbeams. I have included a general introduction to the three Asian species that are suitable for bonsai, but it mostly is what has been published elsewhere, since I have only one non-American hornbeam in my collection.
82.1.1.1 American Hornbeam
These are shade-loving understory trees that prefer moderate soil fertility and moisture. It has a shallow, wide-spreading root system. The leaves are 1-3 inches long, finely serrated, and deeply veined. Here in the Piedmont they are common in riparian areas around streams and swamps. They are not too fussy though, and similar to bald cypress, I have seen trees planted and growing happily in curbside gardens and yards. In fall, the leaves turn yellow, blushing red.
82.1.1.2 Japanese Hornbeam
This species is native to Japan but is cultivated widely as an ornamental. The leaves are glossy dark green and deeply corrugated, but a bit too big for traditional or smaller bonsai. This species is probably best used for very large trees, unless you happen on a smaller-leaved specimen. It grows in similar conditions as our American hornbeams.
82.1.1.3 Oriental Hornbeam
This hornbeam is native to Hungary, the Balkans, Italy, Turkey, Iran, and the Caucasus. In its native range it grows in hotter, drier sites than either American or Japanese hornbeams. Often they grow as shrubs rather than having a single leader. The leaves are very small, 1.5-2 inches long. This species has become very popular for bonsai.
82.1.1.4 Korean Hornbeam
This hornbeam is a native of N. China, where it is common in open woodland and scrub, as well as in the understory. A small, dumpy, low-mounding tree reaching about 20 ft high in the wild. It has the smallest leaves of all hornbeam species, and ramifies easily. The trade-offs are that they are fussier about growing conditions. They also grow about half as fast as the other species.
In its native range, they have longer, colder winters and shorter summers. Like most hornbeams they are understory trees. This species depends on a hard frost to develop much color. Without any early frost, the leaves will be a neutral yellow. At the same time, it needs to be protected from extreme temperature swings in winter.
82.2 Soil: Moist Deciduous Mix
Ratio: 4 parts bark, 1 part grit
Add-ins: dolomitic limestone (~5 lbs/yd), Tree-Tone (15 lbs/yd)
pH: ~6.5 (slightly below neutral)
Moisture: Average
Usually American hornbeams are happy growing the same soil and light conditions as Japanese maples. Both are native to moist woodland species.
82.3 Light
Routine: American hornbeam likes dappled shade to partial sun. In my experience, American hornbeams in shallow pots or training containers will leaf burn in mid-day sun, but trees in open ground can tolerate full sun with no problems.
Korean hornbeam has an attractive open habit in total shade, and will tend to form a dense canopy in full sun. That is hard to do here though. In Piedmont NC the Korean hornbeam is very prone to leaf burn; mine gets badly scorched by early July. I’ve read similar complaints from owners in Maryland, so I suspect we are 1-2 USDA climate zones further south than it likes.
Summer: Definitely provide both American and Korean species with some shade from mid-day sun in summer.
82.4 Cultivation Notes
Hornbeams have three traits that make styling challenging. First, they do not handle defoliation well. They do not put on a secondary flush of growth, and the defoliated branch can die back. Second, hornbeams do not heal over pruning scars. The cambium does not roll in even after repeated stimulatin. This also means they do not graft well. Third, bent branches are VERY slow to set in place. It is not unusual for me to keep a large branch on an American hornbeam wired for 2 years. Rather than risk scarring their thin bark, I usually hold branches in place with stakes and padded guy wires or string.
82.4.1 Potting & Root Pruning
American hornbeams adapt very well to shallow containers, and do not seem to mind root pruning. They can be repotted in early spring, but often I will let them wait while I spend my time in early spring repotting the maples and other fussier trees instead. By the time I get to the hornbeams it is mid-spring, well after they have fully leafed out. They do fine as long as I remove enough top growth to balance what I remove from the roots, keep them in the shade for a couple weeks to recover, and keep the soil moist.
One of their quirks is that when you cut a root, new roots only emerge at the cut end; the roots do not ramify from behind the cut. I use this to my advantage if I have a tree with long woody roots and few smaller side roots. I cut a wedge or slice away about 1/3 of the bottom surface of a long woody root close to the trunk, then cut back the longer end as far as I dare. New roots will form both at the cut end and along the wedge or slice line. Once new roots have emerged closer to the trunk, I can cut the longer woody part back harder.
Korean hornbeam prefers a deep pot and will want minimal root pruning. Plan to repot every 2-3 years in early spring using a basic soil mix.
82.4.1.1 Top & Branch Management
Major developmental pruning should be done in late winter, before bud break. Pay attention to the angle and position of the scar, because it will not callus over. The best times for minor pruning is early spring. For American hornbeams, clip back emerging shoots all summer as they start extending beyond the crown line. I keep a pair of sharp scissors with me when I water so I can snip them off as I see them.
Do not defoliate a hornbeam; it is very likely to die back. They will tolerate fairly hard pruning as long as you keep at least 1-2 pairs of leaves to draw nutrients to the branches.
Unlike maples, American hornbeams like to send out long unbranched shoots rather than forming fine ramification. Fine branching requires using a “grow-clip back-regrow-clip again” approach over several seasons. Get the main branches in place before starting to develop any ramification.
Korean hornbeams are the opposite in they ramify naturally on their own. The trade-off is that as they age, branches tend to die back for no discernable reason.
82.4.2 Feeding
Feed hornbeams more heavily for the first two months after bud break, then cut down to half-strength organic feed every 2-3 weeks until early September.
I feed my hornbeams the same way as Japanese maples. In March or early April I apply a slow-release pelleted shrub and tree fertilizer (18-6-12) developed for nursery production, at half the recommended rate. From July-early September I feed stock trees with 0.25x Miracle-Grow monthly. Mature trees get Epsoma Tree-Tone in July; this is organic granular plant food that must be broken down by soil microbes, so provides nutrients for the rest of the season.
Other options for mature trees are liquid organic fertilizers at 0.5x, or 0.25x Miracle-Grow. Liquid options are:
- Fish emulsion (1x = 5-1-1, 2-4-1, or similar; depends on brand)
- Agro-Thrive Organic (1x = 3-2-2)
- Miracle-Gro Liqua-Feed (1x = 12-4-8)
- The pre-mixed liquid needs to be diluted at rate of 1/2 tablespoon/gallon to get 1x.
- Can pre-dilute 1:4 with water then put in a dial-set hose-end sprayer set to 1/2 Tbsp./gallon.
- Miracle-Gro water soluble (1x = 24-8-16)
- For 1x mix add 1 tablespoon of crystals per gallon of water. For 0.25x, dilute to 4 gallons water.
- Can pre-dissolve crystals in water then put in a dial-set hose-end sprayer. Mix 16 tablespoons of crystals into 64 oz. (1/2 gallon) of hot tap water & mix to dissolve. Add to spray bottle. Set sprayer to add 1 oz/gallon.
82.4.3 Pests & Problems
I have not had significant pest problems on my hornbeams. By mid-summer, there are some chewed leaves from random insects, but nothing seems to attack them with regularity. I have had to spray trees in the ground with rotten egg solids to keep rabbits from gnawing the bark.
82.4.4 Winter Storage
Store in mulch outdoors. Try to place in a shaded area so buds do not break in a mid-winter warm spell. No other special precautions are needed.
82.5 Propagation Notes
American hornbeams can be field collected or grown from seed. If you are field collecting, try to select the tree in early fall and root prune it, then wait until early spring to lift it. Wild trees should be dug in early spring before the leaves open fully, and preferably before the buds break.
American hornbeams are extremely easy to grow from seed. In mid-summer find a mature tree in an open area, and look for clusters of dry seed bracts on the end of branches. If you find a hornbeam planted as a landscape tree in full sun, there likely will thousands of seed clusters on it. Pull entire clusters and place them in a plastic or paper bag. At home let them dry for a couple days, then refrigerate until mid-fall.
When nights start to cool, remove the clusters from refrigeration and rub them between your hands to crush the bracts and loosen the seed. There is no need to soak the seek or remove the chaff; I keep it with the seeds so I can see where I sow them. Spread seed mix (topsoil+bark) in a deep tub. Leave 1 inch of space between the soil and top of the tub. Spread the seed on the surface, then cover with 1/2 inch of soil mix, bark fines, or vermiculite. Water thoroughly and place in a shady spot to over-winter.
Some guides say hornbeam seeds take up to 2 years to break dormancy, but I have not seen that. Most of the time my seed germinates by mid-spring. I leave the seedlings in the tube for a full year to develop stronger roots, then pot them up a year later. In the seedling stage I keep my hornbeams where they get sun in the morning, and are shaded all day after 11am or so.
Cuttings are the best way to propagate Korean hornbeam. Cuttings from mature wood are capable of flowering in a year or two as opposed to seven years or more for seedlings. To obtain cuttings, take clean scissors and obtain a three inch piece of mature wood, dip in rooting hormone and place in consistently moist, good draining soil. Place in a shady area to promote root development.
82.6 Artistic & Styling Notes
I am going to go out on a limb here and say I think American hornbeams should NOT be styled in the traditional Japanese forms. I have a lot of them in my collection, in all stages of development, and American hornbeams do not respond well to the growing techniques required for the traditional styles. They do not ramify well, so it is hard to create tight foliage pads. The leaves do not reduce much in size; the best you can do is remove the largest ones and let the smaller leaves remain. They do not cover scars, so it is nearly impossible to remove the traces of the artist from a tree.
I also have spent a lot of energy trying to make them look like Oriental or Korean hornbeams, without ever being satisfied with the outcome. I’ve found that the traditional styles do not accentuate their best features or reflect how this species grows naturally. Our native hornbeams may be genetically related to their Asian counterparts, but in the wild they have an elegantly sweeping, airy shape and texture that looks very different from Japanese styles. I became much happier with my hornbeams when I stopped fighting them and started trying to capture how they look naturally.
One of the ways I style American hornbeams is in a “stream-side style.” It is a modified version of slanting style, bordering on a semi-cascade. The tree seems to be bending out towards the light above a stream. The nebari is not one tight mass, but rather has several large roots radiating in the opposite direction from the top of the tree. Most of the branches are hidden in the unified foliage mass, which is pruned into an asymmetric triangle. Unlike Japanese styles, there are no obvious breaks in the canopy.
Ideally the tree would be situated with soil level on the “stream” side well below the soil level on the “bank.” I have experimented with ways to build this, but have not yet found an artistically pleasing solution. Even without it though, I like how this style works with the natural habits of our native hornbeams rather than fighting them.
82.7 Additional Information
82.7.1 Natural History
Hornbeams are also called blue beeches or musclewood. The former name comes from the bluish-grey, smooth bark, and the strong resemblance of hornbeam leaves to beech leaves. It took a while to learn how to tell them apart. Beeches have single-toothed edges, while hornbeams have smaller serrated edges between larger teeth. Also, beeches have fine hairs that stick out from the edges of leaves that hornbeams lack.
The latter name comes from their natural habit of forming waves or ripples in the wood of trunk that look like muscles under a smooth skin of bark.