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Tree of the Month - April 2002: Chinese Elm
Chinese Elm or Ulmus parvifolia
Since we have a workshop on this tree coming up this Sunday with Colin Lewis, it seems to be the right time to highlight U. parvifolia as our tree of the month. Chinese Elm has a wide climate range, being native to northern China, Taiwan, Japan, and Korea, the Chinese or Lacebark Elm generally has a broad, vase-like shape (Broom style) with pendulous branches. In warmer regions it may be evergreen, never shedding its leaves, and can reach heights of 60 feet . The small, leathery, dark green leaves are smooth and shiny on top and have small blunt teeth. The fruit mature in fall much later than most other elms.
The bark of this elm can have different textures depending on the variety. It can be rough barked, smooth barked, or exfoliating to show orange or tan under-bark. Varieties commonly used as bonsai include the dwarfs Yatsubusa and Catlin, and varieties Hokaido, Seiju, Cork Bark, Korean Weeping, and Semper Virenss
Growth care for the Chinese Elm seems to be full sun and high light levels for indoors. Chinese Elms prefer full sun when outdoors or bright light when indoors. The smaller leafed varieties, like Seiju, must have full sun or they will not thrive. Use caution when moving from indoors to outdoors. Two or three weeks in shade when going outside will prevent sunburn. do not leave Chinese Elm in the shade or densely ramified specimens may start to loose interior branches through die back. Too much shade will also lead to larger leaves and longer internodes. If summer temperatures are way to hot then some shade may benefit some individual trees. you probably should not put them where the air conditioner will blow on them. just shading the pot may help by keeping the soil temperature lower, or you could plunge the pot in mulch or a growing bed. Hot temperatures may cause a growth check.
If you plant them directly in the ground, Chinese Elms grow quickly and can increase trunk caliper and height in a short time. You can use this to produce very stocky specimens by alternately growing and sacrificing the apex over a few years. One need only be concerned with the trunk during this time as branches may be formed later after the tree is planted in a bonsai pot. In a bonsai pot, their trunks will not thicken very quickly, but the shoot growth can be remarkable if well-fed.
Chinese Elms will grow in nearly any soil medium and their roots should stay moist, but they will get root rot if they are continually wet. An open, granular medium which allows for good drainage is best. The more inert grit used in the soil means that much more attention to the moisture levels. Our club mix should do well for these trees.
These elms may be repotted in either Spring or Fall, but Spring is always best. If you do repot in Fall, you should have a place ready for it inside that Winter. At a Spring repot, do so just before the new buds open when one can be quite ruthless in cutting back the roots, especially the big ones. Elm roots can thicken up quickly and it is therefore wise to repot at least every other year. Repotting every year may be necessary in some cases. You can wash the rootpad clean so that you can see everything that is in the root structure and make your cut selections with a clear plan.
When pruning the roots, use cutters that are very sharp. The roots of Chinese Elms are pulpy and can be crushed if care is not used. Crushed roots can lead to root rot. When cutting away very large roots, it is usually advisable to then clean up the initial cut with a sharp grafting knife so that the wound edges are smooth and clean.
Imported Chinese Elms often have really bad structure of the surface roots. I don’t know if this is a cultural esthetic or if the exporters just think Americans don’t give a rat’s ass about nebari, and send us the rejects. You will often find these trees with ugly, tangled masses of ropy roots well above the soil . This is a common trait with imported elms and one to be avoided. If a nice nebari is not present, it can be manufactured by air layering or ground layering; an operation that is quite easy with elms. If you don’t have a reverse taper problem you can always try starting the root cuttings from these as a way of increasing your stock.
Water daily throughout the growing season. If the tree is maintained in a warm climate or indoors where it acts like a tropical, this will be year round. If the leaves have dropped, water sparingly but do not allow soil to become completely dry. DUHH! Start daily watering again at first signs of new growth .
Chinese Elms should be fed monthly or every two weeks (more or less depending on the organic component of your soil) from the time the first spring leaves harden off through early Autumn. When new leaf buds are in abundance, and in the spring for outdoor trees, fertilize every two weeks. A good water soluble all purpose fertilizer is fine, slow release fertilizer tablets are also acceptable and can be used to supplement regular feedings. Keith Scott recommends a dilute weekly application from mid May to October for our climate in Zone 4 or 5. More developed trees should be fed less to keep the twig ramification in balance. Keep track of the moisture uptake for the tree. If the soil begins to take longer to dry out in hot weather, cut back on the fertilization. It probably means the tree is having a growth check.
Chinese Elms may be maintained outdoors year round in zone six or above, however, some sources say they will tolerate temperatures down to 20 degrees F. The fleshy roots can be damaged if they are allowed to get too cold or have to endure a regular freezing and thawing cycle. If the tree gets too cold for too long, loss of some of the fine ramification may result. In any case, in Columbus this means some serious winter protection or greenhouse care. In Winter, they may keep some or all of their leaves. If this is the case, be sure to keep them in good light to avoid losing their finer shoots. It has been suggested that even if they are bare, shade may contribute to shoot dieback. If the tree is not completely dormant there is some photosynthesis in the twig bark so this could be credible.
Shaping may be done with clip and grow or by wiring. Prune all new growth to two or three leaves. Remove undesired new growth completely. Either method will work well as they bud profusely and, once wired, will hold their shape in a short time. Although wiring may be used to train Chinese Elms, the clip and grow method is preferred due to the fast growth rate. If you do wire the branches, keep close tabs on the growth of the branch as Chinese Elms can thicken quickly, causing the wire to begin to cut in less than a few weeks. It is best to wire shoots just as they harden off if you know what you want at this point. Wiring after leaf drop is often effective as you can easily see what you are wiring and growth will have slowed to nothing at this point. If you wire in late fall, you will need to bring it in for the Winter. Be sure to remove the wire just before the new buds break in the Spring or whenever it starts to move indoors..
Chinese Elms have an alternating leaf pattern and naturally short internodes making nearly any styling aim possible. The trunk form on a smooth barked Chinese Elm bonsai will look convincing with both a tall, thin upright form and a more compact form. The rough barked varieties often look best with larger trunk diameters to account for the coarser bark texture. Like most deciduous trees, it is often grown for its Winter silhouette which can be stunning, so keep this in mind as you do your styling.
Chinese Elms are mostly pest free, but are subject to black spot fungus. This usually occurs when the tree is kept constantly too moist or in times of a wet Spring. Any of the several common fungicides will work, but those with an oil base may burn the leaves, so use sparingly and keep the tree somewhat shaded. Root rot is another water issue.
Keith Scott says Chinese Elms are a magnet for leaf cutters, red spider mite, and aphids. they will attract the beetles that carry Dutch Elm disease, but don’t seem susceptible to that. These pests will usually attack only in times of stress for the tree. Aphids are attracted to new, soft growth and scale insects may be found on woody branches and shoots. Either may be removed by hand or one may use a mixture of vegetable oil and dish soap mixed with water, sprayed onto the tree to remove the pests. Chinese Elms seem to dislike systemic insecticides and may drop their leaves. The leaves will be replaced shortly in most cases. Hmm - wonder if white flies like them?
If you want to increase your stock, elms grow extremely well from seed, but will grow well from hard wood cuttings as well as root cuttings.