There are many varieties of elm from all parts of the world, most of which can be made into bonsai. However, Chinese elm and Japanese elm trees are the most popular bonsai material.
Chinese Elm (Ulmus Parvifolia)
Chinese elm is a deciduous tree native to China, Korea, and Japan. It is a popular landscaping tree because it is hardy, a vigorous grower, and resistant to Dutch elm disease.
This hardy deciduous species is adaptable, resilient to pruning, and thrives in a variety of climates, making it an excellent choice for beginners and experienced bonsai enthusiasts alike.
Over time, it will grow into a magnificent bonsai specimen because of its chunky, exfoliating bark and small leaves. Fall color is yellow, although a few selected cultivars have red foliage; in mild climates, not all leaves will drop.
Catlin elm is a desirable variety of Chinese elm. It is excellent for Shohin (small bonsai) due to its nearly evergreen, tiny, dark green shiny leaves, although fall color is not showy.
Japanese Elm ( Ulmus Davidiana, var. japonica)
Japanese elm is found across China, Mongolia, Korea, Siberia, and Japan. Desirable varieties include cork elms from Hokkaido.
The Seiju variety is perfect for Shohin (small) bonsai because of its tiny leaves. Branches are brittle, making wiring difficult. Wire carefully and use the clip-and-grow approach.
Environmental Factors
These are hardy trees that can take full sun all year round in milder climates. They can be kept in full sun for vigorous growth even in hot and dry climates if the root ball is always kept moist. Provide protection from frost and freezing temperatures.
Sourcing and Collecting
Chinese elm can be found at nurseries, online, and at local bonsai shows selling club members' trees. Starting with a tree with a substantial trunk size could shorten the time needed to develop it into a respectable bonsai. Lower branches will sprout when you top off a stick-like trunk.
Field-grown trees, such as those in your garden, can be dug up in late fall in mild climates or in early spring before the leaves emerge. You will probably have to put the tree in a large training pot to accommodate the root ball. Compensate for root loss by cutting back redundant branches.
Style
Elms can easily be trained into any style but look especially good when trained into a spreading oak/elm tree form with a full, large crown and lower branches radiating outward. Let nature be your guide.
If you are starting with a small-diameter tree, it's best to put it in the ground or a large growing pot for a few years to bulk up the trunk. Since they grow rapidly, take the precaution of putting a plate or tile under the tree to promote lateral root development. Dig the tree up annually during its dormant period and root prune. If these precautions aren't taken, the roots will definitely spread out and down so much that you will lose the fine fibrous mass of roots near the trunk necessary to eventually get the tree back into a pot. While doing this root pruning, also remove any thick side branches. The top leader will provide sufficient energy in the spring for new buds to form all along the trunk. Avoid reverse taper on the trunk by removing "spoke" branches.
Elms in your landscape can be made into nice bonsai. Retain fibrous roots but cut heavy roots aggressively so that the tree can eventually be put into a bonsai pot. Always cut off an abundance of redundant branches to compensate for root loss.
Branches can be brittle, so be mindful when wiring. Use aluminum wire to wrap around branches loosely. Heavy growth during the summer months could embed the wire deep into the trunk quickly. A little wire scarring is okay since it will heal over; deep scarring is unsightly.
Trimming and Pruning
Rapid growth makes spring trimming seem never-ending. Define what you want the bonsai to become and trim off everything outside of your design parameters. Work from the apex down, and also from the lower branches up to enable you to adjust as you go.
Trim back all new growth to the desirable length to force buds to grow and increase ramification. The goal is to create pads (cloud-like masses of foliage along each branch).
Traditionalists recommend sealing cuts to avoid bleeding and rotting. Use a concave branch cutter to cut cleanly close to the trunk.
Watering
Water daily throughout the growing season. On hot days, check if the top of the root ball is supple to make an assessment of whether additional water is needed. When in doubt, water.
Do not worry about overwatering because elms thrive with ample water. Consider using the BH pot, which allows residual water to be absorbed by the root ball over time.
Propagation
Elm trees are easily propagated through cutting, division, root cutting, and air layering.
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