Ulmus crassifolia is found in the Southern U.S., from New Mexico to Tennessee. It is a wonderful species choice for bonsai and like most elms, can survive quite a bit of neglect. It does best in zones 7 thru 9, but is tough enough to do well in other zones as well.
One of its desirable features is its rough, fissured bark. Most specimens are collected from the wild and will invariably have an aged appearance. The branches ramify easily with normal pinching of shoots and the leaves are not over large. This species is a good one for beginners and collecting them is rather easy as well.
Basic care guidelines...
Soil : One nice feature is that they will do well in practically any soil. Good choices are Haydite and an organic component (50/50), or akadama and a grit of some sort (60/40). As their natural soil is often little more than limestone and Juniper bark, keep them around neutral to slightly alkaline ph.
Watering : Cedar elms do not like to be overly wet. Their natural environment is rather hot and arid and they do well to being kept on the dry side of moist. Like most trees, they use more water in the spring.
Exposure : They can be kept in full sun to dappled sun. They have an interesting, if annoying habit when they are getting too much sun. They will rotate their leaves to be edge-up, so as to limit the exposure of the leaves to the sun's rays. If this interferes with your showing of the tree for an exhibition, simply place the tree in light shade for a few days and the leaves will rotate back to their usual flat presentation. If they get too much sun, they tend to go a bit yellow. However, as they are deciduous and elms, they are often best viewed in winter silhouette. In winter, protect them from cold freezes below 20F and cold, drying winds. They are not very subject to dieback of the smaller twigs, but damaged branches or twigs may abort in winter.
Fertilizing : Fertilize in spring after the tree has leafed out and again in late summer or early fall. As their natural tendency is to shut down and cease growth in mid-summer, they require no food during this time. They have a habit of putting on some late growth just as temps are dipping into the freezing zone, so don't give a high nitrogen feed late in the season or you may lose some soft growth to the cold. Chemical fertilizers are fine as are organic ones.
Pruning : Do most pruning in spring or early summer as these trees can have a tendency to be slow in healing later in the year. Calluses can be bulky so reduce wounds properly.
For ramification, pinch new growth back to 2 leaves after 4 or 5 have formed during the spring. It often helps to cut the terminal leaf (of the remaining 2) in half so that the first leaf will have a better chance of producing a bud.
Wiring : Wiring can be accomplished at almost any time, but be very careful about checking the wires as the shoots and branches can swell very quickly. Guy-wires are quite useful with this species. Know that wired branches tend to lose vigor so don't pinch back growth too much on a wired branch.
Repotting : These trees are not fussy about repotting. You can be ruthless in cutting back the roots. Early spring is best, but they do well with a fall repot, too.
Pests : In their native range, they have few pest problems. The worst are caterpillars that can destroy the foliage in very short order. Look the trees over each day, especially the underside of the leaves, for these pests.
