Chapter 133 Ulmus sp.
133.1 Elms
Three species are important:
- Chinese or paperbark elm, U. parvifolia
- Winged elm, U. alata
- Texas cedar elm, U. crassiflora
Forest planting of dwarf-leaf cultivar of Chinese elms on a slab. Link to original image.
133.2 Soil: General Deciduous / Rich Conifer Mix
Ratio: 3 parts bark, 1 part grit
Add-ins: dolomitic limestone (~5 lbs/yd), Tree-Tone (15 lbs/yd)
pH: ~6.5 (slightly below neutral)
133.3 Moisture
Chinese and winged elms like average moisture, but are fairly tolerant of over- and under-watering. Texas cedar elms prefer slightly drier conditions.
133.4 Light
Can tolerate low light, but do best in half-day sun to full sun.
133.5 Cultivation Notes
Chinese elms are one of the most popular species for bonsai, especially for beginners. They tolerate a range of growing conditions, take extremely well to root and top pruning, and can be training into almost any style. Chinese elms grow quickly in open ground so it is easy to develop nebari quickly. Whips also can be fusion grafted to create bigger trunks from young stock.
Many sources say that Chinese elms can be grown as indoor bonsai. I personally have never tried this, but given they are deciduous temperate trees, they definitely must have a dormant rest period in winter. I suspect trees that are not allowed some colder, darker weeks each year decline over time. Trees kept outdoors in bright light have smaller leaves and branch more finely.
Winged and Texas cedar elms are Southeastern natives, and are not often used in bonsai. I find they are slower growing than Chinese elms, but otherwise can be managed in the same way.
Chinese elm forest planting
’Seiju’ cultivar of Chinese elms in a forest saikei. Part of the NC Arboretum collection in Asheville, NC. Link to original image.
133.5.1 Potting
Chinese elms tolerate fairly aggressive root pruning, though not as much as trident maples. For a healthy tree, it is safe to remove up to half of the existing root mass. Removing this much of the root mass should be done just as the leaf buds begin to swell.
Once the first new leaves start to open, Chinese elms should not be root pruned as aggressively; no more than 1/3 of the total root mass should be removed once leaves appear. Trees that have leafed out fully still can be repotted, but less than 10% of the roots should be removed. In my experience, a fully leafed tree sulks if I remove more than what is needed to untangle the roots and even up the root tips.
133.5.2 Pruning
Chinese elms develop twiggy ramification quickly with correct pruning. As shoots emerge, pairs of leaves emerge that tend to be larger towards the growing tip, and smaller near the branch point. To encourage ramification, let new green branches extend until they have 4-6 pairs of leaves (around 6 inches) then cut back to 1 or 2 pairs. Cut back any green branches completely that extend outside of the desired general outline of the crown.
Prune larger branches in mid- to late fall.
133.5.3 Feeding
Chinese elms that are being actively developed need ample fertilizer. Once they are mature, they should be fertilized less, but they still will need regular feeding.
Begin fertilizing in spring after new leaves start to emerge. Use slow-release fertilizers that have about three times more nitrogen than phosphorous. These are all good choices:
- Slow release shrub and tree fertilizer, applied at half the recommended rate.
- Organic liquid plant food that must be broken down by soil microbes, such as fish emulsion.
- Inorganic, water soluble fertilizers at half strength rather than full strength.
To feed more, don’t go up on the concentration of the solution; increase the frequency of feeding.
133.5.4 Pests & Problems
Chinese elms are subject to spider mites in dry weather, but generally are not bothered much by pests.
133.5.5 Winter Storage
Store in mulch outdoors. Try to place in a shaded area so buds do not break in a mid-winter warm spell.
Chinese elm
A bare Chinese elm in winter. The fine branch ramification is the result of several years of consistent, repeated clipping of growing tips. Link to original image.
133.6 Propagation Notes
Most sources recommend propagating Chinese elms from cuttings. This is true if you want to grow one of the miniature-leaved cultivars (cv. ‘Yatsubusa’ for example). I have started elms several times from seeds I collected from landscape trees. In fall I surface sow seeds in dishpans of black topsoil, then leave the tubs outside in shade all winter. The seeds stratify naturally. Most germinate in spring, and are ready to pot up in individual containers by late summer, or the following spring.
I have had mixed results with seeds I collected from Texas elms. According to other sources, cedar elm needs to be collected immediately after seeds mature in the late summer or fall. Seed needs to be cold stratified for 60 days before planting in spring.
I have never found a source tree or vendor for winged elm seed.