Acer buergerianum, the Trident Maple, is grown in many parts of the world as a 'Street' tree due to its tolerance of pruning, dry soil and air-pollution, these attributes also make it excellent material for bonsai culture. Native to Korea, China and Japan, Trident Maples can reach heights of 10metres or more.
In the UK, Trident Maples are the most difficult Acer species to locate. Not stocked in garden centres and tree nurseries, they are also unavailable to collect from the wild. Tridents nearly always have to be sourced from bonsai nurseries as imported stock.
Acer buergerianum differs from its Acer palmatum relatives by virtue of its 3-lobed leaves (as opposed to the 5 or 7-lobed leaves of A. palmatum). Leaves are dark-green above and blue-green underneath, turning beautiful shades of orange and red in Autumn. New buds are a red/brown colour.

Bark is buff and flakes revelaing a soft-orange underbark. In maturity the bark becomes darker.

Trident Maples are a very vigorous species, particularly when grown in the ground and are very suitable for field growing to develop large trunks suitable for bonsai.

BONSAI CULTIVATION NOTES

POSITION Full sun. Provide light shade for weak-rooted specimens during hot summer periods to avoid leafburn. Thick trunked Trident specimens are often planted in relatively shallow bonsai pots that may dry quickly during the Summer; these should also be provided with some shade from the afternoon sun.

Roots have a high moisture content and are susceptible to frost damage. Frost protection should be afforded when temperatures drop below -5°C. Avoid organic/peat based soils as the cycle of waterlogging and frost can damage the roots.

FEEDING Feed once a week for the first month after leaves appear in the Spring to help strengthen new growth. Thereafter feed every two weeks.

REPOTTING Annually as buds extend. Use a very free-draining soil mix. Tridents have very strong and vigorous rootgrowth that is very suitable for the creation of good trunkflare and nebari. Tolerant of aggressive root pruning.

PRUNING Allow new growth to extend to 2-5 leaf pairs/nodes and then prunen back to one or two pairs of leaves (depending on a position on tree) throughout the growing season.

Keep on top of the most vigorous shoots in the apex and upper branches to stop them becoming too thick and weakening the lower and inner branches. Continued pruning of these apical areas will distribute vigour throughout the tree.

Prune all branches back hard after leaf-fall or in late Winter. Removal of large branches or trunk-chopping should not be carried out in early Spring as this can cause excessive sap-loss.

Trident Maples are very suitable for defoliation and leaf pinching techniques. With defoliation and increased ramification of the branches, leaf-size can be reduced dramatically.

Wiring should be carried out with care as bark marks easily and branches thicken quickly.

Trident Maples are very easy to approach and thread graft new branches and roots. However, given their tendency to backbud easily and continually, it can be worth waiting for an appropriate bud to start developing before resorting the grafting.

For further pruning details please refer to the Advanced Acer palmatum article at Bonsai4me.com. The guidelines on Trumming, Defoliation,Branch Pruning, Pinching Back, Internode Distances, Use of Sapdrawers and Positioning as all as equally applicable to Acer buegerianum as they are to Acer palmatum.

PROPAGATION Easy to propogate from seed, sow outside as soon as ripe. Air-layer in late-Spring. Take hardwood cuttings in Winter, softwood cuttings in Summer.

PESTS AND DISEASES Aphids, caterpillars, scale insects, mites and leaf-scorch.

STYLING Acer buergerianum have powerful roots and are particularly suited to root-over-rock forms. Suitable for all forms except formal broom with single or multiple trunks in all sizes.