Elm Basics – Ulmus species
Elms are easy and recommended for beginners. Tolerant of various soil mixes, water quality, and sun/shade conditions. Popular small-leaved species of Ulmus for bonsai include Parvifolia (Chinese Elm), Catlin, Seiju, Nire Keyaki, and Siberian (Hokkaido). Dutch Elm Disease limits available varieties to varieties of Chinese Elm (Ulmus Parvifolia).
Soil, fertilization, and light conditions
My soil mix consists roughly of 60% humus/potting soil and 40% inert sand/pumice/rock, etc.
I fertilize with Osmocote 4-month time release pellets in May or June. I also soak trees in Miracid (evergreens) and half-strength Miracle-Gro liquid fertilizer (deciduous) 3 times in summer. In September, I put 0-10-10 powder on pot soils.
Elms need full sun in spring and shade in hottest summer temperatures. Daily morning watering in summer heat.
Repotting and Pruning
Repotting is best done as buds start to swell. Root ball can be trimmed as much as 50% if needed.
Need repotting with root pruning in spring, yearly for smallest pots. For larger pots, can go as long as 3 to 5 years between repottings.
Need to prune branch tips 2 or 3 times in summer to control shape and promote short twigging.
More severe branch pruning for basic structure shape is done when dormant in December.
Random limb die-back occasionally occurs in bonsai elms.
Over a few years, leaf sizes reduce by as much as 5 to 1 due to pot culture, root and tip pruning.
I spray in summer for insects such as aphids.
Pest control includes dormant oil spray in December and January.
Squirrels dig into pot soil, damaging roots. Worse, they chew bark on lower trunk and exposed roots. The best defense I have found is a quarter-inch plastic gutter screen over pot soil and wrapped around the trunk.
For more details, see the Elm issue of International Bonsai Magazine, 1999, No. 4.