Repot every two to three years in spring, but take care not to cut too far into the old roots. If you want to reduce the heavier roots, do so over a number of years. Use a standard soil mix. Pruning while the tree is growing will encourage the production of masses of unwanted roots. Water carefully during the growing period, keeping the soil moist but never allowing it to be saturated.
Myrtles dislike having their roots wet for too long.
In winter keep watering to a minimum - just enough to prevent the soil from drying out completely. Mist with fresh water daily. Feed with half-strength, balanced feed while the tree is in active growth. Do not feed in winter. The Bonsai Site .