Fertilizing
Feed your cherry blossom bonsai from the end of flowering until late summer, every two weeks with liquid fertilizer or monthly with slow-release organic pellets. Do not feed while the tree is in flower, and stop in autumn and winter so the new wood can harden.
Use a balanced fertilizer in late spring and early summer, then switch to one lower in nitrogen and richer in phosphorus and potassium from midsummer, when next year's flower buds form. Too much nitrogen produces long shoots and large leaves at the expense of flowers.
Placement
Keep the tree outdoors all year in full sun, with light shade on very hot summer afternoons to prevent leaf scorch. Cherries kept in shade flower poorly.
A cold winter dormancy is essential for flowering, so never overwinter the tree indoors. Protect the roots of the potted tree once temperatures fall below about -10°C (14°F), for example in an unheated garage, shed or cold frame. Shelter the swelling buds and open flowers from late frost.
Pruning
Cherries are vulnerable to silver leaf and bacterial canker, which enter fresh wounds in cold, wet weather. Make larger cuts only in warm, dry weather between the end of flowering and midsummer, never in late autumn or winter.
- Structural pruning: remove unwanted branches after flowering or in early summer, and seal large cuts with wound paste.
- Shoot trimming: once new shoots have 4–6 leaves, cut them back to 2–3 leaves until midsummer.
- Flowering spurs: flowers form on short spurs and at the base of last year's shoots, so leave these in place.
- Tools: use clean, sharp tools, with concave cutters for thicker branches.
Wire young branches in spring or summer and check the wire often, as the bark marks easily.
Repotting
Repot every 2–3 years in early spring, as the buds begin to swell; young, vigorous trees may need it more often and old trees less. Use a free-draining mix, such as equal parts akadama, pumice and lava rock.
Prune the roots moderately, removing no more than about a third, and work the soil in so no air pockets remain. Afterwards keep the tree sheltered from wind and strong sun, and wait about a month before feeding.
Watering
Water thoroughly as soon as the topsoil feels slightly dry, until water runs from the drainage holes. In the growing season this usually means daily, and twice a day in hot or windy weather.
Never let the root ball dry out, especially while buds and flowers are developing, but avoid waterlogging, as cherries are prone to root rot. During winter dormancy water much less, just enough to keep the soil slightly moist. Water the soil rather than the foliage to reduce fungal leaf diseases.
Seasonal care
- Spring: Repot in early spring as the buds swell and shelter them from late frost while the tree flowers. Start feeding once the flowers fade and wire young branches if needed.
- Summer: Do structural pruning and shoot trimming in early summer in dry weather, keeping the flowering spurs. Water daily, shade the tree on hot afternoons and switch to a lower-nitrogen feed from midsummer, when next year's flower buds form.
- Autumn: Stop feeding, let the new wood harden and clear fallen leaves from the soil surface to limit fungal disease. Avoid pruning now, as wounds heal poorly in cool, wet weather.
- Winter: Leave the tree outdoors for its cold dormancy and protect the roots below about -10°C (14°F) in an unheated shed or cold frame. Do not feed or prune, and water just enough to keep the soil from drying out.
Common problems
Cherries are more prone to pests and diseases than many bonsai species, so regular inspection pays off.
- Aphids: Black cherry aphids cluster on new shoots in spring, curling the leaves and leaving sticky honeydew. Squash or rinse off small colonies and treat larger ones with insecticidal soap.
- Gummosis and bacterial canker: Amber gum oozing from the bark, often around pruning wounds, signals stress or bacterial canker. Prune only in warm, dry weather with clean tools, seal larger cuts and remove infected wood.
- Silver leaf: Leaves take on a silvery sheen and branches die back after the fungus enters through a wound. Cut out affected branches well into healthy wood in summer and avoid winter pruning.
- Shot-hole disease: Brown leaf spots fall out and leave small holes, especially in wet weather. Water at soil level, improve air circulation and remove affected leaves.
- Few or no flowers: Usually caused by too much nitrogen, cutting off the flowering spurs, too much shade or a winter too mild for proper dormancy. Feed less nitrogen from midsummer, keep the spurs and give the tree full sun and a cold winter.