Fertilizing
Feed every two weeks during the growing season, from spring until late summer, with a balanced bonsai fertilizer. You can use either organic fertilizer pellets or a liquid fertilizer at the dose on the label.
Stop feeding in autumn and do not feed during winter dormancy, so the tree can rest naturally.
Placement
Spirea needs full sun to grow compactly and flower well; in very hot climates, light shade in the afternoon prevents the pot from drying out too fast. It must be grown outdoors all year, as indoor conditions give too little light and no winter rest.
The plant is very hardy, but roots in a pot are more exposed than in the ground. When temperatures fall below about -10°C (14°F), protect the pot in a cold frame, unheated shed or sheltered spot on the ground.
Pruning
The right pruning time depends on when your species flowers. Spring-flowering species such as Spiraea thunbergii flower on last year's wood, while summer-flowering Spiraea japonica flowers on new shoots.
- Spring-flowering species: prune straight after flowering, before new flower buds form in summer.
- Summer-flowering species: prune hard in late winter or early spring, and trim off faded flowers to encourage a second flush.
- Remove some of the oldest stems at the base every few years to keep a clump vigorous.
Wire young, flexible shoots with wire that matches the branch thickness: too thick damages the bark, too thin does not hold.
Repotting
Spirea is vigorous and fills its pot quickly, so repot every 1-2 years in early spring, as the buds begin to swell. It tolerates root pruning well; you can remove up to about a third of the roots of a healthy tree.
Use a well-draining bonsai mix with a slightly acidic to neutral pH (6-7), such as akadama, pumice and lava rock. Secure the tree in its new pot with wire so new roots are not disturbed while they establish.
Watering
Spirea needs plenty of water during the growing season, often daily in summer. Water thoroughly when the soil surface begins to dry and let excess water drain freely from the pot.
Never let the soil dry out completely, especially when the tree is flowering, but avoid waterlogged soil. In winter dormancy water much less, keeping the soil only slightly moist.
Seasonal care
- Spring: Repot every 1-2 years as the buds swell and start feeding when new leaves appear. Prune spring-flowering species straight after flowering.
- Summer: Water daily in hot weather and feed every two weeks. Trim faded flowers on summer-flowering species to encourage a second flush.
- Autumn: Stop feeding and reduce watering as the leaves color and fall. This is a good time to see the branch structure and plan next year's pruning.
- Winter: Leave the tree outdoors for its dormancy and protect the pot when temperatures fall below -10°C (14°F). Prune summer-flowering species in late winter.
Common problems
Spirea is easy-going, but these problems come up from time to time.
- Few or no flowers: Usually caused by pruning at the wrong time or too little sun. Prune spring-flowering species straight after flowering, summer-flowering ones in late winter, and give full sun.
- Aphids: They cluster on soft new shoots and flower buds in spring. Rinse them off with water and treat heavier infestations with insecticidal soap.
- Powdery mildew: A white powdery coating on leaves, especially in humid weather with poor air circulation. Improve air flow, avoid wetting the leaves and remove affected leaves.
- Wilting and leaf scorch: A sign that the soil dried out in hot weather. Water more often in summer and give some afternoon shade in heat waves.