Fertilizing
Feed during the growing season from spring to autumn, using solid organic fertilizer monthly or liquid fertilizer weekly. From the start of the season, a fertilizer higher in phosphorus and lower in nitrogen gives more bracts and less leafy growth.
- Water thoroughly before feeding so the roots do not burn.
- In a cool winter spot (10-15°C), feed only once a month, or stop completely if the tree is resting.
Placement
Give your Bougainvillea bonsai the sunniest spot you have, with at least 6 hours of direct sun a day; it loves heat and flowers poorly in shade. Keep it outdoors from late spring, once nights stay above about 10°C (50°F), until autumn.
It is a tropical plant and does not tolerate frost. Overwinter it in a greenhouse or a bright, cool room at 10-15°C (50-59°F), and avoid moving it around, as it often drops leaves and bracts after a change of position.
Pruning
Do structural pruning in early spring, before strong growth starts. Bougainvillea flowers on new growth, so cut shoots back to two leaves only after each flush of bracts has faded, and do not pinch constantly in summer if you want flowers.
- Seal larger cuts with cut paste, as wounds heal slowly.
- Wire only young, flexible shoots; older branches are stiff and snap easily.
- Wear gloves to protect yourself from the thorns.
The plant buds readily from old wood after a hard cut-back, so a leggy tree can be rebuilt over one or two seasons.
Repotting
Repot young trees every 2-3 years and older specimens every 3-5 years, in spring once temperatures are reliably warm and growth has started. Bougainvillea resents root disturbance: its yellow, thin roots are brittle, and the tree often sheds leaves after a heavy root prune.
- Tease out the rootball gently instead of using a root rake.
- Remove no more than about a quarter of the roots at one time.
- Use a free-draining bonsai mix and keep the tree out of strong sun for a week or two afterwards.
Watering
Water thoroughly when the soil surface has dried, but avoid constantly wet soil, which leads to root rot. Bougainvillea flowers best when kept slightly on the dry side between waterings.
- In hot summer weather this can mean daily watering; in a cool winter spot, water much less often.
- It prefers a pH of 6-6.5, so use rainwater or filtered water if your tap water is hard.
Seasonal care
- Spring: Prune and, if due, repot once the weather is reliably warm, then move the tree into full sun outdoors when nights stay above 10°C. Start feeding with a phosphorus-rich fertilizer.
- Summer: This is the main flowering season, so water generously in hot weather and avoid constant pinching, which removes the shoots that carry the bracts. Cut shoots back to two leaves after each flush has faded.
- Autumn: Let the last bracts finish and water less as temperatures fall. Bring the tree into a greenhouse or bright, cool room before nights drop below 10°C.
- Winter: Keep it bright at 10-15°C (50-59°F), water only when the surface has dried and feed once a month at most. Some leaf drop in a cool winter spot is normal, and the tree leafs out again in spring.
Common problems
Most problems with a Bougainvillea bonsai come from too little sun, too much water or too much disturbance.
- No flowers: Usually caused by too little direct sun, constant pinching in summer or too much nitrogen. Give at least 6 hours of sun, let new shoots grow until they flower and use a phosphorus-rich fertilizer.
- Sudden leaf and bract drop: Bougainvillea reacts badly to being moved, chilled or having its roots disturbed. Keep it in one sunny spot, avoid cold drafts and prune the roots lightly when repotting.
- Root rot: Constantly wet soil, especially in a cool winter location, rots the fine roots and turns leaves yellow. Water only when the surface is dry and use a well-draining mix.
- Aphids, mealybugs and whiteflies: These sap-sucking pests cluster on soft new shoots, especially indoors in winter. Rinse them off and treat with insecticidal soap or horticultural oil.
- Snapped branches: Older wood is stiff and breaks easily when wired. Wire only young, flexible shoots and reposition older branches gradually with guy wires.