Getting bougainvillea to flower

The bougainvillea complaint we hear most is a healthy, green, fast-growing plant that refuses to flower. It is almost never ill. Bougainvillea flowers when it is a little stressed: plenty of sun, a pot on the dry side, and lean feeding. Give it comfort, with daily water and rich feed, and it responds by growing leaves. These are the five things we change, usually in this order.

Count the sun hours

Bougainvillea needs at least six hours of direct sun to flower well, and eight is better. Before you change anything else, watch the spot for a day. A railing that looks bright may only get three hours once the building next door blocks the afternoon. If the balcony faces north, or the plant sits in the shade of a grille or overhang, move it to the sunniest stretch you have. On many balconies that one move is enough.

In Delhi, winter sun is weak and fog cuts it further, so expect little colour in December and January wherever the plant sits. It flowers again from February as the light returns. In Mumbai and Bengaluru, a plant in full sun can flower on and off for most of the year, with the strongest flushes before the monsoon and again after it.

Stop overwatering

A bougainvillea watered every day stays in leaf mode. Let the top two to three inches of mix dry out before watering. In a 14-inch pot in April and May, that is usually every three to four days; in winter, about once a week; in the monsoon, often not at all. A slight droop on a hot afternoon that recovers by evening is fine. The plant takes it as a signal, and flowering usually follows a short dry spell.

If the house help waters everything on the balcony daily, give the bougainvillea its own instruction: only when the pot is dry.

Hold back the nitrogen

Nitrogen makes leaves. Urea, fresh cow dung and general green-leaf feeds all push soft new growth at the expense of bracts. Stop them. From February, feed every three weeks with a bloom fertiliser that is low in nitrogen and higher in phosphorus and potassium, or top dress once a month with a small handful of bone meal and wood ash. If it shares a trough with other plants, feed those separately.

Prune after each flush

Bougainvillea flowers on new growth, so pruning at the right time brings more colour. When a flush fades, cut the stems that flowered back by about a third, just above a leaf node. New shoots appear within two weeks and carry the next round of bracts. Do not prune just before the season, and do not keep trimming the tips all year, which removes the wood that would have flowered. Wear gloves; the thorns are sharp. Tie long canes loosely to the railing with soft twine, and on a high floor keep the plant compact so the wind does not snap them.

A little stress helps

Keep it slightly root-bound. A bougainvillea in an oversized pot puts years into roots and leaves. We rarely go beyond a 14 to 16-inch pot on a balcony, and only move up when roots come out of the drainage holes. Between a sunny spot, a dry pot, lean feeding and pruning at the right time, most plants we adjust start showing colour within six to eight weeks. Be patient in between: the plant needs a few weeks of new growth before it has anything to flower on.