Hawaiian hibiscus are pruned in spring at the beginning of new growth. This is because these plants are very sensitive to ‘cold snaps’ after pruning, and it is also easier to see just where to prune. In temperate areas, leaving the pruning of Hawaiian hibiscus until the first weekend in October is a good rule of thumb.
Immediately before new growth begins, lower leaves turn bright yellow and fall from the plants, and growth buds swell. Pruning hibiscus during First Quarter phase will result in faster recovery from pruning. Remove all withered branches with sharp secateurs, and also remove any branches pointing towards the centre of the plant. Then reduce the size of each plant by a third, cutting above an outward-facing growth bud.
Finally, give the plants a deep watering, a generous feeding of poultry-based organic fertiliser and a drink of seaweed extract tea. Add some compost to the soil surface, if you have it, and cover it with a 7-8 cm thick layer of organic mulch. Hawaiiian hibiscus are heavy feeders, and will continue to supply large blooms if further light applications of organic fertiliser are given every 6 weeks, or so, until May.
All species of hibiscus other than Alyogynes can be pruned at the same time as Hawaiian hibiscus. Alyogynes are pruned lightly after summer flowering.