Tropical hibiscus plants are often treated with a growth retardant that eventually wears off; then the plants begin to grow in their normal lanky form. There are two approaches to pruning. One is to cut the overall plant back quite hard in late winter to early spring, and/or in the fall when you bring it inside. This controls the overall size, but it also delays flowering. The other way to prune is to selectively remove longer branches from time to time. This will not interrupt flowering but does not control the size quite as well. As it regrows you can also pinch the tips periodically to encourage branching and help keep it denser.
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