The GARDENA gardening expert
Dear Ms. L., which variety of hibiscus do you mean exactly? I'll assume it is a rose mallow, hibiscus rosa-sinensis. This plant definitely won't cope with frost, which is why you can only put it outside on frost-free days, and only leave it out after the night frosts are over, after the middle of May.
When you put it outside, please put it first in a warm, half-shady spot, and not in full sun, or the leaves will feature burnt areas from the strong irradiation. Only after an approximately ten to fourteen-day period of getting used to being outside can the plant slowly be introduced to full sun (in a warm place, protected from the wind). However, in particular in a warm and sunny location, it makes sense to replant the hibiscus into a pot with a water reserve so that it is always supplied with sufficient water – it needs a lot of water but must not get too wet. It would be a good time to plant it in fresh quality soil anyway, combined with a high proportion of clay. I myself like to use the citrus and pot plant soil by frux for all pot plants. When planting, you can immediately add three grammes of slow-release fertiliser (6-8 months) per litre of soil. Repotting is necessary every two to three years, depending on the pot volume. When doing so, reduce the soil around the roots by approximately a third. Add slow-release fertiliser each year at the end of March.
In warm, damp summers, the slow-release fertiliser, amongst other things, is used up more quickly. For this reason, I generally only use slow-release fertiliser as a deposit fertiliser, and add a little liquid fertiliser (flowering plant fertiliser, “half quantity”) when watering. With this double strategy (and keeping a close watch out for any over-fertilisation or fertilisation deficits), my plants in pots and containers always remain fairly well fertilised.
As far as pests are concerned, please keep an eye out for infestation with aphids or whitefly; hemispherical scale infestation occurs more rarely.
After blossoming, shorten the shoots to approximately five leaves and let them bud again so that the plant becomes bushy. Should the plant become too bushy, prune it somewhat.