hibiscus turning yellow

This is a bit of a fussy plant - it likes humus enriched soil which is well drained, but doesn't like to be too dry, and needs the shelter of a wall and a sunny position. If you planted it this year, or recently, have you been making sure its watered enough? Leaf yellowing at this time of year might simply be autumning, particularly if the plant is very dry at the roots. One other thing, though - is your soil chalky or very alkaline? If so, you could try giving the plant a shot of sequestrene mixed in a watering can - garden centres sell it. Don't buy one of the acidifying feeds though, it shouldn't be fed this late in the year.