It may still seem like summer but the rains can't be far away now and prevention is always better than cure, especially when waterlogging is involved. Plant roots need air to survive although only in tiny amounts. If the soil gets very wet, the plant can drown.
Signs of waterlogging include reduced growth, yellowing of the older leaves, drooping and blackened, smelly roots. Once the roots are damaged the plant is unable to absorb nutrients or take up water and the leaves develop chlorosis and drop.
To prevent soil becoming saturated, dig in plenty of compost. The action of digging also helps to break up the soil and prevent compaction. Mulching is an important way to help condition the soil as it encourages worms to do their work. Pot plants should be raised on 'feet' of some sort so that excess water can drain out from the bottom.
Also check that guttering and downpipes are cleared so that water cannot overflow onto your plants and drown them.