Saxifrage is Just one of a number of ground-hugging plants which spread out from the centre, seeking fresh nutrients in new ground. The healthy young growth is at the outer edge, and in time, the centre of the clump dies back. This is a natural tendency, and you can maintain a tight cushion only by the artificial means of controlled and regular feeding.
After each flowering, sprinkle some fine bone meal into the soil at the centre of the plant. Use a level tablespoonful for a plant about 9in across, proportionately less for small plants. Do not overfeed, though, or you’ll get abnormal growth at the centre and ruin the cushion effect.
The stems at the centre also become straggly as they get older. The heart of the plant is the oldest part, where you get a build-up of fading leaves. To cure this, remove any fading leaves with a pair of tweezers, and each spring sprinkle some fine, soil-based compost into the centre of the plant. Eventually, a hummock of soil will build up, giving the roots more space and restoring the plant’s cushioned shape