Single trees or specimen shrubs should be mulched to about the radius of the canopy. Avoid the mulch coming in direct contact with the stem.
Bear in mind that you may need to water plants more than usual to get moisture to the roots – you have effectively made the soil deeper.
If you want to mulch a rock garden, remove the weeds and water the soil as before, then apply a layer of gravel around the plants. It should be about 2 to 4cm deep.
Only reapply mulch when it has completely rotted down.