IT is useful to move or to divide herbaceous plants while they still have foliage and stems. That way we see very readily the spread and height of a plant, exact details which will be quickly blurred or forgotten if we leave all moving and division until next spring.
A great many plants like autumn division and moving. There are exceptions - ornamental grasses and the aster family do not enjoy being meddled with now and it will be safer to leave them until spring, likewise late flowering things such as crocosmia (montbretia) and kaffir lilies.
Otherwise we can dig, divide and replant until the soil gets really wet and cold. The sooner plants are settled in the better they will like it. Most herbaceous plants need division to maintain vigour and to encourage the development of healthy young roots.
Division can be by pulling the root apart and this is easily achieved with a clump of bulbs or corms or with most roots of the daisy family. Other roots, like hostas, must be cut up into pieces with a knife or a spade.