Nowadays you only need to split most if your plants are either losing vigour, or if you want to propagate more. As to when, the broad rule is that perennials that flower in the first half of summer (ie before the end of July) are divided in autumn - allowing them plenty of time to recover. Similarly late-summer or autumn performers aren’t divided until spring. Split them just when as new growth is beginning to get going.
However those of you who garden on heavy, waterlogged clay will lose plants if they’re divided in the autumn. So if you’re on heavy clay you should wait until spring before lifting anything, or indeed planting anything. Basically you need to divide just as the new foliage emerges. So earlier flowering plants which shoot from the base first, can be divided in early spring. Later flowering plants, which shoot later, should be left until late spring.
Looser rootstocks respond to being levered apart with two forks. Simply dig up your clump and place two forks in the middle, back to back, and lever them apart by pulling the handles in opposing directions. Select the vigorous outer pieces for replanting.
Larger pieces separated like this can be planted straight back into ground enriched with a slow release fertiliser - like blood, fish and bone. Smaller pieces are often best potted up in soil based John Innes number 2. These can be planted out properly once the roots reach the bottom of the pot - usually after 4 - six weeks.
Woodier rootstocks will need cutting with a large kitchen knife, or saw. Or they can be chopped up with a spade. These woodier roots are often best done in late spring when the soil is warmer. Hemerocallis, kniphofias and agapanthus fall into this category. Achilleas and all irises are also best divided in late-spring as well.
Some plants are tap-rooted and therefore can’t be divided successfully. These include most verbascums, most eryngiums, hollyhocks, oriental poppies, acanthus and lupins. However tap rooted plants usually produce copious amounts of seed. Root cuttings can also be taken in late winter or very early spring. Dig the plant up and cut away a section of pencil-thick root. Cut it into one-inch pieces and place them, the right way up, in trays or pots of compost.
Most plants can stay undivided for many years without being a problem. But heleniums and phloxes are the exception. They need regular division every second or third year to maintain vigour. I like to replant small pieces back into new ground where possible.
The most difficult plant of all to divide is the oriental hellebore. A large plant can be cut in two after flowering and then replanted. But smaller pieces often fail to grow at all. So this is one plant best left to its own devices.