It’s a myth that plants benefit from regular division. Many can be left for several years before they need dividing. Division is done for two reasons, to get more plants, or to restore vigour. Generally, those plants that form a dead patch in the middle need rejuvenating.
There was a time when every herbaceous perennial was divided yearly - but this isn't the case now. Nowadays you only need to split most if your plants are either losing vigour, or if you want to propagate more.
When to divide: the general rule
The well-accepted rule regarding dividing hardy herbaceous plants is:
If it flowers before Midsummer’s Day (generally June 21) you should divide it in autumn, allowing them plenty of time to recover.
If it flowers after Midsummer’s Day you should divide in spring, just as the growth re-shoots. Keep the divisions well watered in dry springs.
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.
The exceptions to the rule
A lot of gardeners divide and lift these in autumn. They seem to thrive on this treatment, although your asters need to be done in spring.
Hostas, astilbes and hemerocallis
These can be divided in the autumn and into winter as long as the soil is not freezing. You will need a spade to chop them up into sections. Replant into soil enriched with compost or green waste.
These should be divided and replanted in October and November ideally. Pick out the plumpest tubers with the fattest buds and discard the rest.
These are best propagated after flowering. Discard the old rhizomes and replant the firm new rhizomes which look paler in colour. Place the rhizome in sun, with the trimmed fan of foliage behind the rhizome so the plant’s toes are catching the sun. Cover them with an inch of soil and let them push themselves upwards.
Many grasses do not divide well. If you tackle them do so in spring and take off large sections and pot them up, or replant them. Stipa is particularly tricky.
These form long strings of corms, with the newest at the top. Replant these new corms to a depth of at least four inches. Discard the older corms. Only divide crocomsias if needed. They prefer to be left to their own devices.
What not to divide
Hellebores do not respond well to being divided. If you do so, cut into large sections and not small noses.
Dieramas hate being divided. Leave them to get on with it.
Japanese anemones, which have roots like thin shoelaces, place themselves. Lift them in late winter and early spring and lay the roots on to trays of light compost. Cover lightly.
Some plants are tap-rooted and therefore can’t be divided successfully, however they usually produce copious amounts of seed. Tap-rooted plants such as most verbascums, most eryngiums, hollyhocks, most poppies, acanthus and lupins are raised by root cuttings taken in 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 of compost.
The hardest thing is recognising what the right way up is. To distinguish between the top and bottom, make a slanting cut at the lower end and flat top at the top to tell the ends apart. Leave in trays or pots until they shoot.
What plants needs regular division
Most plants can stay undivided for many years without being a problem. But some, such as 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.
Do follow your instincts because plants can vary in vigour. If a plant is doing well, leave it well alone.
Plants with fleshy roots, such as heucheras and primulas, are best divided regularly once the plants become lax and leggy.
How to divide
Arm yourself with two border forks, a spade and a tarpaulin.
Lift the whole plant and drag it onto the tarpaulin.
If the roots can be teased between the fingers (as they can with many perennials) pull away some of the outer pieces, because these are the most vigorous, and discard the middle section.
Shake off the soil and replant individual pieces. Water well.
If your plant resists your fingers, use two forks back to back and lever pieces away by pulling the handles in opposing directions. Shake off the soil and replant the vigorous outer pieces.
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 four to six weeks.
If the rootstock is solid or woodier, as with some hemerocallis, agapanthus and kniphofia, try the spade. Failing this, use a large knife or a saw to cut into the rootstock. Don’t cut solid rootstocks into small pieces. These woodier roots are often best done in late spring when the soil is warmer. Achilleas and all irises are also best divided in late-spring as well. Replant sizeable chunks.
Dividing tender plants
Divide tender plants in late-spring, when the soil is warm. Agapanthus, kniphophia and alstroemeria come into this category. Keep well watered.
Taking daisy cuttings
Members of the daisy family can also be raised from basal cuttings taken from the new growth. Aim to take some cuttings when the new growth is three inches in length. Just take a knife and remove the shoot above a node, the bumpy bit where the leaves emerge. Pot the cutting up into gritty compost, either in a pot or in small seed tray.
Read our guide to taking root cuttings.