How to divide perennials, and when to do it
Most perennials are best divided in spring or autumn, when the soil is moist and workable and the plants aren't in full flower. Lift the clump, split it into healthy pieces with roots and shoots, discard any tired woody centre, and replant at the same depth, watering well. Done every three to five years, it keeps plants vigorous and flowering, and gives you free plants for the rest of the garden.
Why divide perennials?
Many clump-forming perennials grow outwards over time. The young, vigorous growth sits around the edge, while the centre becomes woody, crowded and less productive. You'll often see a ring of foliage with a bare or weak patch in the middle, or notice that a plant that used to flower generously is producing fewer blooms.
Dividing refreshes the plant by putting the young outer growth back into good soil with room to grow. It's also one of the most satisfying jobs in the garden, because one well-established clump can easily become three, five or more new plants. That's how you build up the generous drifts and repeats that make a border look settled, something I talk about in my post on planting a border that looks good all year.
When to divide
The simplest rule is to divide in spring or autumn, when plants are not in active flower and the ground is moist but not waterlogged.
Autumn: ideal for many summer-flowering perennials on free-draining soil. The soil is still warm, so roots settle in before winter. See my October garden jobs for timing.
Spring: best on heavy or wet soils, and for late-flowering perennials and grasses. Divide just as new shoots are emerging.
After flowering: early spring flowers, such as primulas and pulmonarias, can be divided once they've finished.
Bearded irises: divide in summer, a few weeks after flowering, replanting the rhizomes so their tops sit at the soil surface.
Grasses are worth a special note. Warm-season grasses such as Miscanthus, Panicum and Molinia are best divided in spring as they start into growth, not in autumn.
How to divide perennials
Water the day before if the soil is dry, so the clump lifts more easily and the roots are well hydrated.
Cut back any tall top growth so you can see what you're doing and the new divisions lose less water.
Lift the clump by working a fork or spade all the way around it, a little way out from the crown, then levering it up.
Split it. Small, fibrous-rooted plants can be teased apart by hand. For bigger clumps, push two garden forks in back to back and lever them apart. Tough, woody clumps may need a sharp spade or an old bread knife.
Choose the best pieces. Keep healthy sections from the outside, each with a good set of roots and several shoots or buds. Compost the old, woody centre.
Replant promptly at the same depth as before, into soil refreshed with some compost or a peat-free soil improver. Firm in and water well.
Smaller divisions can be potted up in peat-free compost and grown on for a season before planting out, or shared with friends. Keep new divisions watered through their first dry spell.
Plants that divide easily
Plenty of reliable border perennials respond really well to division. Hardy geraniums, asters, heleniums, hostas, astrantias, nepeta, achillea, persicaria, rudbeckia, daylilies and phlox are all good candidates. Hostas are easiest in spring, just as their shoots are pushing through.
Sanguisorba, one of my favourite plant groups, can also be lifted and divided in spring or autumn once a clump is well established. It's a lovely way to get more of varieties such as 'Cangshan Cranberry' or 'Tanna' to repeat through a border. There's more on growing them in my Sanguisorba plant of the month post.
Plants to leave alone
A few perennials prefer not to be disturbed. Peonies are happiest left in place for many years. Plants with deep taproots, such as Eryngium, Baptisia and oriental poppies, don't divide easily and are better propagated by root cuttings or seed. Hellebores can be divided with care, but often sulk afterwards, so it's usually best to leave a happy plant alone.
Dividing perennials checklist
Divide every three to five years, or when the centre of a clump dies out.
Choose spring or autumn, avoiding frozen, waterlogged or very dry soil.
Divide grasses such as Miscanthus in spring.
Keep healthy outer sections with roots and shoots, and compost the woody centre.
Replant at the same depth and water well.
Leave peonies and taprooted plants undisturbed.
Quick answers
Can you divide perennials in summer?
It's best avoided for most plants because of heat and dry soil, though bearded irises are traditionally divided in summer after flowering. If you must move something in summer, water it very thoroughly before and after.
How often should perennials be divided?
Most vigorous perennials benefit from division every three to five years. Slower plants can go much longer, so divide when you see a weak centre or fewer flowers rather than to a strict timetable.
Will a divided plant flower the same year?
Often, yes, especially larger divisions made in spring or early autumn. Small pieces may take a season to build up before flowering well.
