A herbaceous clump that has been in the same spot for years starts to go bald in the middle. The outside is still healthy, the centre is woody and bare, and the flowering drops off. That's not the plant dying, it's the plant asking to be split, and splitting it costs nothing.
Why it happens
A clump-forming perennial grows outwards from the middle. The centre is the oldest tissue: over a few years it gets congested and woody and stops throwing up new shoots, while the young growth keeps migrating outwards. All the vigour ends up in a ring round the edge.
Split it, throw the tired middle away, and replant the outer sections. Each one is a young plant again — which is also why replanting them back into the same border works perfectly well. It was never the soil that gave out.
When to do it
Spring or autumn, when the soil is workable — not frozen, not baked, not sodden. The plant needs to get its roots going before it's asked to do much else.
Spring is the safer of the two for most things and the one I'd default to: the soil is warming, growth is about to start, and there's a whole season ahead to recover. Autumn works too while the soil still holds warmth, and suits things that flower early in the year.
Avoid splitting anything in full flower or in the middle of a dry spell. You're asking a plant to rebuild its roots at the moment it can least afford to.
Grasses are two groups, not one
This catches people out, because "divide grasses in spring" is only half the advice:
Cool-season grasses — Carex, Festuca, Stipa — go as they come into growth, late winter to early spring.
Warm-season grasses — Miscanthus, Pennisetum, pampas — want late spring, around May. They don't start moving until then, and lifting one in March means it sits cold and wet doing nothing.
Either way, not autumn.
How often
Every two or three years suits most clump-formers. Slower growers — hostas, agapanthus — are content left four or five. In practice, do it when the plant tells you:
The middle has gone bare or woody.
It flowers noticeably less than it used to.
It's outgrown the space and is crowding its neighbours.
If none of those is true, leave it. Splitting a happy plant is work for the sake of it.
The ones that want different treatment
Peonies resent disturbance and are the exception to the spring default — if one has to be moved or split, do it in late autumn. Depth is the thing that matters most: no more than an inch (2.5cm) of soil over the buds. Plant a peony too deep and you get perfectly good leaves and no flowers. It's fixable — lift it in autumn and replant it shallower — but you lose a year or two doing it, and a division normally takes about two years to flower anyway.
Bearded irises are lifted and split after flowering, in summer. The rhizome sits on the surface on heavy ground, or just under it on light sandy soil, where it would otherwise work its way up.
Oriental poppies can be divided in spring, but root cuttings in late autumn or early winter are the more reliable route. That's the useful part: a snapped piece of root is a new plant rather than a loss.
Hellebores divide fine — early autumn, or straight after flowering — but they're slow to settle afterwards, so don't expect much of them the following year.
Woody shrubs are not perennials. If it has a permanent woody framework, this article isn't about it.
Doing it
Nothing here needs special kit:
Water the plant well the day before if the ground is at all dry.
Lift the whole clump with a fork, working round it and levering rather than hauling on the top growth.
Split it. Small clumps come apart in your hands. Tough ones want two forks pushed in back to back and levered apart, or a spade straight through. It looks brutal and the plant doesn't mind.
Keep the outside, bin the middle. Aim for pieces with three to five good shoots each and roots to match. Smaller than that and you're nursing something through a season for nothing — that criterion, not the size of the original clump, decides how many plants you get.
Replant at the same depth it was, not deeper, and firm it in.
Water it in properly, and keep watering through the first dry spell. This is the part people skip, and it's the part that decides whether it takes.
Cutting the top growth back by about half as you replant takes some of the load off the roots while they re-establish. Worth doing on anything leafy.
Bottom line
When the middle goes bald, lift it in spring, split it into pieces with three to five shoots each, and replant them at the same depth. Water them in and keep watering. Done properly you get two or three plants back for one.
If a border has got beyond splitting and into needing clearing back first, that's the job to do before this one. And a freshly split border is a good moment to get a mulch on while the soil is open.
Border work is a good part of what I do around Ledbury, Ross-on-Wye and Upton-upon-Severn. Give me a call.