How to Divide and Replant Ornamental Grasses

Why divide ornamental grasses, and the signs it is time

Ornamental grasses are among the most generous plants a British garden can hold. They shrug off wind, need little feeding and look good well into winter. But they are not immortal in one spot. Left alone for four or five years, most clumps develop a dead, woody centre with a ring of tired growth around it. Flowering thins, the stems flop after rain, and the whole plant starts to look like a doughnut someone sat on.

Division fixes all of that. You cut away the exhausted middle, keep the vigorous outer pieces, and replant them into soil you have improved while you had the chance. The result is a tighter, brighter clump, plus spares to fill gaps elsewhere or pass on to a neighbour. It is also the cheapest way to make more plants, since grasses rarely come cheap at the garden centre.

Watch for these clues that a clump is ready for the fork:

• A bald or hollow centre when you part the stems
• Flower heads smaller and fewer than in previous years
• The clump splaying open in wind instead of standing upright
• Roots pushing out of the soil surface, or the crown sitting proud like a pudding

Timing: get this right and the rest is easy

Grasses divide happily, but only if you catch them at the right moment. The rule is simple: divide when the plant is about to grow, not when it is trying to rest or flower.

• Deciduous cool-season grasses such as Deschampsia, Molinia and Calamagrostis: divide from late February to early April, just as new shoots break through the soil. They recover fast in cool, damp conditions.
• Warm-season grasses such as Miscanthus, Panicum and Pennisetum: wait until late April or May, when you can see fresh green growth. Lift too early and the roots sit in cold wet soil and sulk.
• Evergreen grasses such as Carex, Festuca and Stipa: late spring to early summer, ideally May or June. They have less stored energy to spare, so avoid autumn and winter division altogether.

Whatever the type, do the job on a dull, damp day and water the clump thoroughly the day before. Never divide during a heatwave or in the middle of a dry spell, and never leave lifted roots lying in the sun.

Cutting back and lifting the clump

Start by reducing the top growth so you can see what you are dealing with. On deciduous grasses, cut the previous year's stems down to about 10cm from the ground using secateurs or shears. Evergreen grasses should only be trimmed lightly, as cutting into old brown growth can leave permanent stubs. Tie the stems into a loose bundle before you cut if you want to save the seed heads for decoration.

Then lift. Water the plant well, wait an hour, and push a sharp spade vertically in a circle about 15cm out from the edge of the clump. Work around it, levering gently as you go. Slide the spade underneath and lift the whole thing onto a tarpaulin — grass roots are surprisingly shallow, but a mature Miscanthus clump can weigh as much as a bag of cement, so ask for help if it refuses to budge.

Once it is out, shake off excess soil so the structure of the root ball is visible. This is also your chance to check for vine weevil grubs, which love grassy crowns.

Splitting the clump: tools and technique

Small clumps can be pulled apart by hand. Larger ones need persuasion. Two garden forks pushed back to back into the centre of the clump and levered apart is the traditional method, and it works beautifully on fibrous roots. For tough, woody crowns, use a sharp spade, a pruning saw or even an old bread knife, cutting straight down through the middle as if you were slicing a cake.

• Replant only the outer, healthy sections — a fist-sized piece with strong roots and visible shoots is perfect
• Bin the woody centre; it will not regrow well
• Aim for three to five divisions from a decent-sized clump
• Trim any damaged or circling roots with clean secateurs, and shorten the top growth to about 15cm to reduce water loss
• Sit spare divisions in pots of multipurpose compost in the shade and water them while you finish planting

Replanting into improved soil

This is the part most gardeners rush, and it is the reason divisions sometimes fail. While the plant is out, dig the planting hole to roughly twice the width of the division and fork the base to break up any pan. Mix the excavated soil with two or three handfuls of garden compost or leafmould, and add a handful of horticultural grit if your soil is heavy clay. Grasses hate sitting in soggy ground over winter.

Plant so the crown sits at exactly the same level it was before — no deeper. Backfill, firm gently with your heel, and water in with a full watering can, even if rain is forecast. Space divisions according to their eventual spread; if in doubt, leave more room than you think, because a healthy grass will double in size within two years.

Finish with a 5cm mulch of composted bark or leafmould, keeping it clear of the crown itself. A scatter of bonemeal or fish, blood and bone at planting time gives roots a gentle start, but skip anything high in nitrogen, which produces floppy growth at the expense of flowers.

Aftercare, and when to leave well alone

New divisions need watering for their first full growing season. In dry spells, give each plant a thorough soak twice a week rather than a daily sprinkle — deep watering builds deep roots. Expect a little sulking in the first year: flowering may be modest, and the clump may look loose. Resist the urge to feed it heavily. By the second summer it will be back to its full glory.

Not every grass benefits from division, though. Short-lived species such as Stipa tenuissima are better raised from seed every few years. Running grasses with spreading rhizomes, and anything grown as a specimen for its arching habit, are usually best left undisturbed or lifted only when they outgrow their space.

Divide a few clumps each spring rather than all of them at once, and you will always have a garden full of vigorous, well-behaved grasses — and a few spare plants to trade.