How do I cut back ornamental grasses without killing them?

The one test that separates the grasses you cut to the ground from the ones you comb through, when to do each, and why cutting an evergreen grass down often kills it.

Difficulty: beginner
Time: 40 min
Read: 5 min

Short answer

Part the clump in late winter and look for green at the base. All brown, no green means deciduous — cut the whole clump to a hand's height in late winter, just before growth starts. Green mixed with brown means evergreen or semi-evergreen — comb the dead material out with a gloved hand or a rake and leave the living blades standing.

Ornamental grasses are lumped together in most garden advice and they divide sharply into two groups that need opposite treatment. Get it right and both improve every year. Get it wrong and an evergreen grass cut to the ground either dies or spends years as a bald stump, while a deciduous one left uncut becomes a tangle of dead thatch that smothers the new growth.

What you'll need

Shears, secateurs or a hedge trimmer for large clumps
Stiff gloves — many grasses have sharp edges
Spring-tine rake (optional)
String, to tie a large clump before cutting (optional)

Cut it or comb it: the two kinds of ornamental grass

Step by step

Do the test in late winter.Part the clump with your hands and look right down at the base. Living green blades among the brown means evergreen or semi-evergreen. Uniformly brown and dead throughout means deciduous. This single observation decides everything.
Deciduous grasses: cut the whole clump down.Miscanthus, Calamagrostis, Panicum, Molinia, Pennisetum, Deschampsia and the deciduous sedges. Cut to about a hand's height above the ground in late winter or very early spring — February or March — just before the new shoots emerge.
Leave the dead growth standing until then.It is one of the best things in a winter garden, holding frost and low light, and it shelters overwintering insects. Cutting deciduous grasses in autumn throws away four months of their value.
Tie a large clump before cutting it.Wrap string round a big miscanthus and tie it before you start. The whole thing then comes off in one manageable bundle rather than blowing across the garden.
Cut before the new shoots appear, not after.Once new growth is up, shears cut the tips off it and the plant carries brown-tipped blades all season. Late February is usually right; in a mild early spring, act sooner.
Evergreen grasses and sedges: comb, do not cut.Stipa, Festuca, Carex, Luzula and Anemanthele. Put on a stiff glove and run your hand up through the clump, or use a spring-tine rake, pulling out the loose dead material and leaving the living blades. Do it in spring.
Understand why cutting the evergreens down is fatal.Their growth points are on the living blades and at the base of them. Shearing the clump to the ground removes them, and the plant either fails to regrow at all or takes several years to recover as a bald stump.
Trim only the visibly dead tips on an evergreen if it needs tidying.Cut individual brown blade tips or remove whole dead blades at the base. Do not shear across the top of the clump, which leaves an obvious flat cut line on every blade.
Divide congested clumps in spring, not autumn.Most ornamental grasses are best split when they are coming into growth, in April or May. An autumn division of a warm-season grass often rots over winter.
Feed sparingly.Most ornamental grasses come from poor ground and are better on a lean diet. Rich feeding produces soft floppy growth that collapses, which is the commonest reason a miscanthus splays open.

Tips

Stipa tenuissima looks dead by late winter but is usually alive at the base. Comb it out rather than cutting it — though it is short-lived anyway and self-seeds prolifically, so replacements are free.
Pampas grass is a special case: it is cut back hard in early spring and the old material is very sharp. Wear gauntlets and eye protection, and note that burning it, once traditional, is not a sensible domestic practice.
Grass blades on many species are genuinely sharp. Stiff gloves are not optional on miscanthus and pampas.
The cut material is excellent brown compost material and shreds well if you have a shredder.

Common mistakes

Cutting an evergreen grass or sedge to the ground — The growth points go with it. The plant often dies, or spends years recovering as a bald stump.
Cutting deciduous grasses in autumn — You lose the entire winter display, which for most of these plants is half the reason to grow them, and you remove insect shelter.
Cutting after the new shoots have come up — The shears take the tips off the new growth, and the plant carries brown-tipped blades all season.
Feeding grasses generously — Most want poor ground. Rich feeding produces soft growth and a clump that splays open in the first summer storm.

If it doesn't work

A grass cut to the ground has not regrown by June

Cause: It was an evergreen and the growth points were removed — Fix: Water and feed lightly and give it a full season. Some recover slowly; some do not. Comb rather than cut in future.

Dead thatch smothering the new growth

Cause: A deciduous grass never cut back — Fix: Cut the whole clump to a hand's height in late winter and rake the thatch out.

Clump splayed open with a hole in the middle

Cause: Congestion, or over-feeding — Fix: Divide in spring, replanting the vigorous outer sections and discarding the dead centre. Stop feeding.

Brown cut tips on every blade all summer

Cause: Cut too late, after new growth started — Fix: Nothing to do this year. Cut in February next time.

Questions people ask

How do I know whether to cut or comb a grass?

Part the clump in late winter and look at the base. Green among the brown means comb it. All brown means cut it to a hand's height.

When should I cut back miscanthus?

Late winter or very early spring, February or March, before the new shoots emerge. Leave it standing through the winter for the display.

Can I cut back Stipa tenuissima?

Comb it rather than cutting. It looks dead in winter but is usually alive at the base, and shearing it to the ground frequently kills it.

When do I divide ornamental grasses?

Spring, as they come into growth, rather than autumn. Warm-season grasses divided in autumn often rot over the winter.