Why the autumn tidy-up can wait

There is a strong instinct in October, once the borders start to collapse, to get out the shears and tidy everything down to the ground. Resist it. For most deciduous ornamental grasses, an autumn cut is the single worst thing you can do. You remove the bleached stems and arching seed heads that carry the garden through to March, and you expose an open, soft crown to months of cold and wet. Miscanthus, calamagrostis, panicum and deschampsia all resent it, and the result is usually a rotted, hollow centre and a shabby clump the following summer.

The one genuine exception is tender material that you are treating as seasonal bedding, but even then, leaving the top growth in place over winter gives the crown a little insulation. In short: leave it all standing, and save your effort for a single decisive cut in early spring.

Which grasses to leave standing, and which to handle gently

It helps to sort your grasses into two rough camps before you do anything.

• Deciduous grasses — Miscanthus, Calamagrostis x acutiflora, Panicum virgatum, Deschampsia cespitosa, Molinia caerulea, Hakonechloa macra, Sporobolus. These die back completely. Leave them alone all winter and cut them hard in early spring.
• Evergreen grasses and sedges — Festuca glauca, Carex oshimensis, Carex morrowii, Stipa tenuissima. Never cut these to the ground. In spring, comb through the clump with your fingers or a rubber rake to pull out the dead blades, and trim only the worst of the tips with scissors.
• Semi-evergreen types — Anemanthele lessoniana, Stipa gigantea. Tidy lightly. Cut out the old flowering stems of Stipa gigantea back to the base in late winter, but leave the foliage alone.

Getting this division right matters more than any other detail. Cutting an evergreen fescue to ground level will leave you with a dead patch in the border, while leaving a Miscanthus uncut for three years gives you a thatch so dense that new shoots struggle to push through.

Seed heads, birds and winter structure

The wildlife argument for leaving grasses standing is a strong one, and nowhere more so than in a garden with a pond or boggy corner. The seed heads of Miscanthus, Deschampsia and Calamagrostis are stripped by goldfinches and other finches through the hardest weeks of the year, and the hollow stems make excellent overwintering quarters for ladybirds, lacewings and small solitary bees.

Dense clumps at the water's edge also give frogs, newts and ground beetles somewhere damp and sheltered to sit out the cold. Add the way a frosted seed head catches low winter sun, and you have a plant doing three jobs at once. Cutting it down in October throws all of that away for the sake of tidiness.

Cutting back: timing and technique

The signal to act is new growth at the crown, not the calendar. In most of the UK that means late February to mid-March; in colder northern gardens, lean towards early March. Look for the first green shoots pushing up through the dead material, then move quickly, before they are tall enough to be damaged.

The trick that makes the job bearable is to tie the clump before you cut it. Wrap a couple of turns of twine around the middle, or slip a bungee cord over it, then cut horizontally about 10–15cm (4–6in) above ground with sharp shears or a hedge trimmer. The whole bundle lifts away in one piece and you are not chasing stems across the border for the rest of the afternoon.

• Never cut into the crown itself — leave a short stubble of stems.
• Rake out the centre of the clump afterwards so air can circulate and damp does not sit in the base.
• Clear every last clipping; decaying debris around the crown encourages rot.
• Wear gloves and long sleeves. Miscanthus leaves will slice through bare skin.

Grasses and sedges at the pond edge

Wetland marginals need the same patience, with a few extra rules. Carex elata 'Aurea', Phalaris arundinacea 'Picta' and other deciduous sedges and grasses can be cut back hard in late winter, but always cut above the water line. Clippings that fall in will rot, deoxygenate the water and feed algae, so gather them as you go and take them to the compost heap.

Leave a proportion of hollow stems standing around the pond margin — damselfly larvae and other invertebrates use them to climb out and emerge in spring. Reedmace and other Typha species can be cut in March once the seed heads have been picked over by birds. Evergreen sedges such as Carex oshimensis should only be combed through, never cut down. And resist the urge to tidy bog plants sitting in permanently wet ground in autumn; their crowns are far more likely to rot in winter than in spring.

Aftercare: dividing, feeding and mulching

Early spring is also the moment to deal with clumps that have gone bald in the middle or spread further than you wanted. Lift the whole plant and split it with two garden forks placed back to back, or with a sharp spade. Throw away the woody, exhausted centre and replant the vigorous outer pieces, watering them in well.

Mulch around — never over — the crown with well-rotted compost or leafmould, and give a light scatter of balanced granular feed. Hold back on anything high in nitrogen, particularly in damp ground, or you will get soft, floppy growth that falls apart in the first autumn gale. In wet gardens, plant grasses on a slight mound with a handful of grit in the planting hole. Do that, and your grasses will reward you with a winter's worth of structure and a clean, vigorous start every spring.