Advice
RHS Help & Advice
Tidying up perennial grasses
Grasses are like other herbaceous perennials: their flower stalks can be cut off or allowed to go to seed.
The spent flower stalks of grasses, whether evergreen or deciduous, keep their structure well so are often left uncut over winter, and only removed early the following year. The stalks may be bleached of colour but still provide height, structure and movement in the garden when most perennials are invisible.
They look beautiful covered with hoar frost, and winter wildlife appreciates both the shelter they offer and the food the seeds provide for birds and small mammals.
Evergreens
Small evergreen grasses (right), such as Festuca glauca, can be trimmed in spring. Remove any brown tips and cut back the dead leaves that usually collect around the base.
Larger evergreen species, such as Stipa gigantea, also just need the dead leaves removing.
However, Cortaderia selloana (pampas grass) (left), benefits from a hard annual pruning, cutting back as far as possible without damaging new growth. Cut tough stems with loppers, wearing goggles and thick gloves as leaf blades have sharp edges. Burning pampas grass is no longer common practice as the crown can be damaged. After trimming, weed the crown, pulling out loose blades of grass, and mulch around it with organic matter or gravel.
Deciduous
Some deciduous species, for example Stipa tenuissima, Calamagrostis x acutiflora 'Karl Foerster' and Deschampsia cespitosa 'Goldtau' (left), should be trimmed to ground level before growth starts in early spring.
Other deciduous grasses, such as Pennisetum orientale, do not produce new growth until later in the season. The culms (old stems) will protect the crown, so delay clipping these types until late April.
This method is also appropriate for Miscanthus, which has structural stems that persist over the winter (right). These should be pruned away individually with secateurs to ensure the new, green shoots are not cut off in the process.
Sedges
In general, sedges don’t respond well to hard pruning and should just have the brown tips removed. Alternatively, lift clumps of wood sedge (Luzula sylvatica), the brown Carex buchananii, and striped C. oshimensis in spring, to remove dead or poor sections before replanting.
Other grasses
Some grasses will need extra attention. Those that seed readily, such as Stipa tenuissima, may need to be thinned out, while a number of Pennisetum and Elymus produce underground runners. Use a trowel to expose unwanted clumps and either dispose of them or transplant to new areas.
Attend to the popular but tender Pennisetum setaceum ‘Rubrum’ when it begins to shoot in the greenhouse. Cut away any dead leaves and, if necessary, gently pull or cut out dead sections. Apply a liquid feed and, once it’s growing rapidly, place this grass outside in May.
Further information
For a step-by-step approach to cutting back grasses click here