Herbaceous perennials and grasses can be cut back in late autumn, when seasonal flowering has ended, or left until the early spring.
Cutting back in autumn
In gardens the cutting down and removing of dead or dying stems in the autumn restores tidyness. Separate and burn any diseased material (showing signs of leaf-spots, mildew or rusts, for example). Add all other material, excepting ripening seed heads, to the compost heap. Fork out weeds, which can be difficult to control if such material is allowed to accumulate, and apply a light mulch or feed in early spring.
Cut back perennials that produce leaves and flower stems from below the soil level, such as crocosmia and paeony, back to soil level. Cut back less severely perennials showing new basal shoot growth, asters and Sedum spectabile for example. Any attractive dead stems or flower heads can be left until early spring.
Cutting back in spring
Many gardeners choose to leave dead herbaceous plants and grasses over winter to provide structure to the garden, as well as food and shelter for wildlife. However, more care is needed when cutting back in spring to avoid damaging new shoot growth. Most gardeners start cutting back from March onwards.
The bleached flower stalks and foliage of large ornamental grasses such as Cortaderia selloana (pampas grass) and Miscanthus offer height and screening, while cardoons and ornamental fennel provide architectural interest even when bleached and dead. Other perennials are left to develop seedheads, to harvest seeds for propagation or to provide food and shelter for wildlife. More tender plants, such as penstemons, are left so that the old stems protect the crown from frost. Leave pruning of these and other boderline-hardy perennials until the risk of frost has passed, usually April or May.
With the arrival of spring, new growth arises from the base of herbaceous plants (including grasses) that die back to ground level over winter. Evergreen perennials, or those such as penstemons, that should survive the winter in milder areas, may either shoot from the base or from branching points higher up the old shoots.
Where growth arises purely from the base, cut the old stems back entirely to within a few centimetres of the ground, leaving the new shoots open to light and air. Tidy up the base of the plant, removing debris and replacing mulch as necessary. Tender plants, such as grass Pennisetum setaceum ‘Purpureum’, should remain under cover to protect the new shoots from frost.
Where growth arises higher up, simply shorten the old stems, cutting to just above a healthy leaf, branch or bud.