When to cut back perennials
Cutting back herbaceous perennials during autumn restores order and tidiness to the garden. However, this removes potential winter interest, in the form of height and structure, plus food and habitat sources for wildlife so many gardeners delay the cut back until spring.
Selective cutting back in autumn can retain the dried, bleached flowerheads of plants, while removing material showing signs of decay or fungal growth. Examples include: such as Eryngium (sea holly), Phormium (New Zealand flax) and the foliage and flowers of ornamental grasses.
More tender plants with woody stems, such as penstemons, are left so that the old stems protect the crown from frost. Leave pruning of these and other borderline-hardy perennials until the risk of frost has passed – usually April or May.
Evergreen perennials such as certain Kniphofia and ornamental sedges are not cut back, but are tidied during spring and summer by removing dead foliage.
After cutting back, mulch and fertilise to promote growth and flowering.