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Perennials are not demanding plants, but trimming them after flowering finishes in autumn helps improve their appearance and flowering. However, you can leave some stems over winter to provide homes and food for wildlife, and then trim back in spring.
Cutting back perennials. Credit: RHS Advisory.
All herbaceous perennials and ornamental grasses that die back to soil level.
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.
Although the general principles are the same, there are a few differences depending on what season the work is carried out. This is explained below.
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:
Early-flowering perennials such as geraniums and delphiniums are cut to near ground level after flowering to encourage fresh foliage and late summer flowering. These are then cut back again in autumn or spring.
Sometimes gardeners are caught out by earlier-than-expected growth of perennials in spring. In these cases, rather than cutting out new growth, merely tidy up the plants by pulling out dead stems.
In very wet winters, the soil can be too wet to access plants without compacting soggy soil. Rather than damaging the soil, it is best to wait until it is drier in spring; and then tidy up the plants by pulling out dead stems. Where soils are prone to waterlogging and damage, carry out cutting back in autumn.
Borders: revitalising an over-mature bed
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