Are you cutting back?
To cut back or not to cut back, that is the question. Now, I’m not talking about reducing spending on your favourite plants - perish the thought! I’m talking ‘cutting back’ in the sense of should you remove dead tops of herbaceous perennials, and, if so, when?
In this post I’m looking specifically at herbaceous perennials rather than shrubs and climbers. I’ll be covering other types of cutting back (ie pruning) in future blog posts, at the appropriate time.
Back to herbaceous perennials. To begin with let’s look at when is the best time to cut back. Well, there are two schools of thought.
The first is that, generally speaking (and there are always exceptions), perennials should be cut back in the autumn, after the flowers have faded and the leaves have browned. This certainly brings an overall tidiness to the garden. In addition, you can be sure that any material that is diseased or is likely to harbour detrimental pests is removed, making sure that you’re not storing up problems for next year.
There are some perennials that should be cut back in the autumn. These include: Paeonia and Crocosmia which produce leaves and flower stems from below soil level; also Phlox, Monarda, and some species of Symphyotrichum (Aster) because they are prone to powdery mildew.
The second school of thought is that you can leave any cutting back until the spring – this is the one that I tend to favour, with the ‘definitely autumn’ exceptions above.
Some plants like Penstemon and other borderline hardy perennials are best left until the spring simply because the old stems help protect the crown of the plant from damaging frost.
There are two main reasons why I like to leave my cutting back till spring, however. The first is that there are many perennials that have lovely winter interest. The renowned Dutch landscape designer Piet Oudolf describes such plants as those that ‘die well’. You only have to look at some of his planting designs in the depths of winter to see what he means. One trick to find out if your plants will ‘die well’, is to take a black and white photo of them when they are still going strong. Obviously the flowers will be fuller, and the foliage more lush, but it will give you a pretty good idea of how they will look when the colour has faded and you’re just left with the structure and form of the plant. Imagine spent flowers of Achillea, say, covered in frost sparkling and glistening in the winter sun? Or the cones of Echinacea, bereft of their tutus of petals, but still beautiful in their naked state?
The second, and perhaps more important reason, is that spent flowers and foliage provide food and habitats for various forms of wildlife over winter, from birds finding seeds in desiccated flowerheads, to groups of ladybirds huddling together in shrivelled foliage.
Another reason has to do with the weather – surprise, surprise. If we have a particularly wet autumn it might be impossible to get to the plants without compacting and damaging the soil, in which case it is best left until spring when, hopefully, the soil will have dried out a little.
That’s the ‘when’, what about the ‘how’?
Whether you cut back in autumn or spring, the process is the same: cut the stems close to the base or crown of the plant using secateurs or a sharp knife. If, however, there is any young growth, don’t cut into it, cut just above it.
There are exceptions. For example, if your Penstemon that we mentioned earlier have survived over winter and they show new growth from the base of the plant, cut out the old stems to within a couple of centimetres of the base. If there is no growth from the base, but shoots are appearing further up the old stem, cut the old growth back to where the new shoot has appeared. This is also true for any other evergreen or semi-evergreen plants.
The other exception is perennials that flower early in the season, such as Delphinium and some Geranium. These can be cut back hard after flowering – this will encourage new growth and you will get a second flush of flowers later in the season. Then you can either cut them back again in autumn or wait until spring.
Grasses should only be cut back in spring, and then it depends on whether they are evergreen or deciduous. Evergreen grasses only need to be tidied up, removing any dead material; deciduous grasses, however, can be cut back hard.
That’s it. As you can see, there are no hard-and-fast rules. Like many other aspects of gardening, find a routine that suits you and your garden, and you can’t go far wrong.