The Chelsea chop stems from the famous flower show: any plants that hadn’t made the grade would get a hard haircut, prompting a new flush of growth and, with luck, flowers. Thus something that was spent became sellable. It’s a good trick and can be employed for all sorts of perennials. It’s easy to remember, too: once Chelsea is over, start the chop.
There are a number of benefits to chopping back. Spring-flowering perennials can be persuaded to have another go – pulmonarias and Siberian bugloss (Brunnera macrophylla) are good examples. As long as the plant hasn’t set seed (and this is key), it will flush with new leaves and try to flower again. Bergenias that have just finished flowering can be given a similar haircut. Have a go with wallflowers (both the normal biennials and the perennial ‘Bowles’s Mauve’). Be hard but kind: follow with a good soak and a little feed or mulch.
Chop back now to limit overall height and manipulate plants into a compact, bushier shape that is less likely to flop at the end of summer. Choose late-flowering perennials for this method. If you were to attack something that wants to flower in June, it’s likely to sulk, and though it would recover, you’d miss out on flowers altogether. However, plants that flower in August and September have plenty of time to reboot. In fact, for these plants a light chop now is no different from being munched by a herbivore.
Plants that respond best to such treatments tend to be prairie or meadow plants that have evolved to be eaten at some point. Heleniums, asters, eupatorium, rudbeckias, echinacea, sedums and the milky bellflower (Campanula lactiflora) are ideal.
Chop back by a third, though take the tallest back to half. The harder you chop, the smaller the flowers, though you often get more flowers as a result. It does involve some experimentation – soil, aspect and rainfall will all affect regrowth.
You can’t go wrong with sedums, either ice plants (Sedum spectabile) or orpine (S. telelphium). Pinch both back by half and the result is a sturdier plant that won’t flop to show its unsightly middle. Likewise Phlox paniculata responds well to being cut back by a third.
Be pretty brutal about it and use shears if you’ve got large spaces to tackle, or carefully snip through: the results tend to look identical. Or get fancy and try a layered look, cutting the front of the clump lower than the back, effectively making the plant stake itself.