You may hear gardening programmes refer at this time of year to the Chelsea Chop which quite simply is a pruning technique that you can use on some of your herbaceous perennials to delay or extend, the flowering period, and to create sturdier plants that need less staking. The only reason it is called the Chelsea Chop is that the pruning is normally carried out around the end of May which coincides with the staging of the Chelsea Flower Show.
Making a Chelsea Chop in order to delay the flowering of perennials can delay those blooms by four to six weeks. There is no one method of chopping at the time of Chelsea: you can either prune all the stems on a clump, which delays all the flowers, prune down half of them, or prune a third of the stems by a third, and another third by two thirds. This spreads the plant’s flowering over a longer period by delaying flowering in those stems, and makes the plants more compact so you avoid leggy stems.
Why would you want to delay flowering in your garden?
Chopping some plants is useful in a border where all the plants bloom at the same time because then they all finish blooming at the same time, leaving you bereft of colour. Pruning back half the stems will extend the display as some plants will still flower in their natural season, while the rest will just come into bloom when the first ones have finished. Alternatively if you have several of the same plant you can cut some back and leave others untrimmed to stagger the flowering period of your favourite herbaceous perennials.
Plants that respond well to the Chelsea Chop
Plants that respond well to the Chelsea Chop include salvia, euphorbia, campanula, penstemon, delphinium, phlox, echinacea, achillea, nepeta, sedum (now known as Hylotelephium), and aster. The Chelsea Chop is not suitable for flowers which flower only once during the growing season such as peonies, irises, and aquilegia.
On tall, leggy plants, doing a Chelsea Chop enables the sideshoots to branch out so plants will need less staking as they will be shorter, bushier, and more compact. It is also a useful method of keeping tidy plants which tend to be sprawling in habit.