May is the traditional time to consider the Chelsea Chop, so called because it's often done around the time of the Chelsea flower show. It is a means of delaying the flowering of summer plants, and making the plant more compact by cutting back, either the whole plant, or parts of it. Cutting the plant back delays flowering because the plant needs to grow on more before flowering. It also causes the plant to bush more so it is good for plants which have a tendency to become leggy later in the summer, such as Nepta Cat Mint illustrated below.
Chelsea chop can be done in a number of ways, and in fact, not just in May depending on the plant, see below. One method of using the Chelsea chop is to cut the whole plant down by about 1/3 and this delays the flowering of all of the plant and makes the plant more bushy, (you are in effect pruning it during it's growing season.)
Alternatively, you can chop some of the plant's stems by a third, but don't chop other stems on the same plant, which will stagger flowering. The unchoped part will flower first and later the Chelsea chopped part. Also, if you have two or more plants of the same variety, chop one but not the other, the options are varied.
Question : is the Chelsea Chop worth it?
I think it is; it makes the plant bushier with more flowers and, especially towards the end of the season, when some plants, left unchecked tend to get leggy which means it can be used to good effect. Below are two Nepeta late in the season, the one on the left was left alone and it appears more spread and sprawling, less bushy. The one on the right was Chelsea chopped and cut down by about a third across the whole plant and it looks bushier.
Also as a border plant get more established some will grow larger each year and this may not be ideal. Chelsea Chop is one way to stop the plant overshadowing its neighbours; chop it in May or June and it will flower on shorter bushier stems. Later flowering perennials can be back in July and for early flowering plants use the Chelsea Chop in May or June. Some plants will benefit from being cut back hard to encourage fresh new growth or a second flush see list below
Plants suitable for the early Chelsea Chop (late May early June) include:
Echinacea purpurea Artemisia Sedum
Phlox Helianthus Rudbeckia
Plants to cut back for a second flush of flowers
Plants and Perennials are suitable for chopping in July to encourage a second flush of flowers. In these cases cut after flowering, and feed. A second flush is not always successful, and the flowering will not be as strong. In each case just cut of the flower head stalk which has flowered to encourage side shoots.
Plants to cut back to get new foliage
Plants and Perennials are suitable to cut back almost to the ground again after flowering in July. This is good where the foliage looks tatty and within 14-21 days new fresh foliage will shoot up which will look more attractive for the rest of the summer.
Use the Chelsea chop cuttings to propagate new plants
Lots of plants are suitable for the Chelsea Chop and it works well. In addition with some plants, the soft new growth which has been removed will make good cuttings and root very well.
Sedums must be the easiest plants in the world to propagate by cuttings and to make new plants in this way. If you have one Sedum you shouldn't have to buy any more!
It is just so easy; push the tip which has been cut off into the ground, and if kept moist it will root where it is, with no more effort than making a hole with a dibber. This has been done on the image to the left. The large plant is the original, smaller is last years cutting and the very small one at the front has been Chelsea chopped in May and the cutting has rooted.
These cuttings are very easy to do and will make sturdy plants for next season's garden. Quite easy to root as well are left over choppings from Nepeta which are best placed in a small pot with a poly bag over to retain moisture, or in propagator away from sunlight.
It is good fun and worth experimenting with the Chelsea chop and cutting back some plants by a third, others just some individual stems by a third, or on the same plant cut some stems and not others.
Chelsea Chop is not suitable for flowers which flower only once such as Peonies, Irises and Aquilegia.