Chelsea Chop and other methods of extending the flowering season
Many gardeners who are happy, even gung-ho, with the secateurs when pruning shrubs and climbers are surprisingly reluctant to take the shears to herbaceous perennials. Maybe this is because it just doesn’t seem quite right to be cutting back all that new growth that has sprouted so valiantly after winter dormancy - almost like cutting them back in their prime. However, there are some useful benefits to be had if you can bring your secateurs to bear on a select group of flowers.
Delaying flowering is very straightforward and often referred to as the ‘Chelsea Chop’ because late May (when the Chelsea Flower Show is staged) or early June is the best time to carry it out. There are several approaches you can take, depending on the plants you are cutting back and the effect you are trying to create - as well as how brave you are feeling. The simplest method is to cut all growth back, but this can leave big gaps in your borders, so many professionals opt to cut alternate plants or cut back just one-stem-in-three on prominent plants (see ‘Relay flowering’ below). Another option is to cut back stems that are out of sight at the back of a clump. If this is all too drastic for you, you can achieve similar results with some perennials by regularly pinching out the growing tips before they flower. Bear in mind that the nearer to flowering time you leave any of these cut backs, the later the subsequent flush of blooms will be produced.
You can use this technique to keep tall perennials more compact and less liable to collapse in a summer storm. This is particularly useful for plants, such as aster and campanula, which are prone to flopping all over the place even when the weather is good. Sedums, too, are worth pinching back early in the season to prevent the clumps opening up and sprawling over their neighbours when they come into flower as summer turns into autumn. A few perennial giants, such as eupatorium and veronicastrum, can be kept in proportion to the other perennials in the border by cutting back now. Not only will this keep the overall display balanced, but you will be able to cram in a few more plants.
Some multi-stemmed plants which produce lots of flowers on stems produced during the current season, can be deliberately pruned to encourage some stems to flower earlier or later than others, thereby spreading the flowering display over a longer period. A few clump-forming perennials that produce a massed display of blooms, such as coreopsis, phlox and helenium, can have their flowering spread over a longer period by selective cutting back. All you need to do is cut back by half about one-third of the stems before they produce flowers. These will then produce several sideshoots, each of which will flower later in the summer, effectively extending the season of colour. You can use the technique with some climbers and shrubs too. Some clematis are a case in point [link to clematis pruning info], but this technique can also be tried with vigorous shrubs such as lavatera or the butterfly bush (buddleja) which are normally cut back hard each spring. In this case, instead of cutting all the stems back hard, cut back about two-thirds, leaving the others to flower earlier than the hard-pruned stems. Perennials that produce a single main stem of flowers such as foxgloves, delphiniums, hollyhocks and lupins, can be encouraged to produce a second flush of blooms by pruning off the main flower spike as it fades. Carefully make the cut just above the top developing sideshoots low down on the plant, which will then grow and flower in mid- to late summer. With this technique it is important to leave the foliage intact even if it is looking a bit tatty.
Fresh foliage and a flower bonus
Other perennials are best cut back later in the summer after their main flush of flowers is over. Flowering perennials, such as astrantia, hardy geraniums, lamium and nepeta, can start to look a bit tatty towards the middle of the summer as the effects of drought and pest and disease attack take their toll. You can improve the overall appearance of the plants and, if you are lucky, get another flush of blooms in late summer by cutting all the old foliage right back almost to ground level using a pair of shears. Some gardeners, who’ve got a lot of plants to cut back, even use a nylon-line trimmer complete the task. Whichever method you use, the resulting new growth that emerges looks fresh and neat until the autumn. Water plants thoroughly after cutting back and give them a boost by applying a high-potash fertiliser such as rose or tomato food. Plants grown mainly for their attractive foliage, such as artemisia, will maintain a neat habit and fresher looking foliage if they are prevented from flowering altogether. In this case, trim the flowers from the plant before they start to open.