No, it's not a martial arts maneuver or the latest dance craze. The Chelsea Chop is a special pruning technique, named after the annual Chelsea Flower Show in England. It's a method that limits the size, controls the flowering season, and often decreases the flopping of a number of herbaceous perennials. It's used to reduce the height of fall-blooming plants before they set buds by clipping off the top third of its flush of spring growth. The cut is made above a leaf node, which encourages lateral branching stems and more (but smaller) flowers.
What to do: take a large clump of a suitable herbaceous plant and selectively prune growing stems to form a layered effect. Flowering can be delayed by about six weeks; pruned sections are shorter when they flower, so a bushier plant is created.
When to do it: from mid-May onwards, just as flower buds first begin to form. If you prune later, plants may take longer to recover and flowering may not avoid the first frosts.
Tip: You can save some of the cuttings to root and make new plants! New England asters cuttings placed in a glass of water will quickly produce new roots. They can be potted up to share or planted out in the garden where they often bloom the first year.
How to do it: Cut back all the stems by about 1/3. Timing should be as above, when the plant has reached about a third of its normal height.
Suitable plants include: tall Symphyotrichum spp spp such as New England aster and tall Solidago spp. such as stiff goldenrod.
Alternative method: cut one in three stems to stagger flowering
Rather than cutting down sections of the perennial, reducing stems selectively (thinning) throughout the whole clump can provide a softer, more natural effect. Try cutting back one stem in three; cut stems should grow back without being masked by the earlier-flowering, uncut stems and thus will prolong the overall flowering period.
More info: Check out Monty Don's demo video How to do the Chelsea Chop https://www.gardenersworld.com/plants/how-to-do-the-chelsea-chop/