You'll often hear this being talked about by gardeners, especially at this time of year: but what is it, and why do we do it?
There are various theories about the origins of the phrase: some say that that after the Chelsea flower show, any plants which hadn't been sold would be brutally dead-headed, often with shears, in order to promote a flush of new flowers, so that they could be sold later in the season. Others suggest that it was part of the plant-torturing used by exhibitors at Chelsea, to get their plants to be in optimum shape for the show, by cutting them back much, much earlier in the year, to promote flowering at just the right time.
But over time, it has come to mean something different: a pruning method for getting more out of late-season perennials. It is applied to anything which flowers tall and late, such as the meadow or prairie plants like Asters, Rudbeckia, Heleniums, Eupatorium, Echinacea - basically, anything which has evolved to cope with being grazed on.
By cutting these plants down by a third or a half at this time of year - mid to late May - they will respond by branching out, giving you several, shorter, flowering stems, which will often flower slightly later than if you had left them uncut.
Why would you want this?
The idea is that, if you have a large clump of tall perennials, they will flower all at the same time, all at the same height, which can be a bit of a one-hit-wonder. This technique can give them a more interesting appearance, and can increase the number of flowers, as well as prolonging the display.
Of course, you might prefer having tall elegant stems to your perennials, but sometimes it's good to change things a little, and if your flowers are prone to flopping - Sedum are particularly bad at this - then making them shorter can be a way to keep them strongly upright, without having to put in lots of plant supports.
The classic Chelsea Chop means cutting everything in the clump: you can do it individually with secateurs, or you can get the shears out and chop the lot.
For a classier look, you can go for the layered effect: cut the front few stems down to ankle height, leave the back ones as they are, and cut the middle ones halfway between.
Later in the year, instead of having a mass of green, with flowers on top, you will have a bank of flowers.
As for Sedums, I prefer to cut out the stems in the middle of the plant, right down low, to prevent that “falling apart” effect: by the time the outer stems are flowering and falling outwards, the inner ones are filling the gap and thinking about flowering.
That's pretty much all there is to it: and because plants are (thankfully!) so resilient, you can do experiments at home by making notes of how hard you cut which plants, and what the results were, so that after a couple of years you will know exactly how to get the best display in your borders.
But don't try it on your Aconites! (Ask me how I know....)