Pruning Herbaceous Perennials “The Chelsea Chop”

By Marilyn Homewood, Master Gardener

As the spring bulbs fade and we move past our “last frost” date (or so we hope!), the perennials are starting to grow by leaps and bounds. This can lead us to consider ways to manage their size or bloom time. The Chelsea Chop is a method of pruning that limits the size of a plant, controls the flower season (which can assist in creating peak season bloom combinations) and often decreases the floppiness of a number of herbaceous perennials.

In England, the time for this type of pruning is carried out now which is around the same time as the Chelsea Flower Show is held, hence the name.  In our neck of the woods, timing would be most appropriate in late spring or very early summer when the plant has a fairly substantial amount of vegetative growth.  When I am going to do this, I like to have it done before the onset of our hotter, drier weather so that it does not stress the plant overly (on average by mid June).

Phlox chopped in front to extend blooming season

Plants that have received the Chelsea Chop are not as tall or leggy, so that they may not need supporting.  Flowers are smaller but are more numerous (removal of top shoots encourages branching of laterals).

It should be noted that you can not chop all summer blooming plants.  Woody sub-shrubs do not respond well.  If the spring has been dry, drastic pruning could shock your plants so a light hand is recommended in those years.

Many summer and autumn flowering perennials are good candidates for the chop.  These plants include:

  • Garden phox (Phlox paniculata)      
  • Yarrow (Achillea spp.)
  • Bellflower (Campanula spp)
  • Aster (Symphyotichum spp)
  • Coneflower (Echinacea spp)
  • Black-eyed Susan (Rudbeckia spp)
  • Upright Sedum Hylotelephium spp)
  • Penstemon (Penstemon spp)
  • Goldenrod (Solidago spp.)
  • Shasta daisy (Leucanthemum superbum)

This list is not complete.  Try experimenting with some of the vigorous plants in your garden.  I have a cultivar of catmint (Nepeta “Six Hills Giant”) that is large and spreading.  In one area of my garden, I prefer it to stand a little more to show the blooms to effect so I chop it back by about one third in the last week of May.

Nepeta more upright and floriferous due to chopping

The chop is done in two ways depending on the effect one desires.  In the first method, clumps of perennials are cut back by one third to one half.  This will delay flowering and keep plants shorter and more compact.  The second method involves cutting back only half of the stems on a plant, this has the effect of extending the flowering season over a longer period.  I commonly used the second method on my Garden Phlox keeping the plants in bloom for a longer time. Pruning can be done with sharp shears or with secateurs.  Garden shears are often faster when there is a large volume of pruning to be done.  Try out the chop and see if you can alter the form or flower of some of your favourites!


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