The ‘Chelsea Chop’ and seasonal pruning

ON GARDENING

Sea chamomile (Anthemis maritima). A herbaceous perennial, native to the Mediterranean region, grows up to 16 inches tall, with bright yellow flowers on numerous stems. A member of the daisy plant family (Asteraceae), it attracts bees and butterflies. CHRISTIAN FERRER VIA WIKIMEDIA COMMONS

Purple coneflower (Echinacea purpurea). This perennial is native from the central to southeastern United States. It typically grows to 2-4 feet tall. The showy flowers, 5 inches wide, bloom during summer on stiff stems with coarse, dark green leaves. In the fall, goldfinches and other birds visit the blackened seed cones. The genus name comes from the Greek word for hedgehog or sea-urchin, referring to the blossom’s spiny center cone. BURKHARD MUCKE VIA WIKIMEDIA COMMONS

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At this time of the year, from late May to early June, nature offers gardeners the opportunity to enhance a selected category of plants.

The focus is on widely popular garden plants, specifically herbaceous perennials that typically bloom in the summer.

Examples of these plants: achillea, anthemis tinctoria (golden marguerite), artemisia, aster, campanula, cranesbill (hardy geranium), echinacea (coneflower), eupatorium (Joe Pye weed), helenium, helianthus (perennial types), iberis (candytuft), monarda, nepeta (catmint), penstemon, phlox paniculata (garden phlox), rudbeckia (black-eyed Susan), sedum (upright forms), solidago (goldenrod) and veronicastrum.

This column’s image gallery includes selected plants in this category, with images from Wikimedia Commons.

Pruning this category of garden plants at this time is called the “Chelsea Chop,” referring to the schedule of the annual flower show held by the Royal Horticultural Society in Chelsea, a borough within Greater London.

The basic method of the Chelsea Chop involves removing one-third to one-half of the plant’s stem while it is young and actively growing. This can be done with pruning shears or secateurs (which are more precise).

Removing the top of the stems has the effect of interrupting apical dominance, a process in which the stem’s topmost bud, called the apical bud, suppresses the growth of lower buds. Removing the apical bud allows the lower buds to sprout, creating a bushier, branched form of the plant.

Gardeners have these objectives for this seasonal pruning.

Control overall size: For plants that grow larger than desired relative to the surrounding landscape, pruning can help maintain a manageable size that does not block neighboring plants’ visibility or access to sunlight. Candidates for this treatment include artemisia, nepeta and anthemis.

Control height: Some plants tend to grow so tall that they can flop in the late summer or fall. The Chelsea Chop stimulates side shoots with additional buds, allowing the plant to remain upright. Candidates for controlling height include eupatorium, helianthus, helenium and veronicastrum.

Delay flowering: This seasonal pruning “restarts” the normal blooming schedule, so that flowers appear two or three weeks later to display during a social event planned for the garden, or to coordinate blooms with other nearby perennials.

This timing trick can be applied to coreopsis, phlox paniculata, monarda, heliopsis, nepeta, Shasta daisies, asters and helenium.

Extend flowering: A variation on the basic approach to pruning involves cutting back one-third of the stems by half, one-third by a third and leaving one-third of the stems uncut. Applying this plan to plants like those for delaying flowering (above) creates a layered effect with flowering over a longer period of weeks.

More seasonal pruning ideas

Even after the Chelsea Chop season, gardeners can pursue two other pruning projects to enhance the garden.

Remove spent flower stalks: After blossoms fade on some plants, cutting the flower stalks to the ground can stimulate an additional round of flowering, or at least make the garden look neater and more vigorous. Cut the spent stalks to the ground for foxgloves, delphiniums, lupins, hollyhocks, daylilies and campanulas.

Refresh tired foliage: Some summer-blooming, mounding perennials can look unattractive by midsummer.

Shearing these plants can promote new foliage and, hopefully, a second round of blossoms. Candidates include cranesbill, nepeta, achillea, lamium, aquilegia, threadleaf coreopsis, alchemilla, epimedium, pulmonaria and brunnera.

Advance your garden knowledge

For a virtual visit to this year’s Chelsea Flower Show, browse to tinyurl.com/5btaphd2 for a series of episodes describing this extraordinary garden show by the Royal Horticultural Society, held in Chelsea.

The first event, called the Great Spring Show, was held in 1862 in Kensington.

Previously, the society had held flower shows from 1833 to 1862 in Chiswick.

This week in the garden

Walk through your garden with pruners in hand and consider pruning for improvement opportunities.

Enjoy your garden!

Tom Karwin is a past president of Friends of the UC Santa Cruz Arboretum and the Monterey Bay Iris Society, a past president and lifetime member of the Monterey Bay Area Cactus and Succulent Society, a Lifetime UC Master Gardener (certified 1999-2009), past board member of the Santa Cruz Hostel Society and active with the Pacific Horticultural Society and other garden-related societies. To view photos from his garden, visit facebook.com/ ongardeningcom -566511763375123. To review the archive of recent On Gardening columns, visit santacruzsentinel.com and search “Karwin.” Go to ongardening.com to review columns from 2012-2020 (and soon) from 2025. Send comments or questions by email to gardening@karwin.com.