URBANA, Ill. – The time between Mother's Day and Father's Day is ideal for pruning many garden perennials, providing more branching and even more flowering in the fall. Overall, proper pruning can result in shapelier and more floriferous perennials with blooms that may be smaller in size, but more numerous.
“Gardeners can think of late spring/early summer pruning as pre-emptive because it takes place before a perennial achieves its full potential growth and before bloom,” notes University of Illinois Extension horticulture educator Candice Miller. “This type of pruning is not technically necessary to the survival of our perennials, but instead becomes more of an art, allowing the gardener to be creative in the garden.”
Mid- and late summer perennials that could benefit from early pre-flowering pruning include chrysanthemums, “Autumn Joy” sedum, monarda, or New England aster. It's common in many years for these perennials to flop open in the center later in the season, making them rather unsightly. A pre-emptive pruning earlier in the season could prevent this.
“I often do this on the ‘Autumn Joy’ sedum and asters in my garden, and it works wonders,” Miller says. “Knowing when to prune is the most critical part.”
Mid- to late summer bloomers could be pruned until early summer.
“Just keep in mind that you are manipulating the plant and can expect a slightly later bloom time in most cases,” notes Miller.
The amount to cut off depends on the vigor of the perennial. On a more vigorous perennial, it might mean cutting back one-half to two-thirds of the foliage. On a less vigorous plant, no more than one-third of the foliage should be pruned off at a time. Cuts should be made back to a lateral flower, bud, or leaf so that the new growth will hide those cuts quickly.
The intended goal in pre-emptive pruning is to cut back those perennials before flowering for height control, and to stagger or delay bloom time. Pruning is a great way to control when a plant will bloom.
“There may be plants in your garden that you do not want to flower at the same time because of a bad color combination,” Miller suggests. “Staggering the bloom time is a great way to avoid clashing colors.”
Some spring flowering perennials could benefit from a pruning after flowering, as well. The following is a short list by Minnesota Extension of spring-blooming perennials that could benefit from this type of post-bloom pruning: columbine (Aquilegia hybrids), rock cress (Arabis caucasica), siberian bugloss (Brunnera macrophylla), ground clematis (Clematis recta), maiden pink (Dianthus deltoides), evergreen candytuft (Iberis sempervirens), bearded iris (Iris hybrids), spotted deadnettle (Lamium maculatum), catmint (Nepeta mussinii), wild sweet William (Phlox divaricata), moss phlox (Phlox subulata), and wooly thyme (Thymus praecox).
“If you've never done this practice before in your garden, do some experimenting this season and see what results,” Miller encourages. “Try pruning a portion of the plant and see what happens. Perennials are resilient, and through trial and error, you might discover that you can create a much more appealing plant with a little manipulation.”