Published in the six Nashoba Publishing papers on Friday, 14 June 2002

A line of fall-blooming asters grows in front of my big bed of iris. As the iris come into bloom I cut back the aster plants hard -- to a few inches high -- the easier to see the iris. By the time the asters get tall again, the iris are done blooming, and the iris leaves are starting to yellow with Iris Leaf Spot, a fungal disease.

Because I don't use chemicals, I control the disease by keeping the iris bed open and airy, by removing dead leaves (both from the iris and from trees), and by trimming off iris leaves, or parts of them, that are badly infected. But the asters growing up in front of them also screen them from view.

Cutting plants back must be done early in the season, to give them time to recover: in general, by the fourth of July. Cut every stem, just above an outward-facing leaf node. Plants cut back end up lower; they don't catch up to how tall they would have been. This is useful in blending the shapes of different plants together in a bed, and to avoid having to support plants that otherwise grow gangly and flop over.

After being cut back, a plant also grows more "bushy": the stems branch more and the plant grows more densely. Hedges are cut to produce this effect — but in perennials, which start anew each year from the ground up, it doesn't result in the harm it does to shrubs. It does result in smaller flowers — and more of them. For example Sedum 'Autumn Joy' cut back hard in June results in a low mound full of color in September.

(Pinching is another pruning technique used to increase bushiness, less drastic than cutting back: you simply remove a leaf node or two, by pinching the stem between thumbnail and finger. Chrysanthemums are usually pinched back twice.)

Finally, cutting back delays flowering. One year I cut back two thirds of a bed of Shasta daisies, so they had three different heights: low in front, high in back. The ones in back bloomed first, then as they were fading the ones in the middle took over the show, and then the front — prolonging bloom to three times what it would have been.

Not all plants respond well to cutting back. But you can experiment: the worst that's likely to happen to a robust, well-established plant is that it won't flower that year.

Next Date

For more information:

The Well-Tended Perennial Garden, by Tracy DiSabato-Aust

On Iris Leaf Spot

• Iris Leaf Spot — by Paula Flynn, Department of Plant Pathology, Iowa State University, Ames, Iowa.
• Iris Leaf Spot by Mary Ann Hansen, Extension Plant Pathologist, Department of Plant Pathology, Physiology and Weed Science, Virginia Tech