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Time to Prune Perennials

© Mary Henry

This next week is the best time for northern gardeners to summer prune most perennials. Why would you want to do that? Well, there are several reasons. Pinching mums in early summer is a time honored tradition to produce stockier, more floriferous plants in the fall, but mums aren't the only perennials that profit from such treatment. Several years ago, I learned that many sun-loving perennials grown in part shade would get leggy and flop if I didn't cut them back in late June to give them some backbone. You can also stagger bloom times with summer pruning and even use this technique to layer the flowers in mass plantings or to cover the bare legs of stalky perennials.

I was taught to pinch the mums approximately three times beginning when the plants were about six inches high and ending by the Fourth of July. No later, or I would chance having the blooms so late that frost might come first. When I was a professional grower, we didn't have time to spend doing it three times, so around the first of July we cut the plants back by half their height with hedge shears. I do my tall asters at the same time. Some of the shorter ones like 'Purple Dome' don't need it, but 'Hella Lacy' and 'Patricia Ballard' become ever so beautiful and full for me when cut back in early summer.

If your garden is small like mine, some of the taller growing perennials are a better size when cut back too. Some of these are echinacea, rudbeckia (especially 'Herbstonne', 'Golden Glow' and laciniata), boltonia, Joe Pye weed (Eupatorium), sunflower (Helianthus), and false sunflower (Heliopsis), The mums, asters and boltonia do very well with shearing, but plants that are not so bushy should be cut stem by stem with a hand pruner or scissors (depending on the toughness of the stems) for a less shaggy appearance.

My entire lot has more shade than sun, so I cram the sun lovers into even the smallest scrap available. That means that many (if not most) don't really get as much as would be best. This leads to lanky floppy stems that will be strengthened by cutting back. My Sedum'Autumn Joy' was always beautiful until it began to bloom. Then the heavy flowering heads all flopped outward and left the whole thing looking like an elephant had stepped on it. Cutting this plant back by half will make it look ridiculous for a week or so, but then it will fill out and be shorter and sturdier for the remainder of the season. It's actually the only way to enjoy this plant in less than full sun. I read last year that you could do the same to the balloon flower (Platycodon) to prevent flopping, so I did it this week, but as I was completing my research for this article, I found that early June was recommended for them. Oh well, stay tuned. I'll let you know later how this experiment comes out.

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