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Ask the Gardener: Garden phlox is a pain, so what changed my mind?

Ask the Expert Gardening
Phlox-Stock-Art-Pink
Garden phlox can be tiresome to take care of, with the deadheading, the mildew, and the rogue offspring. Adobe Stock

What to do this week: Grass barely grows in summer, so don’t let lawn care companies mow when it’s not needed. Mowing high (2½ inches) produces healthier soil and deeper grass roots that need less watering. Don’t remove more than a third of the lawn’s top growth at one time or you’re scalping it. Sow seeds of beets, chard, spinach, and lettuce for a fall harvest. Water tomatoes twice a week early in the day. Try not to wet the foliage. Harvest corn when the silks turn brown and cucumbers before their skins turn yellow. Schedule garden projects for the fall. Concentrate on watering.

 

Q. When you write about cutting back phlox by a third, is that to encourage more flowers or for a different reason? After flowering, when should they be cut back? Personally, I would just leave them, but they look grungy if they develop mildew.

J.C., Dover, N.H.

A. Garden phlox is a fragrant native wildflower that has been called “the backbone of the summer garden’’ since Colonial times because it can bloom from July though September with attentive watering and deadheading. This also makes it important for pollinators. I advise gardeners to cut out a quarter of the stalks each plant produces in May or June to improve air circulation and also to cut back stalks by a third to increase branching and thus the number of flowers. Like you, I remove and destroy foliage that’s grungy with mites or white powdery mildew fungus, the bane of garden phlox. (There are other kinds of native phlox that bloom earlier and don’t need such pruning or coddling.) It is even more important to snip off spent flowers within a week or two of blooming to encourage new flowers and prevent unwanted seedlings.

Garden phlox is a surprising amount of work! I almost stopped growing them, but everything changed when a sharp-eyed horticulturist spotted a towering white phlox growing by a parking lot. She helped propagate it and named it “David’’ after her husband. The Perennial Plant Association named it its Perennial Plant of the Year in 2002. David is such a great plant that it changed the fate of phlox, long regarded as an old-fashioned garden plant of fading reputation.

With its love of low humidity, cold winters, and cool summer nights, phlox would not seem to have much of a future on rapidly warming Planet Earth, but now it’s suddenly a hot plant. Breeders are producing dozens of new largely disease-resistant varieties. “Shortwood’’ was the top-performing new garden phlox at the Chicago Botanic Garden plant trial, followed by variegated “Frosted Elegance,’’ tough “John Fanick’’ from Texas, lavender “Katherine,’’ and striped pink and white “Peppermint Twist.’’

A phlox trial at the Mt. Cuba Center in Delaware found “David,’’ coral-pink “Glamour Girl,’’ and pink-and-white “Delta Snow’’ to be the best performers there. But what about butterflies? “Jeana,’’ a small-flowered phlox found in Tennessee by a woman named, well, Jeana, got five times as many butterfly visits as any other phlox variety at Mt. Cuba. That’s the one for me! It also blooms three months without mildew.

Should you replace your old phlox? Probably. Here’s a dirty phlox secret: When named selections are not deadheaded soon after blooming, they produce seedlings that are much more vigorous than their parents, who are soon crowded out. So if your old garden phlox is a muddy magenta, you are growing these mutts now even if you started out with pedigreed parents. If you don’t want to start over, just keep the mutts and enjoy a less attractive but also lower maintenance garden. But do add one David cultivar to enjoy its fragrant white clouds of August bloom. David is sold at most garden centers, but look to mail order for rarer cultivars.

Events

The Heritage Museums & Gardens in Sandwich officially opened The McGraw Family Garden of the Senses on July 13 during the organization’s 50th Anniversary Gala. It is designed on two of the garden’s 100 rolling acres to provide universal access to all visitors, regardless of age or abilities. For more information, go to heritagemuseumsandgardens.org.

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