Dr. Leonard Perry, Extension Professor, University of Vermont
Garden phlox are one of the staples of the summer perennial garden with their large flower clusters in many colors, and have been used in gardens since Colonial times. They’re native to many parts of the eastern U.S., are long-lived, low maintenance, and provide color in late summer when many other perennials have finished bloom. There are many choices, with some having good resistance to the white powdery mildew disease.
Phlox comes from the Greek word for flame, referring to the brightly-colored flowers. The main species (paniculata) of most cultivars (cultivated varieties) refers to the dense, flower clusters or panicles on tops of the stems. Although these can sometimes be heavy and cause stems to droop a bit, usually stems are sturdy and upright. Another species (maculata)—the early or meadow phlox—has more elongated and cylindrical flower clusters. A hybrid (x arendsii) between the garden phlox and the blue phlox (divaricata) has good mildew resistance. You may see all these species referred to collectively as “border phlox”.
The phlox are in the phlox family—the Polemoniaceae—just as is the Jacob’s Ladder (Polemonium). They combine well with bee balm (Monarda), Joe-Pye (Eupatorium), ornamental grasses such as the switchgrasses (Panicum), daylilies, rudbeckia, Shasta daisies, and meadowsweets (Filipenula). They make good cut flowers, and in fact some of the newest cultivars (the Feelings series from Holland in particular) were bred for this purpose.
Phlox grow and bloom best in full sun to part shade—6 hours or more of sun a day. They prefer moist, well-drained soils but will tolerate some drought once established. Although listed as hardy in USDA zones 4 to 8 (to -30 degrees F in winter), they prefer cooler nights as in the north. “Deadhead” or pick flowers off once bloom is finished to improve appearance, and to keep them from seeding around the garden. Plants won’t come true from seeds. They seldom need division unless too large, or blooms get smaller and fewer.
In early summer, you can cut plants back by about one-quarter to one-third, which will promote branching and even later bloom. I learned this trick from the deer, which love to graze the tender shoots. If you want phlox to bloom in summer rather than fall, and have deer about, you may need to consider some form of deer repellents or fencing.
About the only problems of this perennial are spider mites and powdery mildew. The mites are very small, so you’ll likely need a hand lens to find them on the undersides of leaves. What you can see is the stippled yellowish leaves, which are puckered and very yellow when severe. They’re more of a problem in hot, dry weather, and can be controlled with both organic and synthetic sprays—just make sure to get the undersides of leaves, and to apply according to label directions.
Powdery mildew is an appropriately named fungal disease, often a problem in humid conditions and with poor air movement. It is more a problem in some areas than others, some years than others, and with some cultivars than others. Although there are sprays for this, and providing good spacing with good air movement can help reduce this, selecting resistant cultivars is perhaps easiest.
In trials in Chicago, three quarters of the cultivars were impacted in some years by powdery mildew. Those with the greatest resistance in all years included ‘Sherbert Cocktail’, ‘Flower Power’, ‘Becky Towe’, ‘Frosted Elegance’, ‘Goldmine’, ‘Natural Feelings’, ‘Peppermint Twist’, ‘Rubymine’, and ‘Shortwood’.
Overall, ‘Shortwood’ was the top cultivar in all respects in these trials, the only one in fact to receive a five-star excellent rating. With its rosy-pink flowers, similar to the species, it is taller than many of the newer selections bred for a more compact habit. Its name is deceiving, as it reaches 4-feet or more tall, being named instead for the garden of plant expert and garden writer Stephanie Cohen. It originated as a seedling of the white-flowered ‘David’, discovered in Pennsylvania by grower and breeder Sinclair Adam, Jr.
‘David’ is arguably the most popular white-flowered cultivar, and although in previous Chicago trials was listed as fair for mildew resistance, often gets no mildew in other areas. It was selected by the Perennial Plant Association as Perennial of the Year for 2002. ‘David’ was discovered as a seedling in a native population at the Brandywine Conservancy in Pennsylvania, and the above grower was instrumental in its introduction. The garden phlox ‘Katherine’ rated good for mildew resistance in these trials. It has lavender flowers with a white central “eye”.
Some phlox have solid color flowers, mainly pink, but also many in purples, with some red, lavender, white, and even blue. Others have this central contrasting eye of a different color. Some of the top-rated phlox with such flowers include ‘Orange Perfection’ which is orange with magenta eye, ‘Norah Leigh’ which is pale pink with darker eye, ‘Little Boy’ which blooms violet purple with white eye, ‘Frosted Elegance’ blooming pale pink with darker eye, ‘Delta Snow’ which is white with purple eye, and ‘Miss Karen’ which blooms bright pink with darker eye.
For more unusual flowers there is ‘Peppermint Twist’ with pink and white stripes like a peppermint. ‘Sherbert Cocktail’ opens pink, but turns white with a pink blush and yellow tips to the petals. The Feelings series, such as the deep pink ‘Fancy Feelings’, has straplike petals if any, other flower parts such as sepals being what you see instead. Perhaps the most unusual is ‘Blue Paradise’, whose violet flowers start as blue buds, and then fade to blue.
While most phlox have green leaves, some are variegated with creamy white margins. ‘Norah Leigh’ has wider variegation than ‘Crème de Menthe’ or ‘Frosted Elegance’. ‘Goldmine’ has broader golden to creamy variegation than ‘Giltmine’. While ‘Rubymine’ has purplish highlights to its creamy variegation, reddish highlights are on the yellow-rimmed leaves of ‘Becky Towe’ and cream-variegated ‘Harlequin’. Unfortunately, as with many perennials, the variegation often is not stable. Over a few years, most or all leaves may revert to green.
Much more on all the 75 cultivars, including the 27 top-rated, in the trials at the Chicago Botanic Gardens is online (www.chicagobotanic.org/research/plant_evaluation/#notes). One of the more extensive sources of phlox, both for information and ordering online, is Perennial Pleasures Nursery in Vermont (perennialpleasures.net). Grower and manager Rachel Kane specializes in garden phlox, having 137 cultivars along with other heirloom perennials. Many of these are scarcely available, if at all, anywhere else in the country. If in central or northeast Vermont in early August, make sure to check out her Phlox Fest. http://perennialpleasures.net/phlox-fest
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