Fabulous phlox

Garden phlox, a staple of summer gardens since colonial times, are some of the most recognizable and popular perennials today. Their telltale magenta flowers enliven native landscapes for many weeks from summer to fall, while a myriad of cultivars in a rainbow of colors grace gardens everywhere. They owe their prodigious nature as much to the oversized flower trusses that crown tall stems as to the vivid pinks, purples, reds, and oranges of the flowers.

Phlox paniculata, garden or summer phlox, is one of 60 or so species of phlox native to North America. P. paniculata is native to the eastern United States, from New York to Georgia, and west to Illinois and Arkansas. It has been in cultivation in Europe since the 1800s and many of the early cultivars originated in England and Germany.

Phlox maculata, early or meadow phlox, is a native of the eastern United States, too. While similar in habit to garden phlox, its flowers are borne in elongated cylindrical clusters earlier in the summer. The species name reflects its purple-maculated or spotted stems. Phlox × arendsii, a hybrid between P. divaricata (blue phlox) and P. paniculata, boasts greater mildew resistance than garden phlox. P. paniculata, P. maculata and P. × arendsii are collectively referred to as border phlox.

Flowers are without a doubt the main ornamental attribute of the border phlox. A pavonine palette of flower colors is available including shades of pink, salmon, orange, red, purple, lavender, blue and white. While the magenta-pink flowers of wild P. paniculata are unmistakable, modern cultivars offer a variety of distinctive colors such as voluptuous cherry red (‘Miss Mary’), whimsical pink and white (‘Peppermint Twist’) and delicate pink and yellow (‘Sherbet Cocktail’). Flower size varies from ½-inch to almost 2 inches wide, whereas the panicles typically range from 4-6-inches tall and 6-8-inches wide. Flowers are fragrant, although the degree of fragrance is variable among cultivars.

Evaluations
From 2001 through 2009, the Chicago Botanic Garden (USDA Hardiness Zone 5b) evaluated 78 Phlox taxa in full-sun trials. The study concentrated on cultivars of P. paniculata but included P. × arendsii cultivars and unspecified hybrid cultivars.

Plants were regularly observed for descriptive traits such as flower color, bloom period, foliage color and plant habit. Data also was collected on disease and pest problems, winter injury and habit quality and plant health issues related to and/or affected by cultural and environmental conditions.

While 27 phlox received high ratings for outstanding flower production, good health, and strong habits, only P. paniculata ‘Shortwood’ received a five-star excellent rating for its exceptional overall performance and superior resistance to powdery mildew and spider mites. Rosy-pink ‘Shortwood’ is a chance seedling of white-flowered ‘David,’ discovered by Sinclair Adam in Pennsylvania and named for the garden of Stephanie Cohen, perennial plant expert and garden writer. At 50 inches tall, ‘Shortwood’ is anything but short. In fact, with its large size and vivid flower color, somewhat of a throwback to the species, ‘Shortwood’ defies the recent trend in phlox introductions. But its noteworthy resistance to powdery mildew sets ‘Shortwood’ apart. 

Other border phlox that fared well were P. maculata ‘Flower Power,’ P. paniculata ‘Bartwelve,’ P. paniculata ‘Frosted Elegance,’ P. paniculata ‘Lichtspel’ and P. paniculata ‘Peppermint Twist.’

Colorful flowers
Uniquely different from other garden phlox, ‘Sherbet Cocktail’ opens pink, eventually turning white with a pink blush and yellow petal tips. Its maroon calyces appear almost black from a distance, further enhancing the floral display.

The pretty pink-and-white striped flowers of ‘Peppermint Twist’ were unique in the trial and reminiscent of a Starlight mint. Salmon-pink cultivars such as ‘Candy Floss,’ ‘Bartwelve,’ ‘Becky Towe,’ ‘Fairest One,’ ‘Pink Attraction’ and ‘Rainbow’ seemed to alternate between looking pink one day and orange on another. Purple-flowered ‘Blue Paradise’ opened from blue buds and faded to blue. Paradoxically, ‘Ending Blue’ had violet flowers streaked with white but did not turn blue at the end.

Foliage and habit
Foliage was predominantly medium green with some shade variations observed —’Graefin von Schwerinn’ and ‘Miss Universe’ had lighter green leaves than other cultivars. It was not uncommon for the terminal leaves just under the developing inflorescences to be purple, especially early in the season. Among the cultivars with dark-purple terminal leaves were ‘Blue Paradise,’ ‘Düsterlohe,’ ‘Iris,’ ‘Jubilee,’ ‘Karmin Grand,’ ‘Little Princess,’ ‘Ping Pong,’ ‘Robert Poore,’ ‘Rubymine’ and ‘Tenor.’ Among the variegated cultivars in the trial were ‘Crème de Menthe,’ ‘Frosted Elegance’ and ‘Norah Leigh,’ which had irregular creamy-white margins, although ‘Norah Leigh’ had a wider variegation than the others.

Unfortunately, reverted stems were not uncommon on these cultivars. ‘Giltmine’ was weakly variegated with narrow yellow edges; an abundance of non-variegated green stems were produced each year, and by the third year one variegated stem remained in one plant only. By contrast, ‘Goldmine’ displayed broad golden to creamy-yellow margins; variegation on some leaves was fully and deeply yellow in late June, becoming creamy yellow in August. Interestingly, the tiny leaves within the inflorescences were bright gold when the variegation was creamy yellow, giving ‘Goldmine’ a singular look. Like ‘Giltmine,’ there were no variegated stems remaining on ‘Goldmine’ by the fourth year. Several variegated cultivars were prominently tricolored with purple to red overtones. ‘Rubymine’ featured purple highlights over creamy variegation, while yellow-rimmed ‘Becky Towe’ and cream-variegated ‘Harlequin’ were generously brushed with red. Sunscald was common on all variegated cultivars in late summer.

Plant habits were generally good with strong, erect stems; however, occasionally stems would be pulled down by the weight of the flowers. Among the cultivars with especially sturdy stems and robust habits were ‘Barci,’ ‘Dunbar Creek,’ ‘Eden’s Smile,’ ‘Flower Power,’ ‘Inta,’ ‘John Fanick,’ ‘Lichtspel,’ ‘Miss Margie,’ ‘Miss Mary,’ ‘Shortwood,’ ‘Uspekh’ and ‘Violetta Gloriosa.’

Disease issues
Powdery mildew can be a significant problem for border phlox and Phlox paniculata cultivars in particular. Resistance or susceptibility to mildew infection is regional and seasonal in nature. Weather and cultural conditions may be more conducive to mildew infection in one year but not in the next. Although observed in all years of the trial except 2002, powdery mildew was a significant health issue in 2004, with 84 percent of the trial group infected at some level.

Over the course of the nine-year trial, 57 of 75 taxa were impacted by powdery mildew in one or more years. No cultivar had mildew in all years of its trial; only P. × arendsii ‘Babyface,’ P. paniculata ‘Barci,’ ‘Barfourteen,’ ‘Eden’s Smile’ and ‘Jubilee’ recorded mildew infection in three out of four trial years.

Taxa displaying the greatest resistance to powdery mildew in all years of the trial included ‘Candy Floss,’ ‘Sherbet Cocktail,’ ‘Flower Power,’ ‘Bartwelve,’ ‘Becky Towe,’ ‘Frosted Elegance,’ ‘Giltmine,’ ‘Goldmine,’ ‘John Fanick,’ ‘Lichtspel,’ ‘Natural Feelings,’ ‘Peppermint Twist,’ ‘Pleasant Feelings,’ ‘Rainbow,’ ‘Rubymine,’ ‘Shortwood,’ ‘Swirly Burly’ and ‘Wendy House.’

Garden phlox are valued for their impressive floral displays, sweet fragrance, and long-blooming nature. The increasing desire for stronger, trouble-free garden phlox will undoubtedly fuel the market for these popular garden plants for many years to come. Through breeding and selection, new introductions in coming years will likely surpass many of the current offerings. In the end, testing border phlox in a variety of geographical areas will ensure that the best selections reach gardeners.

Richard Hawke is the plant evaluation manager at the Chicago Botanic Garden; www.chicagobotanic.org.