Garden phlox, when well grown, is one of the glories of the midsummer garden, its large heads of flowers adding color, fragrance, and stature to flower beds as the many early summer blooming perennials are fading into foliage. Unfortunately, this North American native has an Achilles' heel -- its susceptibility to powdery mildew, a fungal disease that can leave its foliage disfigured with an ugly white coating.
To help gardeners select phlox varieties and cultivars that make the best garden performers, the Chicago Botanic Garden recently concluded a multi-year evaluation of a number of types of border phlox. Border phlox is a category that encompasses the familiar garden phlox, Phlox paniculata, as well as the similar but earlier blooming meadow phlox, P. maculata, and the hybrid P. x arendsii, which shows greater resistance to mildew than P. paniculata.
From 2001 to 2009, 78 Phlox cultivars were evaluated in the Botanic Garden's Zone 5b gardens. They were grown in full sun and were given minimal maintenance, in line with what they'd likely receive in the average home garden. Plants were rated on a scale of one star (very poor) to five stars (excellent).
Only one cultivar garnered a top five-star rating, Phlox paniculata 'Shortwood'. This 50-inch tall cultivar was covered from mid-July to early October with large, rosy-pink heads of flowers, showed excellent resistance to powdery mildew, and had no problems with spider mites, another common border phlox woe. 'Shortwood' is a chance seedling of the white, mildew-resistant cultivar 'David', itself an excellent garden performer that strangely is missing from this evaluation line-up.
A further twenty seven cultivars received a four-star ″good″ rating, including such readily available choices as P. paniculata cultivars 'Katherine', 'Laura', 'Robert Poore', and 'Orange Perfection'. The uniquely colored garden phlox 'Peppermint Twist' with pink and white striped flowers also received four stars.
To read the entire comparative study of Phlox cultivars, go to: Chicago Botanic Garden.