Garden phlox is one of the stalwarts of the summer garden, providing color and stature during the midsummer months. But, although lovely, it isn't always carefree. Too often powdery mildew mars its leaves during warm, humid weather and spider mites leave leaves yellowing and misshapen. Fortunately, breeders have been working to overcome these problems and varieties are now available that show good resistance to these threats.
To help gardeners choose the best from among the many garden phlox cultivars on offer, from 2001 through 2009 the Chicago Botanic Garden conducted an evaluation of 78 different kinds of phlox, including cultivars of Phlox paniculata, P. x arendsii, as well as some hybrids. All plants were grown in full sun at the Botanic Garden located in USDA Hardiness Zone 5b and AHS Heat Zone 5. Plant were given minimal maintenance, receiving mulch and overhead watering as needed, but no fertilization, pest or disease control, or winter protection. Plants were evaluated for bloom period, flower coverage, mildew resistance, and degree of mite infestation and given an overall rating of one to five stars.
Only one cultivar rated top honors. Rosy pink Phlox paniculata 'Shortwood' took the prize for its exceptional performance and floral display coupled with excellent powdery mildew and spider mite resistance. Fortunately 27 other cultivars received high 4-star ratings, so there are plenty of good choices that offer a wide range of colors and heights.
To read the entire comparative study of garden phlox cultivars and see all the cultivar ratings, go to Chicago Botanic Garden.