The scented flowers of wild blue phlox (P. divaricata) are the strands from which some of my earliest memories are woven and renewed. Not sweet like fragrant lilies, and not spicy like clove-scented pinks, the phlox's pleasant, mild perfume is part of a spring walk in the woods in many areas of the country. The scent was familiar to me when, as a child, I collected flowers for May baskets. The place was a perfect setting for colonies of this phlox to flourish, along a small stream under a high, but not dense, canopy of deciduous trees where the sun filtered down through the branches.
Close contact with the flowers -- and a child's inclination to bury her nose in any bunch of blooms -- firmly affixed the scent in my memory. Today, wafted on the breeze, it is a welcome part of spring gardening in my area of Vermont. Continuity with past pleasures is only one of many reasons for planting P. divaricata in the garden, however.
Clusters of delicate 3/4- to 1-inch flowers come in violet-blue to white shades. Even within one colony, shades vary from deep to pale. Smaller than the tall summer phlox (P. maculata and P. paniculata), yet taller than the creeping species (P. stolonifera and P. subulata), wild blue phlox has slim, leafy stems to about 1 foot tall. It grows well in moist, partly shady locations of USDA Hardiness Zones 4 through 7, though not in hot and humid locations. Northern strains of P. divaricata (those that are native to zones 5 or colder) are easily grown where temperatures reach -40°F. and colder, if they are nestled between rocks or trees that mitigate the ferocity of winter winds.
Depending on where you live, you may know P. divaricata as wild blue phlox, woodland phlox, or timber phlox. Because the branched clusters of flowers remind some observers of sweet William (Dianthus barbatus), this phlox has another common name: wild sweet William. That branching form, in fact, was the basis for its species name, divaricata, from the Latin for "divergent" or "spreading."
Alan L. Summers, president of Carroll Gardens in Westminster, Maryland, recommends 'London Grove Blue' as the most desirable cultivated variety. It has large periwinkle blue blooms and a stronger fragrance than the wild blue phlox does. Bright white flowers are found on 'Fuller's White'. 'Dirigo Ice' and 'White Perfume' are silvery blue.
The individual flowers of the purple-blue kinds are lovely, but usually smaller than those of other varieties. Ainie Busse, owner of Busse Gardens in Cokato, Minnesota, recommends 'Chattahoochee' as a vigorous grower there (zone 4). All bear flower clusters atop 10- to 18-inch stems; bloom lasts for three to four weeks.
For gardeners interested in the history of their plants, P. divaricata is the subject of a surprising tale. Some of these plants are believed to have survived the hot, dry periods between ice ages. One colony, with unnotched flower petals, spread westward from areas in what is now Missouri to areas of present-day Nebraska and Kansas. These plants are now designated P. divaricata laphamii. Another colony with notched petals survived in the Appalachian highlands and, with the return of favorable growing conditions, spread throughout the eastern part of the United States. The two colonies have mingled in overlapping ranges, with variations in the flowers' shape and color.
From Pennsylvania southward, native stands of P. divaricata include pink-flowered forms mingled with the blue. Charles Oliver of Scottdale, Pennsylvania, has bred large-flowered and pink types, not yet introduced to the nursery trade but expected to be available in a few years. Oliver has also developed hybrids of P. divaricata and P. amoena, a species native to southeastern states. One such cross, 'Charles' Passion', has glowing purple flowers. It, too, is still rare in cultivation. The plant I have grown for two years has been hardy where I live in zone 4b, despite its southern heritage. (Because soil acidity, moisture, and fertility all influence P. divaricata hybrids' flower color, before new varieties are named, they are grown long enough to ensure that their features are uniform under all conditions.)
Woodland garden planners prize P. divaricata. For a natural look, grow it on wooded slopes, in shady rock gardens, in front of shrubs in transition areas between garden and woodland, or among shaded ferns. Integrate its light violet-blue flowers with an assortment of spring bulbs such as gold daffodils (Narcissus), or early-bloomin perennials such as goldenstar or green-and-gold (Chrysogonum virginianum).
Or pair it with pastel tints of foamflower (Tiarella cordifolia). This northeastern native has runners that will root and form a ground cover; a quick-growing variety is T. 'Slick Rock', with smaller leaves and light pink flowers. Others do not form runners, but do supply abundant foliage. Because wild blue phlox loses some foliage in midsummer, other shade-loving plants can fill in the gaps in greenery. T. 'Pink Bouquet' yields a true bouquet of rich light pink blossoms with darker-hued buds. Companion plants with attractive foliage in all seasons go well with this phlox. One of my favorites is a cross between coral bells (Heuchera) and foamflower: Heucherella, 'Pink Frost', has pink and white blooms.
Quick Facts about Wild Blue Phlox
Charlotte, Vermont-based Dorothy J. Pellett is the proprietor of Rock Crest Gardens and a garden writer.
Photo courtesy Ohio Department of Natural Resources