Some species of phlox have been mainstream garden plants for many years, but at this time there is little literature devoted to the group from a horticultural point of view. Neither of the two recent horticultural books on phlox is in English (Fuchs 1994, Bendtsen 2003). Wherry's systematic monograph of the genus is 55 years old. James Locklear's new book, Phlox: A Natural History and Gardener's Guide, gives a wealth of information on the ecology and habitats of the wild species, along with botanical descriptions and colorful descriptions of the landscapes the plants inhabit.

Many phlox species have great horticultural potential but are virtually unknown to the general gardening public. These web pages are an attempt to bring some of these plants to the attention of gardeners and commercial growers. They are essentially in first draft form. I have worked with phlox off and on for the last 25 years; most of my experience has been with the species of the Appalachians.

Overview

Phlox is a genus of sixty-some species. There is a current list of taxa at the USDA Plants Database, including synonomy, common names, and geographic ranges. With the exception of one species found in northern Asia, the group is exclusively North American. The genus shows great diversity in growth form from creeping mat types to erect plants well over a meter tall. Almost all are herbaceous or semi-woody perennials – a very few are annuals. Phlox inhabit a diverse range of habitats from marshes and riverbanks to prairies, deserts, and tundra and a wide geographic range from Alaska to New England and south to Mexico and Florida.

In general the larger herbaceous types are grow in moist, open habitats of the eastern and central US and southern Canada. The forests and prairies of this general area also support herbaceous species of small to medium size (15-40cm tall). Barrens in the East and semi-arid and alpine habitats in the West support many species of low or medium size (5-45cm tall).

The flowers of all phlox are attractively colored and showy. They have been grown as garden plants since the 17th century, when Phlox paniculata was sent from the American Colonies to Europe. A few other species, such as P. subulata, P. stolonifera, and P. drummondii, have become garden subjects, but surprisingly few of the wide array of wild phlox have come into mainstream horticulture. Of these, only a couple have been the subjects of extensive breeding and selection programs. Many cultivars of Phlox paniculata have been developed primarily in England and The Netherlands, and these have become important elements of the formal perennial border. P. drummondii, an annual of the Texas plains, has been bred for a wide range of flower colors and is a popular bedding plant started from commercial seed.

Classification

Phlox is a taxonomically confusing group. The best reference and the only book devoted to the systematics of the genus is Edgar T. Wherry’s The Genus Phlox, published in 1955. Luckily, there has been more recent work on the genetics and evolution of phlox by Donald Levin and his students at the University of Texas. Their publications help greatly with the relationships of the eastern species, at least. The most active worker in the field at the present time is Carolyn Ferguson at Kansas State University.

Wherry divided Phlox into three main sections, based primarily on floral and seed characters. Section Protophlox is characterized as having short styles united for ¼ to ½ their length and relatively large seeds with large embryoes. Section α-Phlox has styles united for more than ½ their length, with the styles often exceeding the length of the floral tube and relatively large seeds with large to medium sized embryoes. Section Microphlox is composed of small western and arctic species with styles united more than ½ their length and relatively small seeds with small to medium sized embryoes. Wherry's arrangement divides the eastern moss phlox species, which would otherwise seem by general appearance to be closely related, into two different sections of the genus. All of the western moss phlox are considered to be in a separate section from those of the East.

More recent work by Carolyn Ferguson and others (Ferguson et al. 1999, 2002) using DNA studies indicates (my interpretation) that the eastern moss phlox are all closely related and are relatively close to the western Subsection Nanae. Subsections Divaricatae and Drummondianae are coherent and related, as are the Ovatae and Paniculatae. Hybridization data (Levin 1963, 1966, and my own observations) support these findings. I have seen no data on genetic relationships of the western microphlox to the eastern moss phlox, but the existence of fertile garden hybrids that appear to involve P. subulata and western species indicate that relationships are much closer than Wherry's classification. The forthcoming Flora of North America treatment of Phlox should give a better rearrangement of the genus.

Since we are primarily interested in phlox from a horticultural point of view, we will simplistically divide the species into three groups based on horticultural use, which is itself a function primarily of growth habit and also of habitat requirements:
Tall – This group is made up of the relatively large herbaceous species, from about half a meter to over a meter tall, most of which are found in the East, e.g. P. paniculata, P. maculata, P. pulchra, P. ovata. These species tend to die back to basal foliage in winter. They are used as garden plants primarily in sunny border settings.
Medium – These average smaller and often are semi-woody at the base. The P. pilosa-divaricata group is included as well as the Texas annuals like P. drummondii. P. stolonifera and P. adsurgens, creeping plants of the woodlands of the East and West, respectively, can go here and the western species like the P. speciosa group and P. grayi are probably best included. Except for the annual species, plants in this group are usually grown as part of native habitat (e.g. woodland or prairie) gardens. Many of the less well known species are probably grown almost exclusively by rock gardeners, if they are cultivated at all.
Small – These are generally known as moss phlox. They are semi-woody mat- or cushion-forming plants with needle-like, more or less evergreen foliage. In the East there are several species inhabiting sandy and rocky barrens, in the West there are dozens of species in semi-arid and montane habitats, and in the Arctic there are species growing in tundra. Selections and hybrids of the eastern species (P. subulata and P. nivalis) are commonly used as landscape plantings where a low, dense groundcover is needed in full sun. The western species are very difficult to grow in the eastern US; they are grown by rock gardeners in the West and in Europe.

Phlox pages copyright February 2014 Charles G. Oliver