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Introduction

Horticultural History

Basis of Breeding

Accounts of Species and Cultivars

References

Vegetative
The leaves of phlox are opposite and pinnately veined and vary from relatively large and sword-shaped in the tall, herbaceous species to tiny and needle-like in the smaller moss phlox. The largest leaves are those in the P. paniculata group (Paniculatae). Here the leaves are as much as 170mm long and 80mm wide in the case of P. amplifolia, which has the largest leaves in the genus. Those of P. paniculata are smaller, to 150mm long and 50mm wide, and roughly elliptical. These large leaves are relatively thin, with sparse fine bristles. The P. paniculata group uniquely has well defined lateral veins. 

In the P. glaberrima group (Wherry’s Ovatae) leaves are relatively smaller and, except in P. ovata, relatively narrower. Shapes vary from broadly elliptical in P. ovata and P. pulchra to narrowly lanceolate or even linear in some forms of P. carolina and P. glaberrima. The leaves tend to be thickened and often glossy.

The P. pilosa group (Divaricatae) has leaves that are similar in general shape to those of the P. glaberrima group, but with the exception of P. divaricata, they are almost always hairy. The western mid-size phlox have leaves that are linear to lanceolate and vary from smooth to hairy. The P. stolonifera group is unusual in that the leaves are smooth to sparsely pubescent and basally obovate to nearly round.

The moss phlox have foliage that is greatly reduced in size. The cespitose (cushion) growth form has been derived from that of the larger species by shortening of the stems between leaf nodes, making the stems semi-woody, and thickening and narrowing the leaves.The leaves are compressed and thickened into the shape of an awl (subulate) and borne in tufts along the stems. The leaves in each tuft are closely spaced and overlapping, and the stems grow laterally rather than upright.

In each group of phlox growth habit and leaf size is closely related to habitat conditions of soil moisture and weather conditions. The tall phlox grow in moist soils generally in the East. Most of the species of the Phlox pilosa group grow in drier conditions, and the pubescence of their foliage is an adaptation to prevent dessication. The overall reduction in size and hardness in texture of foliage and stems of the moss phlox is an adaptation to drought and harsh winds. This parallels the evolution of growth habit of alpine and arctic species in many other plant genera.

foliage photos

cutaway flowers

seed capsule and seeds

Reproductive

In contrast to the great diversity of vegetative structure in the genus, floral structure in phlox varies relatively little. The flower has five petals united at least half of their length into a narrow tube. Nectar is borne at the bottom of the tube. The petal lobes (what we think of as petals) are salverform (i.e. like a flat plate) and vary in width from overlapping to resembling wide wheel spokes. The outer margin may be notched. Corolla color may be purple, pink, white, or occasionally red or yellow. There is often an eye-ring or contrastingly colored lines around the opening of the tube. This coloration probably serves as nectar guides for pollinators. 

The style may be very short in relation to tube length, as long or longer than the tube, or of intermediate length. The five stamens vary in length and are all contained within the tube in short-styled species or may project from the tube in long-styled species. Lepidoptera appear to be the main pollinators of phlox. Probably they are the only pollinators of the short-styled species, which have tubes with such narrow openings that only a butterfly or moth proboscis can reach the nectar. In species with anthers and stigma presented at the mouth of the tube bees may collect and transfer pollen but are unable to reach nectar; large bees often bite into the base of the tube to reach nectar without being pollinators. Hummingbirds do visit P. paniculata, at least, and may serve as pollinators.

The calyx consists of five sepals, united into a tube for 3/8 to 3/4 of their length. Characters of the calyx, pubescence of the corolla, and proportions of the style and anthers have taxonomic importance.

Phlox seed capsules contain one to several seeds. When the capsule is ripe, it breaks into three sections, scattering the seeds. Seeds of the eastern tall and woodland species do not live long if dry stored, and need to pass the summer and next winter in the soil. The seeds of the drier habitat P. pilosa group species and moss phlox do tolerate some dry storage.

Phlox inhabit a wide range of habitats and the diversity in their growth habits and vegetative morphology reflects this. In contrast the species are relatively conservative in pollination strategy and the range of floral morphology reflects this. Interestingly, Primula sieboldii, a species of East Asia, greatly resembles Phlox divaricata in flower morphology, inhabits similar habitats, and uses similar pollinators. [previous page]