Spreading Phlox

Gardeners often use the tall garden phlox (Phlox paniculata) as a key species in perennial borders, but not as many grow the low-growing, mound and mat forming species.

Credit: Richard Hebda

Gardeners often use the tall garden phlox (Phlox paniculata) as a key species in perennial borders, but not as many grow the low-growing, mound and mat forming species.

Within this group of excellent rock and alpine garden beauties can be found a remarkably showy native British Columbian – spreading phlox (Phlox diffusa). Loose colourful patches of spreading phlox are a sight to see. The 5- to 10-cm-thick (2- to 4-in.) mats grow mainly from a perennial taproot that probes deep into the soil. Rootlets arise along the relatively thin scrambling stems, holding them to the surface. Pairs of short, opposite, narrow leaves cover the branches of this sub-shrub. Each toothless leaf in the pair fuses at the base with its partner, where there may be a mass of spidery hairs. The leaves are otherwise smooth. Masses of whitish to bright lavender flowers cover the surface of the mat so closely that often only the blooms are visible. On southern Vancouver Island you cannot miss the joyous splashes of colour in the mountain meadows.

The spreading phlox flower is five-parted, typical of plants from the Phlox Family (Polemoniaceae). The sepals form a narrow, whitish tube with five sharp teeth. The petals erupt from the mouth of the tube. Like the sepals, they are fused at the base but separate into five showy, rounded lobes or segments in the upper portion. These stick out at right angles to the floral tube, forming an open flower 1 to 2 cm (1⁄2 to 3⁄4 in.) across. The opening into the floral tube looks tiny, but crammed inside are five stamens of varying length and below them an ovary with a three-parted stigma. A pollinating insect must have quite a task forcing its way into the flower and certainly will pick up pollen to pass on to the next bloom. Flowers open from late spring and sometimes continue into August. In British Columbia the species’ range includes the mountains of the east and central parts of Vancouver Island and the adjacent mainland for a couple of hundred kilometres (125 mi.) inland. This truly western North American species extends southward as far as California and eastward into Idaho and Montana. It is a mid- to high-elevation species favouring forest openings, slopes and meadows on scree and even on rock outcrops. Several colours of spreading phlox are available in the horticultural trade, but you may need to ask for it specifically at the garden centre or nursery.

It may be sold under its old name, Phlox douglasii, rather than P. diffusa, and it is still known by the old name in a recent edition of The Royal Horticultural Society Gardener’s Encyclopedia of Plants and Flowers. Apparently, plants can be raised reliably from the rather shyly produced seed, sown outdoors in a sandy or gritty medium in the fall. New-growth cuttings will root, but skill and patience are required. The plants can be propagated most easily by gently teasing away the weakly rooted scrambling stems and replanting them in an airy, moist, but not wet, medium. Spreading phlox performs well in the rock garden, left to conform at its own leisurely pace to the place provided. The site must be in full sun or light shade and be airy. The soil must have excellent drainage, but as with many alpine and sub-alpine plants, it must also be loose, porous and moist.

Phloxes are most widely valued for their horticultural uses. But a peculiar story relates to the group. A long-recognized native medicine to treat tape and roundworm problems with pink root (a species of Spigelia, a low perennial native to the U.S. south) was regularly adulterated with phlox roots. A good thing perhaps, because the pure medicine could cause death if used in too high a dose! Next time you consider phlox for the garden, think small not tall. Try our native spreading phlox, a true beauty from the mountains of southwestern B.C. The following plants are hardy to the zone number indicated: Phlox diffusa – zone 6 or perhaps hardier An expert on native plants, Richard Hebda is curator of Botany and Earth History at the Royal British Columbia Museum in Victoria.