Smooth phlox (Phlox glaberrima) is an upright, clump-forming, lipomatous perennial plant species. Smooth phlox grows in moist meadows, low woods, and along riverbanks. This species attracts butterflies, moths, and bees.
I. Appearance and Characteristics
Phlox glaberrima, commonly called smooth phlox and sometimes marsh phlox, is a species of flowering plant in the phlox family. It is native to the Midwestern and Southeastern United States where it is found in moist to wet areas. It can be found in both prairies and forests, where it is an indicator of high quality habitat.
Phlox glaberrima (Smooth Phlox) is a clump-forming, rhizomatous perennial with large, pyramidal clusters, up to 12 in. long (30 cm), densely packed with sweetly-aromatic, tubular, rose to purplish flowers in mid-spring to early summer. Blooming for about 6-8 weeks, each blossom, 1 in. across (2.5 cm), features 5 flat, rounded petals that are partially joined. Butterflies and hummingbirds are attracted to the bright flowers which are a source of nectar in spring. The flowers are borne on stiff stems clad with thin, hairless, finely-toothed, lance-shaped, dark green leaves, 4 in. long (10 cm).
The relationships between Phlox glaberrima, Phlox carolina, and Phlox maculata are particularly unclear. There may be hybridization, or an undescribed third species that resembles an intermediate. As of 2014, the phylogenetic relationships of this group of Phlox were under review, with variations in the “Phlox carolina–glaberrima complex…among the most difficult phlox plants to positively identify”.
Botanist Edgar T. Wherry (1955) recognized three subspecies, subsp. glaberrima in its eastern range, subsp. interior of the Plains (the Wabash smooth phlox) and subsp. triflora (syn. P. triflora, the three-flower smooth phlox) in the South, but Locklear (2012) only two, incorporating subsp. triflora with subsp. glaberrima.
The glaberrima and interior variations, along with other taxa in the P. carolina–glaberrima complex and P. maculata, are frequently cultivated as ornamental landscape plants, but triflora rarely is. Notable cultivars include P. glaberrima ‘Morris Berd’, P. glaberrima ssp. triflora ‘Forever Pink’ and P. glaberrima ssp. triflora ‘Triple Play’.
II. How to Grow and Care
Smooth phlox thrives in moist, well-drained soil and requires full sun to partial shade for optimal growth. Special care points include ensuring consistent soil moisture without waterlogging, and providing support during flowering as stems can be tall and susceptible to wind damage. Regular deadheading encourages further blooming and prevents self-seeding in unwanted areas.
Sunlight
Smooth phlox requires full sun or partial sun in order to properly grow and thrive. It should receive at least 5 to 6 hours of direct sunlight during the day, ideally in the morning hours. When grown in full sun, smooth phlox will bloom for weeks. If this plant is placed in too much shade, the blooming time may be significantly shortened or the plant may not bloom at all. Additionally, if the plant is placed in too much shade, the foliage may become disease-prone due to increased humidity levels.
Watering
Thriving in its native moist meadows, smooth phlox has adapted to regular hydration cycles, displaying a preference for consistent moisture. This species is characterized by moderate drought tolerance and benefits from watering every week to maintain its lush growth. For indoor cultivation, smooth phlox excels with higher humidity levels, mimicking its natural habitat’s conditions. As an herbaceous perennial, smooth phlox’s water needs are particularly crucial during its active growing season to support its vibrant flowering.
Soil
Smooth phlox performs best in organically rich, almost constantly moist, well-drained soils.
Fertilizing
For optimal bloom, fertilize smooth phlox in spring with balanced fertilizer (e.g., 10-10-10), then monthly with high phosphorus to boost flowering. Use sparingly; over-fertilization risks growth at the expense of blooms. Adjust seasonally: less in fall, none in dormant winter. Employ slow-release or diluted liquid forms to prevent root burn. Novices: use pre-measured spikes or granules for ease. Veterans: tailor amounts to your smooth phlox’s response and soil test results.
Pruning
Smooth phlox should be pruned after blooming has faded. Generally, the first pruning should take place in late summer (August-September) to remove the faded blooms. At this time, prune back the plant by 1/3 to both promote new growth and reduce the potential for disease.
During subsequent years, do 1/3 pruning every 1-2 years depending on the desired size of the plant. In early to mid-spring, only remove dead branches and leaves to encourage new growth. Do not prune stems too close to the ground as this may prevent new growth from forming.
Propagation
Smooth phlox can be effectively propagated through the method of sowing. This involves preparing a well-draining soil mix that supports the germination and growth of new seedlings. Pre-treatment such as stratification might enhance germination rates. The seeds should be placed just below the soil surface and kept under appropriate moisture conditions. Once seedlings emerge and develop, they’ll need thinning to allow enough space for mature growth. Success with this method requires careful attention to environmental factors such as light and temperature, which support smooth phlox’s developmental needs.
Transplanting
For smooth phlox, early to mid-spring is a splendid time to transplant, ensuring a cool but growing season for root establishment. Choose a spot with well-drained soil and partial shade. Gentle handling of the roots will encourage a smooth transition.
III. Uses and Benefits
Noted for its excellent resistance to powdery mildew and root rot, Smooth Phlox is a fine choice for the perennial border, cottage garden, wildflower meadow, native plant garden, or along ponds or streams. Plant in drifts for great impact and make sure you have enough for the vase!