Creeping phlox (Phlox subulata) is a popular perennial ground cover known for its dense, mat-forming habit. This North American native creates a stunning carpet of color in the spring, typically blooming for several weeks with masses of tiny, saucer-shaped flowers. While propagation often uses cuttings or division, growing creeping phlox from seed is a viable and budget-friendly method. Starting from seed requires a longer timeline and specific preparation, but it allows for the cultivation of many plants to cover large areas, such as rock gardens, borders, or slopes for erosion control.
Seed Preparation and Timing
Creeping phlox seeds possess dormancy, preventing premature sprouting. To initiate germination, the seeds require cold stratification—a period of cold, moist conditioning that mimics natural winter conditions. This process signals that the cold season has passed and growth can begin.
The recommended duration for cold stratification is typically 30 to 60 days. This can be achieved artificially using the refrigerator method: place seeds in a sealed container with a damp medium, such as perlite, vermiculite, or a folded paper towel. The container should be stored in a refrigerator, maintaining 33 to 40 degrees Fahrenheit, for the required period.
An alternative is natural outdoor stratification, or winter sowing. This involves sowing seeds directly into the garden during late fall or early winter. The natural freeze and thaw cycles provide the necessary cold treatment throughout the winter. The best time to begin the process, whether indoors or out, is in late winter, two to three months before the last expected frost date.
Sowing and Initial Germination
After stratification, or when starting indoors without it, seeds are ready for sowing. Use a sterile, light, and well-draining seed-starting mix in trays or small pots. The medium should be thoroughly moistened before planting.
Creeping phlox seeds require light to germinate, so they should be sown very shallowly or pressed gently onto the soil surface. If covering them, use a thin layer of fine vermiculite or mix, no more than 1/8 inch deep, to allow light access. Germination occurs within 10 to 21 days when the soil temperature is maintained between 65 and 70 degrees Fahrenheit.
Maintaining constant moisture is crucial, but the medium must never become saturated, which causes fungal issues. Indoor setups benefit from using a heat mat to regulate soil temperature and a clear plastic dome to maintain high humidity. If sowing directly outdoors, wait until after the last hard frost and keep the area consistently damp until emergence.
Care for Young Seedlings
Once seeds germinate and the first true leaves appear, thinning is necessary to ensure proper air circulation and growth. Seedlings often emerge densely packed in the starting trays. Using small scissors, clip the weaker seedlings at the soil line, leaving the strongest plants spaced one to two inches apart.
When seedlings develop their second set of true leaves, they are ready for nutrients, as seed-starting mixes are low in fertilizer. Apply a weak solution of balanced liquid fertilizer, diluted to one-quarter or one-half strength, to encourage robust leaf and root development.
Seedlings started indoors require hardening off before transplanting outside. Beginning about one week before the planned date, move the seedlings outdoors for increasingly longer periods each day. This gradual acclimation allows the plants to adjust to direct sunlight, wind, and temperature fluctuations, reducing transplant shock.
Establishing and Maintaining Mature Plants
Once seedlings are hardened off and all danger of frost has passed, they are ready for their permanent location. Creeping phlox thrives in full sun, requiring at least six hours of direct light daily; afternoon shade is beneficial in extremely hot climates. The planting site must feature well-draining soil, as the plant is intolerant of consistently wet conditions that cause root rot.
When transplanting, space the young plants 12 to 18 inches apart. First-year plants require consistent moisture to establish root systems, needing weekly watering if rainfall is scarce. Once established, creeping phlox develops good drought tolerance.
Long-term care involves seasonal maintenance to encourage dense growth and flowering. Immediately after the spring bloom finishes, a light pruning or shearing helps remove spent flowers and stimulates tighter foliage. This post-bloom trim is also the time to apply a balanced, slow-release fertilizer if the soil is poor, though established plants require minimal feeding.