The genus Phlox encompasses a diverse group of flowering plants, most commonly grown as garden perennials, celebrated for their vibrant displays of color. These plants are native to North America. Determining the ultimate size of a Phlox plant is not straightforward because the genus includes a wide variety of species, each with a distinct growth habit. Understanding how a specific type of Phlox naturally grows is the first step in setting accurate expectations for its final height and spread in the garden.
Classifying Phlox by Growth Habit
The many species of Phlox can be categorized into three primary groups based on their mature structure and form. These structural differences are the most significant predictor of a plant’s eventual dimensions.
The smallest group is the Creeping Phlox (Phlox subulata), which is characterized by a dense, mat-forming habit. Creeping Phlox grows along the ground, spreading horizontally to create a thick, carpet-like cover. This growth pattern means the plant’s spread is dramatically wider than its height.
In contrast, the Tall or Garden Phlox (Phlox paniculata) exhibits a strong, upright, and columnar growth habit. This species forms distinct clumps of stiff stems, resulting in a mature plant where the height is often significantly greater than its spread.
The third category is the Woodland or Wild Phlox (Phlox divaricata). These plants occupy an intermediate space, often forming loose, low-to-medium mounds that spread slowly by short rhizomes or rooting stems.
Mature Height and Spread of Common Varieties
The specific dimensions of common cultivated species reveal a Phlox plant’s potential size under optimal growing conditions.
The Creeping Phlox (Phlox subulata) maintains a low profile. Its mature height typically ranges from 4 to 6 inches. Its lateral growth is substantial, with a mature spread that commonly reaches between 18 and 24 inches.
The Garden Phlox (Phlox paniculata) is a popular choice for perennial borders. This species develops into a substantial, clump-forming perennial with a mature height generally spanning 2 to 4 feet. Its spread is usually less than its height, typically filling out to a width of 2 to 3 feet. Dwarf varieties have been bred to remain under 2 feet tall while retaining the characteristic upright form.
The Woodland Phlox (Phlox divaricata) prefers shadier conditions and falls between the two extremes. This plant forms loose, spreading clumps with a modest height of 6 to 12 inches. Its spread is proportional to its height, generally filling out to a width of 12 to 18 inches.
Environmental and Maintenance Factors Affecting Final Size
Achieving the maximum size potential for any Phlox variety requires suitable environmental conditions and consistent maintenance. When plants receive insufficient sunlight, they often exhibit etiolation, or legginess, as they stretch to find light, which reduces the plant’s vigor and density. For sun-loving types like Garden Phlox, insufficient light also reduces the number of blooms.
Soil quality and nutrition play a role in determining a Phlox plant’s sturdiness and size. Phlox species thrive in moist, well-drained soil rich in organic material. Over-fertilization, especially with high-nitrogen formulas, can stunt flowering and result in weak, spindly stems prone to flopping, preventing the plant from reaching its full height.
Consistent moisture is necessary, as Phlox is intolerant of drought conditions, which cause growth to become stunted and foliage to brown. While they require regular watering during dry periods, avoiding overhead irrigation is advisable because water sitting on the foliage encourages powdery mildew, a fungal disease that stresses the plant and diminishes its mature size.
Maintenance practices like division and pruning are important for maintaining a healthy size and spread. Mature clumps that become too dense often lose vigor, resulting in smaller blooms and reduced size. Dividing the clump every few years rejuvenates the plant, allowing it to return to its maximum potential size. Removing spent flower heads (deadheading) encourages the plant to put energy into lateral growth and new blooms, leading to a denser, bushier plant.