Tall Phlox, or Phlox paniculata, is a garden perennial prized for its large, fragrant summer flower heads. This plant does spread, but its expansion is generally slow and predictable, making it a manageable addition to the perennial border. Tall phlox is primarily a clump-former, unlike aggressive groundcovers that send out long runners. It gradually occupies more space over time but does not aggressively colonize the garden overnight.
Understanding the Growth Habit
The primary method of lateral spread for tall phlox is through the slow elongation of short, underground stems called rhizomes. These rhizomes stay close to the main root crown, causing the plant to widen its diameter in a slow, outward creep. This action results in a dense, tight clump of multiple stems emerging from the soil as the plant matures.
The individual clump increases in size by a few inches each year, eventually becoming crowded and less vigorous in the center. This slow expansion is predictable, allowing gardeners to anticipate the plant’s spread over several years. When a clump becomes congested, typically every three to five years, division is needed to maintain plant health and control the overall footprint.
Self-Seeding and Unwanted Colonization
While rhizomes cause the slow expansion of the original plant, self-seeding is often the source of unexpected and unwanted colonization across the garden. After the flowers fade, tall phlox produces small seed capsules that dry and burst open, flinging seeds a short distance. This mechanism is responsible for new plants appearing suddenly several feet away from the original location.
The most significant issue with this form of spread is reversion in hybrid cultivars. Most modern tall phlox varieties are hybrids bred for specific colors like pure white, bright red, or clear pink. Seeds produced by these hybrids often carry the genetic traits of the wild species, which typically reverts to a less desirable, often muted magenta or pale lavender color.
If left unchecked, these volunteer seedlings germinate and grow, quickly forming new plants that crowd the garden and detract from the chosen cultivar. The resulting plants may also carry a higher susceptibility to common fungal diseases, such as powdery mildew, compared to the original, disease-resistant hybrids. Therefore, managing seed production is a priority for controlling the quality and location of tall phlox in the landscape.
Managing and Controlling Expansion
Controlling the spread of tall phlox requires a two-pronged approach addressing both underground rhizome growth and above-ground seed production. To manage the size and density of the original clump, divide the plant every three to five years. This involves digging up the entire plant in early spring or fall and splitting the crowded root mass into smaller sections. Replanting only the vigorous outer sections maintains the plant’s health and restricts its size.
To prevent unwanted colonization and genetic reversion caused by self-seeding, deadheading is highly effective. Deadheading involves removing the spent flower heads before the seed capsules fully mature and scatter their contents. Removing these faded blooms stops seed dispersal and can also encourage the plant to produce a second, smaller flush of flowers later in the season.
Proper site selection also helps manage the plant’s density and overall health, indirectly controlling spread. Planting tall phlox with adequate spacing, ideally eighteen to twenty-four inches apart, ensures good air circulation. Improved air flow reduces the likelihood of powdery mildew, which can weaken the plant.