Phlox is a popular perennial flower known for its vibrant, dense blooms and versatility in the garden. Its spreading habit, however, is a common consideration for gardeners who need to manage their planting space and maintain defined borders. Understanding how quickly and by what method different species of Phlox expand is the first step toward effective garden planning and control.
The Two Main Types of Phlox and Their Spread Rates
The speed and manner of Phlox spread depend on whether a gardener chooses a creeping or a tall variety. Creeping Phlox (Phlox subulata) forms a dense, low-growing mat, making it an effective ground cover. These varieties are fast-spreading, achieving a mature width of up to 2 to 3 feet in a few years.
Creeping varieties are typically planted 12 to 24 inches apart to allow them to merge and create a seamless carpet of foliage and flowers. This lateral expansion is their primary method of colonization, quickly filling in gaps on slopes or cascading over rock walls.
In contrast, Tall or Garden Phlox (Phlox paniculata) are classic upright perennials that grow from 2 to 4 feet tall. Their primary growth habit is clump-forming, meaning the plant expands slowly outward from the center. While a mature clump can reach a width of 2 to 3 feet, its lateral spread is much slower than the creeping types. The main concern with tall varieties is their potential to appear aggressively in new locations via self-seeding.
Mechanisms of Expansion
The difference in spread rate is directly related to the biological mechanisms of expansion. Creeping Phlox primarily uses vegetative spread, where the plant’s stems root where they touch the ground. This process creates new, independent plantlets around the original crown, allowing it to form a dense, interconnected mat that quickly covers a large area.
Tall Phlox relies on seed dispersal for expansion outside of its original clump boundary. Once flowers fade, the plant produces inconspicuous, three-valved seed capsules that dry out. When mature, these capsules forcibly “pop open” (ballistic dispersal), flinging small seeds a short distance from the parent plant. These seeds then germinate, often leading to volunteer seedlings appearing unexpectedly across the garden the following spring.
Environmental Factors Influencing Growth Speed
The rate at which Phlox spreads is sensitive to external variables, mainly light, water, and soil quality. Phlox performs best in full sun, which encourages denser growth and more vigorous blooming, leading to faster lateral spread in creeping varieties. Consistent moisture is necessary for robust growth and expansion, particularly for upright varieties, though well-drained soil is required to prevent root rot.
The composition and nutrient content of the soil also dictates growth speed. Rich, well-draining soil encourages rapid expansion, while poor, compacted soil acts as an inhibitor. Over-fertilizing with a high-nitrogen product accelerates foliage growth, potentially making the plant look leggy and reducing flower production. A balanced fertilizer, such as a 10-10-10 blend, supports both root development and flower production for a healthy plant.
Managing and Controlling Phlox Spread
Gardeners can manage the spread of Phlox through several direct interventions. For clump-forming Tall Phlox, the primary method of control and rejuvenation is division. Digging up and dividing the root clump every two to four years in the early spring prevents the center from becoming sparse and keeps the plant contained. Each separated section should contain healthy roots and at least three active offshoots for successful replanting.
To prevent the aggressive self-seeding of tall varieties, regularly deadhead spent flower panicles before the seeds mature and are released. Removing faded flowers prevents unwanted seedlings and can also encourage the plant to produce a second, smaller flush of blooms.
For creeping types, physical barriers can define a permanent boundary. Installing landscape edging or a root barrier that extends 4 to 6 inches deep will effectively halt its low-growing vegetative expansion. Trimming or shearing Creeping Phlox immediately after its spring bloom also helps maintain a dense, compact form and limits its outward reach.