Woodland Phlox (Phlox divaricata) is a native North American perennial prized for its beautiful spring display in shaded gardens. This popular plant is often sought after to create a naturalized carpet beneath trees or along shady pathways. Woodland Phlox does spread, but it does so gently and non-aggressively. It requires dappled shade and consistently moist, well-drained soil rich in organic matter to thrive and slowly expand its footprint.
Defining the Growth Habit
Woodland Phlox is categorized as a clump-forming perennial, meaning its spread is slow and highly localized. It does not possess the aggressive rhizomes that characterize truly running or invasive plants. Instead, the plant expands its colony outwards at a moderate pace, creating a dense but manageable mat of foliage.
Its growth habit is described as “naturalizing,” where the original plant gradually increases its diameter to form a soft, cohesive patch. The species name divaricata refers to a spreading habit, which accurately reflects its slow, steady expansion. A single plant will take several years to occupy a significant area, making it a reliable and easy-to-control choice. The entire colony remains shallowly rooted, which contributes to its manageable nature.
Mechanisms of Propagation
The expansion of a Woodland Phlox patch is achieved through two distinct biological processes: vegetative spread and sexual reproduction via seed. Understanding these methods explains how the plant both thickens its existing patch and establishes new colonies nearby.
The primary method of localized expansion is through nodal rooting, a process similar to layering. Non-flowering stems grow horizontally along the soil surface. When the nodes make contact with damp earth, they produce roots. These rooted sections become new plantlets that remain connected to the parent plant, effectively thickening and broadening the original clump.
The second method involves the production and dispersal of seeds following the spring flowering period. After the flowers fade, small seed capsules form on the fertile stems. As these capsules dry, they release their seeds, often projecting them a short distance away. This dispersal mechanism allows for the establishment of new colonies some distance from the main patch.
Seedlings that germinate from these dispersed seeds may exhibit slight variations in flower color or size compared to the parent plant. This genetic variability is not present in the new plants created by nodal rooting, which are clones. Gardeners can collect these seeds to manually start new plants or allow the natural process to occur to encourage a more widespread naturalization.
Controlling and Encouraging Spread
For gardeners who wish to accelerate the formation of a dense groundcover, providing the ideal habitat is the most effective approach. Maintaining a loose, humus-rich soil, such as one covered with leaf litter, ensures the trailing stems can easily make contact and root at their nodes. Ensuring the plant receives consistent moisture, particularly during dry periods, also supports the vigorous growth needed for quick patch formation.
If the goal is to limit the plant’s spread, remove the spent flower heads before the seed capsules mature to prevent the establishment of distant new colonies. For existing patches that have become too dense, the entire clump can be easily dug up and divided. Dividing and transplanting sections of the colony every few years is usually sufficient to manage its size and rejuvenate the plants.