Phlox species spread through two main biological processes: sexual reproduction via seeds or asexual reproduction via vegetative growth. The specific method depends heavily on the species; for instance, tall garden phlox (Phlox paniculata) spreads differently than low-growing, mat-forming varieties. Understanding these distinct strategies is crucial for managing the plant’s growth in a garden setting.
Spreading Through Seeds
Phlox plants produce numerous tiny seeds within small, three-parted capsules that develop after pollination. For many perennial species, the mature capsule dries out and splits open, sometimes releasing seeds with a forcible, ballistic action. Garden phlox is known for “self-seeding,” where new seedlings often appear near the original clump. However, seeds from hybrid cultivars may not grow true to the parent plant, potentially resulting in different flower colors. Furthermore, many perennial phlox seeds require cold stratification, naturally occurring over winter, before they can successfully germinate in the spring.
Vegetative Spreading Mechanisms
The most aggressive forms of phlox expansion rely on vegetative growth, which results in genetically identical clones of the parent plant. Tall garden phlox (P. paniculata) primarily spreads using short, thick rhizomes, which are horizontal underground stems. These rhizomes grow outward from the main crown, sending up new shoots and creating a progressively denser, wider clump over time.
Stolons and Crown Expansion
In contrast, low-growing groundcover varieties like creeping phlox (Phlox stolonifera) utilize stolons, or runners, which creep along the soil surface. These stolons root at their nodes, establishing new plantlets that quickly expand the colony into a wide, dense mat. Moss phlox (Phlox subulata) spreads more slowly by crown expansion, creating a dense, mounding mat rather than sending out long runners.
Managing and Controlling Phlox Expansion
To prevent unwanted seedlings and the potential reversion of hybrid flowers, gardeners can employ deadheading. Removing spent flower clusters before they form viable seed capsules stops the sexual reproduction cycle and often encourages a subsequent flush of blooms.
Controlling Vegetative Growth
Managing the vegetative spread of clump-forming types like garden phlox requires periodic division of the rhizomatous root mass. Dividing the plant every three to four years, preferably in early spring or fall, controls lateral growth and prevents the center of the clump from becoming woody. For stoloniferous varieties, expansion is controlled by physically pruning runners that extend beyond the desired boundary. Physical barriers, such as metal or plastic edging sunk into the soil, can also contain the creeping stems of aggressive mat-forming species.