Garden Phlox, most commonly the perennial species Phlox paniculata, is a popular garden flower known for its vibrant, fragrant summer blooms. While it does spread, its habit is generally manageable rather than aggressively invasive. This plant typically expands its footprint through two primary methods: a slow, steady expansion of its root system and the germination of dropped seeds. Understanding these growth mechanisms is key to keeping your phlox contained within its desired space.
How Garden Phlox Spreads: Mechanisms of Growth
The most predictable method of spread for perennial garden phlox involves vegetative growth, which results in a widening of the original clump. This expansion occurs beneath the soil surface through short, shallow rhizomes, which are modified stems that grow horizontally. As the plant matures, new shoots emerge around the periphery of the main crown, causing the clump to slowly increase in diameter over several years.
Root expansion is a slow process that allows the plant to form a robust, dense colony supporting its tall, upright flower stalks. However, this growth can eventually lead to a congested center where older roots become woody and less productive. This type of spread makes the plant long-lived but requires periodic maintenance to maintain health and bloom quality.
The second mechanism of spread is through sexual reproduction, or self-seeding, which is often the source of unexpected phlox plants appearing throughout the garden. Once flowers fade, they produce small seeds that drop and germinate in the surrounding soil. While this creates new plants, the seedlings rarely inherit the color or characteristics of the hybrid parent. Instead, they often revert to the species’ wild, less desirable colors, typically a muted lavender or magenta shade, which can crowd out the desired cultivar.
Tall Phlox vs. Groundcover Phlox: Understanding Different Spread Rates
The term “Garden Phlox” can refer to several species, and spread rates vary significantly based on growth habit. Tall Phlox, Phlox paniculata, is the classic upright summer perennial that reaches heights of two to four feet and is considered a moderate spreader. Its growth is primarily clumping, meaning it expands slowly outward from the center, typically gaining only a few inches in width each year. This controlled expansion allows it to remain a reliable border plant for several seasons.
In contrast, low-growing varieties used as groundcovers exhibit a more aggressive spreading habit, though they are usually planted specifically for that purpose. Phlox subulata, or Moss Phlox, forms dense, low mats and expands relatively slowly, creating a tight carpet of needle-like foliage. It is non-invasive and primarily used in rock gardens or along pathways where slow, steady coverage is desired.
Phlox stolonifera, or Creeping Phlox, is a vigorous spreader that utilizes runners, known as stolons, which creep along the soil surface. These stolons root at the nodes, allowing the plant to rapidly colonize an area and weave through other plantings. This method of horizontal propagation is effective for quickly covering bare ground or stabilizing slopes, but requires diligence if boundaries must be strictly maintained.
Controlling and Managing Phlox Spread
The most effective way to manage phlox spread is by addressing both vegetative and sexual reproduction. Controlling self-seeding is achieved through deadheading, which involves removing spent flower clusters before they set seed. Removing the entire terminal flower head ensures no seeds are dropped, preventing the proliferation of volunteer seedlings that may revert to a wild color.
To control the slow expansion of the root clump, plant division is necessary every three to five years. This process rejuvenates the plant and prevents the center from becoming woody and unproductive, which can reduce bloom quality. To divide, carefully dig up the clump and use a sharp spade or knife to separate the younger, healthier sections from the outer edges.
The older, woody core should be discarded, and only the vigorous, younger sections should be replanted. This maintenance task controls the size of the phlox and ensures the longevity and continued robust flowering of the desired cultivar. By consistently removing spent blooms and periodically dividing the clumps, gardeners can enjoy the beauty of garden phlox without it overtaking the planting area.