The Phlox genus contains both annual and perennial varieties, which often confuses gardeners. This diverse group of flowering plants offers varieties suited for various garden uses, from groundcovers to tall border flowers. The difference lies in the plant’s life cycle: annual phlox completes its entire life cycle in a single growing season, while perennial phlox regrows from the roots each spring, returning reliably year after year.
Understanding Annual Varieties
Annual phlox, most commonly Phlox drummondii, is cultivated for its bright, abundant, and season-long floral display. These varieties germinate, flower, set seed, and die within one year, requiring replanting every spring. The growth habit of annual phlox is typically shorter and bushier, usually reaching a height of 6 to 15 inches with a similar spread.
The trumpet-shaped flowers are densely clustered at the stem ends and come in a wide spectrum of colors, including reds, pinks, whites, and purples. These blooms are prolific and often fragrant, providing continuous color from mid-spring until the first hard frost. Although they are robust and can tolerate some soil dryness once established, they often decline significantly in the intense heat of mid-summer without supplemental watering or afternoon shade.
Exploring Perennial Varieties
Perennial phlox varieties are far more numerous and varied, serving as a dependable, long-term foundation for garden design. The two most popular perennial types are distinguished by their vastly different growth habits and preferred placement in the garden.
Tall Garden Phlox (Phlox paniculata) is an upright variety that forms sturdy clumps, growing two to four feet tall, making it a popular choice for the mid to back border. This species is known for its large, fragrant flower clusters (panicles) that bloom from mid-to-late summer, attracting butterflies and hummingbirds. Creeping Phlox (Phlox subulata), in contrast, is a low-growing, mat-forming groundcover that rarely exceeds six inches in height. It creates a dense, spreading carpet of foliage and is valued for its spectacular early spring bloom, cascading over rock walls and slopes. Other perennial types, such as Woodland Phlox (Phlox divaricata), offer semi-evergreen foliage and thrive in dappled light.
Managing Phlox Based on Type
Annual phlox requires consistent deadheading—the removal of spent flowers—to encourage continuous blooming throughout the season. This prevents the plant from setting seed prematurely, which would signal the end of its life cycle. At the conclusion of the growing season, annual plants are entirely removed from the garden.
Perennial phlox requires maintenance actions tailored for long-term growth. Tall varieties benefit from the “Chelsea Chop,” which involves cutting some stems back by half in late spring to delay flowering and produce a bushier plant with a longer bloom time. Over time, established perennial clumps can become sparse in the center, requiring division every three to four years in the spring or fall to rejuvenate the plant. In late fall or early winter, perennial stems should be cut back to the ground once the foliage has died back, and a layer of mulch can be applied to help the plant successfully overwinter.