Green Thumbs Up: Cultivating the showy phlox

In typical summers, many landscapes begin to lose their appeal as the month of August approaches, with parched lawns and fewer blooms to brighten our perennial gardens. The summer of 2009, however, with its cool weather and copious moisture has painted an entirely different portrait with lush green carpets highlighting tropical-like vegetation in our borders. Our perpetual spring-like conditions have delayed the blooming of some perennials and extended the flowering of others, while cloudy, overcast days promote rich, saturated flower colors that begged to be photographed. My perennial gardens continue to provide an abundance of multi-colored blossoms and robust foliage, and should this weather pattern continue, there may be plentiful bloom into September.

While I consider foliage all-important when selecting plants for my garden, I do make exceptions for a few plants that offer exceptional flowers or an extended bloom period. Few perennials rival the summer-blooming garden phloxes (Phlox maculata and paniculata) for color and long bloom in the late summer garden. Dozens of cultivars are available that exhibit spectacular clusters of sweetly-scented flower heads composed of numerous smaller flowers known as pips. The blooms are irresistible to countless butterflies, hummingbirds, and hummingbird moths. Flowers are produced over a long season in a wide spectrum of colors ranging from soft pinks and lavenders to fiery oranges, reds and salmons, glistening white to rich violets and purples. In addition to solid-colored cultivars, many varieties exhibit contrasting eyes in the center of the flower pips.

The earliest to bloom tend to be the meadow phloxes, commonly known as wild Sweet William (P maculata). These members of the phlox family display handsome, pointed, deep green glossy leaves on stems growing 2 to 4 feet in height and produce conical clusters of blooms that are generally less susceptible to mildew than their later blooming cousins. Cultivars include ‘Miss Lingard’ with snow white flowers, ‘Delta’ and ‘Omega’ with white flowers and contrasting darker eyes, and my personal favorite, ‘Natascha’ whose pips resemble pink and white pinwheels. Removal of declining flower heads usually promotes additional bloom as the summer progresses.

The tall garden phloxes (P. paniculata) are some of the showiest of all garden perennials with large flower heads in a multitude of colors on stems up to 4 feet in height. Perhaps the most popular introductions are Phlox ‘David’ with dazzling pure white flowers and ‘David’s Lavender’ both of which demonstrate excellent mildew resistance. P. ‘Franz Schubert’ produces appealing dense clusters of bluish-lavender blooms while ‘Laura’ is striking with violet colored pips highlighted by white eyes. Radiant reds include ‘Red Riding Hood’ and ‘Starfire’ and ‘Orange Perfection’ absolutely glows with salmon-orange flower heads. A newer addition to the color parade is Phlox ‘Blue Paradise’ with reddish-violet pips that become deep bluish-purple in cooler weather. Several cultivars display attractive variegated foliage adding a dramatic accent to the border throughout the summer; ‘Norah Leigh’ offers delightful green and white foliage with flowers that feature pale lilac pips accented by deeper pink eyes while ‘Becky Towe’ exhibits foliage with tints of gold, cream, green, and pink topped by stunning blossoms of rich, saturated salmon with deep rose eyes.

Healthy, hybrid garden phlox are among the most elegant additions to the garden but they can be demanding and often unsightly when suffering from infestations of spider mites or covered with powdery mildew. These hardy perennials derive from species often found growing in moist, riverside environments in the eastern US and consequently prefer to be grown in fertile, moisture-retentive soils in sun or light shade. Stressed plants are more susceptible to insect and disease problems. During periods of drought, spider mites tend to be problematic resulting in mottled yellow, curled leaves. A stiff spray of water early in the day will often dislodge these microscopic, destructive pests and an application of light, horticultural oil will help to reduce or eliminate their population.

Mildew, a powdery white coating that appears on leaf surfaces, is perhaps the most common plague and while it spoils the appearance of the foliage, it rarely leads to the demise of the plants. To minimize mildew on phlox, thin the stems of large clumps in the spring to improve air circulation and avoid overhead watering during the summer months, particularly in the evening. A weekly application of a baking soda spray (1 T/gal of water) will often minimize the appearance of this unsightly fungal disease or applications of fungicides can be effective if applied early and regularly at the very first sign of mildew.

Phlox are heavy feeders and a yearly application of fertilizer is recommended to promote stronger plants. Given the rich, moist soils they favor, they may spread prolifically forming expansive clumps in short order. Divide every third year in early spring, discarding the old, woody centers. Remove spent flower heads promptly to encourage side shoots, prolong bloom, and reduce the production of seed. Despite my best intentions to deadhead, innumerable phlox seedlings appear every season, many with unattractive muddy pink or magenta flowers but a few have been reasonably pretty and have been allowed to linger as long as they remain relatively mildew-free.

Nearly every year I threaten to eliminate all my phlox when their leaves are covered with mildew but when their lovely, fragrant flowers appear and butterflies and hummingbird moths hover at their blossoms, I usually relent, culling out only the worst offenders. The magnificent, colorful, long-lasting floral display of the tall garden phloxes usually outweighs their shortcomings and the extra effort required to grow these stunning additions to the late summer border.

Suzanne Mahler is an avid gardener, photographer and lecturer who has been developing the 1.5-acre property surrounding her home in Hanover for more than 30 years. Her weekly gardening column 'Green Thumbs Up' has appeared in Community Newspapers for more than a decade. She is a member of two local garden clubs, past President of the New England Daylily Society, an overseer for the Massachusetts Horticultural Society and is employed at two garden centers.