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Tall garden phlox for Minnesota gardens

Deborah L. Brown, Extension Horticulturist Department of Horticulture

Count on tall garden phlox (Phlox paniculata hybrids) to provide a colorful summer display in perennial gardens, blooming as much as six weeks or more. Some cultivars begin blooming in mid-summer; others not until late August. Most tall garden phlox grow two to three feet, with some slightly taller. These perennial flowers are often used as background plants in narrow borders or in groups between taller and shorter plants in a wide border. Placing them next to fences or walls where air circulation is poor, however, is an invitation to disease problems.

Garden phlox are available in a wide range of pink, rose, red, lavender, purple, and white, along with bi-colors that have an "eye" in the center of each flower or a contrasting margin. Cultivars range from softest pastels to electric, "knock-your-socks-off" brilliant blooms. In addition to their visual appeal, many tall garden phlox are sweetly perfumed.

Planting and Transplanting

Tall garden phlox are typically purchased as potted plants from nurseries, garden centers, or local farmers' markets. These potted plants may be transplanted into the garden throughout the growing season, right up to early autumn. Set the crown of each plant about an inch and a half below the soil surface.

You can also buy bare root plants from mail-order nurseries, preferably in spring. Fall-planted specimens are more subject to frost heaving, and it's not unusual to receive mail-order plants too late in fall to be planted safely. Whether bare root or potted, it's important to mulch fall-planted phlox with four to six inches of marsh hay, chipped leaves, or pine needles as soon as the soil freezes. After the first year fall mulching is less critical, though it's always a good idea. Mulch not only protects plants from winter's cold, it helps prevent premature soil warming that could result in early spring growth, before weather is reliably mild.

Most phlox need to be divided every two to four years if they're growing well. The best time to do this is in spring, before new growth begins. Each division should consist of three to five vigorous shoots with roots attached. Position them as you would new plants, with the crowns an inch or so below the soil surface. Space them eighteen inches apart to ensure adequate air circulation.

Site Selection

Tall garden phlox grow best in full sunlight, but will tolerate light shade. Shading decreases both bloom quantity and quality, while it increases disease problems. If shade is produced by trees, their roots will also compete with the phlox for moisture and nutrients. Try to steer clear of large shrubs and hedges for the same reason--competition is just too fierce.

Because garden phlox thrive in soil that drains well, yet retains adequate moisture, it's useful to incorporate generous amounts of organic matter into any soil before planting. And it's an absolute necessity if the site has sandy soil. Composted yard waste, peatmoss, or wellrotted manure all work well to improve soil structure.

Have a soil test run to determine the type and rate of fertilizer you might need to add for best growth. In the absence of a legitimate soil test, work some starter fertilizer into the soil at planting time, and follow-up with a light application of 10-10-10 or organic fertilizer as new growth emerges each spring, and then again just before plants begin to flower.

Routine Care

Tall garden phlox grow best when kept moist, so make a point of watering them thoroughly on a regular basis. If at all possible, water the soil rather than phlox foliage, to aid in disease prevention. If you must water overhead, water early in the morning so plants will dry rapidly in the sun. Spread two to three inches of mulch after the soil warms in mid-June or a little later, to help cut evaporative moisture loss and keep the soil more cool and damp. It will also aid in reducing weed growth.

Once stems are about six inches tall, eliminate all but five or six stems per plant, then pinch back the growing tips of those remaining stems. You'll find they become more robust, produce larger clusters of flowers, and are less bothered by powdery mildew because of improved air movement.

Phlox seedlings usually revert to pale magenta. To keep your plants the color you chose initially, it's important to "deadhead" or remove clusters of faded flowers so mature, viable seeds are not dropped to the ground. Some newer phlox cultivars are sterile, but even then, deadheading is a good idea because it results in more attractive plants. And sometimes if you fertilize again, phlox may put out an unexpected second smaller flush of blooms.

Problems

Powdery mildew can be a destructive disease on tall garden phlox. It appears as disfiguring, powdery white spots on the foliage. When these spots merge, they can nearly obliterate any remaining green tissue, moving onto the flowers as well. Maintaining good air circulation will help reduce powdery mildew problems, but the best way to avoid the disease is to choose diseaseresistant cultivars in the first place, then make sure they're neither crowded nor shaded when you plant them. If necessary, you could begin a fungicide spray program using sulphur or chlorothalonil. (sold as Daconil 2787) at the earliest signs of infection.

Phlox may also develop other fungal leaf spots, particularly on lower leaves. However, they are rarely serious enough to warrant treatment.

Spider mites are the most troublesome "insect" pests of phlox. (Mites are not true insects, but are arachnids, similar to spiders.) They insert their sucking mouthparts to remove liquids from phlox leaves, causing pinprick yellow discoloration of the foliage, eventually turning leaves dry and brown. Spider mites are generally worse in hot, dry summers than when it's cooler or more moist. Vigorously growing plants are better able to tolerate mite feeding, so make sure phlox are watered and fertilized adequately to insure their vigor.

An insecticidal soap spray can be moderately effective, particularly if you catch the mite infestation in its early stages. If the mite problem is severe, check your local garden center for Kelthane, a miticide labeled specifically for use on ornamentals or flowering perennials. Apply any spray very early in the day, when temperatures will remain cool for several hours. Spraying during the heat of the day is more likely to damage foliage.

Some Phlox Cultivars Recommended for Minnesota Gardens:

Dozens of different phlox cultivars will grow in Minnesota, but many are quite disease-prone. In their book Growing Perennials in Cold Climates (Contemporary Books, 1998), authors Mike Heger and John Whitman rated the following phlox as "five star" perennials-the best available on the market today. Here, with their permission, is a list of those plants. All are hardy to 40 degrees below zero.

Cultivar Color Height
'Bright Eyes' Pink with red eye 36 inches
'David' White 36 inches
'Eva Cullum' Pink with red eye 30 inches
'Franz Schubert' Lilac 30 inches
'Russian Violet' Violet purple 30 inches
'Sir John Falstaff' Salmon pink 30 inches
'Starfire' Cherry red 30 inches
'World Peace' White 42 inches

Produced by Communication and Educational Technology Services, University of Minnesota Extension.

 

 

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