August 2011 Culture

Perennial Solutions: Phlox paniculata ‘Shockwave’ By Paul Pilon

With its variegated foliage and strong impulse appeal, ‘Shockwave’ is changing the norm for how phlox is used.

Traditionally garden phlox was used as the backbone of the summer landscape, but Phlox paniculata 'Shockwave' is changing the norm for how phlox can be used in gardens and on patios. What sets 'Shockwave' apart from other P. paniculata cultivars? … Its unexpected and stable variegated foliage.

'Shockwave' has deep-green leaves with prominent yellow margins that lighten to a creamy yellow coloration as the season progresses. The variegated foliage alone gives this cultivar strong impulse appeal. As an added bonus, the plant maintains an attractive, compact habit, reaching only 12 to 18 inches tall in the landscape. And if the impressive foliage isn't enough, it produces large clusters of fragrant, lavender-pink flowers with white starburst centers in the late summer.

This cultivar performs well across a wide portion of the United States, throughout USDA Hardiness Zones 4 to 8. Garden phlox prefers full sun, although in the South it performs best when some partial shade is provided. Phlox paniculata is commonly used as an aromatic border plant, for accent plantings, and as cut flowers. It also attracts hummingbirds and butterflies into the garden. With its compact size and attractive foliage, 'Shockwave' is especially attractive in combination planters and patio pots.

'Shockwave' is the only truly stable variegated phlox on the market. Not only does it keep its colors reliably, the foliage also boasts terrific resistance to powdery mildew. The variegated foliage alone gives this plant strong impulse appeal, offers great garden presence and provides a fabulous partner in combinations planters.

Propagation

Phlox paniculata 'Shockwave' is vegetatively propagated. A U.S. Plant Patent has been applied for (PPAF); propagation without permission of the applicant is prohibited at this time.

Production

'Shockwave' can be grown in quart or 1-gallon containers. It performs best in well-drained growing mixes. When transplanting, the liners should be planted so the original soil line of the plug is even with the surface of the growing medium of the new container. To improve fullness and plant quality, it is beneficial to soft pinch the plants when they are 3 to 4 inches tall. Phlox paniculata can be potted in the spring or during the late summer of the year prior to the intended market date. Planting phlox in the late summer allows them to bulk up, increases plant vigor, produce more flowers per plant, and result in earlier and more uniform flowering.

Garden phlox perform best at light to moderate fertility levels. Growers using water-soluble fertilizers either apply 200-ppm nitrogen as needed or feed with a constant liquid fertilization program using rates of 75- to 100-ppm nitrogen with every irrigation. Controlled-release fertilizers can be applied as a top-dress onto the media surface using the medium labeled rate, or incorporated into the growing mix prior to planting at a rate equivalent to 1.0 pounds of elemental nitrogen per yard of growing medium. During production, phlox should be grown with a slightly acidic pH: 6.0 to 6.5.

Grow phlox under 'average' irrigation regimes. It is best to keep them uniformly moist, but not consistently wet. When irrigation is needed, water thoroughly and allow the medium to dry slightly between waterings.

With its compact growth habit, it is usually not necessary to implement height management strategies. If some growth control is necessary, the plants can be toned using spray applications of 2,500-ppm daminozide (B-Nine or Dazide) or the tank combination of 2,000 ppm daminozide plus 3-ppm uniconazole (Concise or Sumagic).

Insects and Diseases

There are several pests and diseases that attack garden phlox on occasion. Aphids, spider mites, thrips and whiteflies are the most prevalent pests; however, caterpillars, grasshoppers, leafhoppers, slugs and spittlebugs are also commonly observed. The primary diseases that commonly infect phlox are Alternaria, Botrytis, Cercospora leaf spot, Fusarium, Pythium, Phytopthora, powdery mildew and Rhizoctonia. Growers can detect the presence of insects and diseases using routine scouting programs and determine if and when control strategies are necessary.

Forcing

Phlox paniculata 'Shockwave' is commonly grown for its attractive foliage and in many cases, can easily be marketed without flowers. Growers producing it in small container sizes or in combination planters often sell plants that are not in bloom. However, if flowering is desirable, 'Shockwave' can easily be forced into bloom.

When growing them in large container sizes, it is best to bulk up the plants prior to forcing. Bulking can occur by planting them in the late summer the year before they are to be sold or by providing a bulking period in the spring. For spring bulking, grow the plants at 12- to 13-hour photoperiods with temperatures of 64 to 70¡ F to keep them vegetative and actively growing. The length of the bulking period depends on the size of the pot; in general, allow four to six weeks for bulking. As mentioned above, the plants can be pinched when they reach 3 to 4 inches tall to promote branching.

Although flowering does occur without vernalization, 'Shockwave' is a cold beneficial perennial and will flower more rapidly and uniformly following a cold treatment. Phlox can be vernalized in the final container or as plug liners prior to transplanting. I recommend growers provide at least six to nine weeks of cold temperatures at 35 to 44¡ F during vernalization.

Garden phlox are obligate long day plants and will not flower when the natural day length is less than 14 hours. Photoperiodic lighting (day extension or night interruption) will effectively promote flowering when the days are naturally short. The highest quality plants are produced in high light environments (minimum 3,000 foot-candles). Under low light intensities, the size and quantity of flowers per plant are reduced and the stems are often weak.

The time to bloom after vernalization and the proper photoperiod provided is a function of temperature. 'Shockwave' takes approximately 12 weeks to flower when it is grown at 68¡ F. Non-flowering containers can be grown in less time; allow five to seven weeks for quarts or six to eight weeks for 1-gallon containers.

Availability

Phlox paniculata 'Shockwave' are marketed by Proven Winners (www.provenwinners.com). Liners and bareroot are available from Walters Gardens, Inc. (www.waltersgardens.com). Liners may also be available through several Proven Winner propagators (www.pwcertified.com/grower/purchase/propagators.cfm).