Astoria Phlox

Chris Schlegel and Jason Ginn

Although a half-hardy perennial by nature, Astoria phlox is grown as an annual crop. It offers an excellent selection of colors for the spring market, blooming early and with continuous color through the summer. Their five-petal, star-like flowers come in an assortment of colors, each complementing the other. With a finished height of 14 to 24 in. and a spread of 30 in., their mounding habit is ideal for both landscaping and containers. These phlox are extremely free flowering, as well as self-cleaning, and thrive in full sun to partial shade in the landscape.

Astoria phlox can be challenging to grow. The following cultural tips provide guidelines for growing a successful crop.

Transplanting and media selection. Rooted cuttings should be potted up as soon as possible to avoid having them become rootbound in the liner trays. They perform best in a well-drained, peat-perlite mix with a pH between 5.5 and 6.2. A media EC of 1.5 to 2.0 is ideal.

Temperature and light. Astoria phlox benefit from being grown in high light levels, both in propagation and finishing. Improved light levels will help in branching and ultimately in flower power. They are not a crop that should be grown under hanging baskets or in shady areas of the greenhouse. The crop is best grown at 59F to 65F (15C to 18C).

Water Management. Watering is a key issue in growing a successful crop. High levels of water may cause root death, so a “little and often” regime is encouraged when plants are very young to give as much air to the crop as possible. Also included with this strict water management plan should be a preventative fungicide program. Treating the liners prior to potting can be extremely effective.

Fertilizer. Astoria phlox is a fairly heavy feeder, so it should be watered with a balanced liquid fertilizer throughout its life. During the propagation phase, fertilizer is applied earlier on phlox than on many crops. Continuous feed levels of 200 to 300 ppm nitrogen are recommended. The use of slow-release fertilizer in combination with liquid fertilizer is another option. Slow-release fertilizer with a longevity of three to four months may be incorporated in the media at a rate of 0.1 lb per cu. ft. (1.5 kg. per cu. m.).

Pinching. This crop may require pinching to encourage the right habit and to force breaks to get a bushy and spreading habit. Normally the young plants are pinched in the liner tray prior to potting. A second pinch after they are established will improve their finished habit.

Pest and Diseases. The most common insect pests on phlox include thrips, whitefly and aphids. They may be controlled through a regular rotation of appropriate pesticides. Fungus gnats and shore flies may also affect this crop, however, proper water management will decrease the likelihood that these pests will pose a problem.

Root diseases are perhaps the main concern in growing this crop. Phlox may be affected by Phytophthora, Thielaviopsis, and Pythium. A preventative fungicide drench applied seven to 10 days after potting will greatly reduce the risk of root problems. Also, the importance of water management cannot be stressed enough when discussing root disease prevention.

Botrytis, Rhizoctonia and powdery mildew are also known to affect phlox. Providing proper light levels and allowing for good air circulation are excellent cultural practices to avoid these problems.
 
Crop times.
    4-in. pot—1 liner; 4-6 weeks with 1 or 2 pinches
    6-in. pot—2-3 liners; 7-9 weeks with 2 or 3 pinches
    10-in. basket—3-5 liners; 12-16 weeks with 2 or 3 pinches  

Chris Schlegel is head grower and Jason Ginn is a grower for D.S. Cole Growers in Loudon, New Hampshire.