May 2008 Culture

Polemonium ‘Bressingham Purple’ By Paul Pilon

Polemonium 'Bressingham Purple' is a compact selection of Jacob's ladder with many appealing characteristics for commercial growers, landscapers and homeowners. The purple-tinted, deep-green fernlike leaves grow symmetrically and are highly decorative. In the spring and fall, the foliage turns vibrant purple. In the late spring, flower clusters of star-shaped rich-blue flowers with bright-yellow anthers are held above mounds of foliage on attractive burgundy stems. Adrian Bloom selected this plant in 1996 for its distinguished purple foliage.

When blooming, the attractive clumps remain compact, reaching only 12-15 inches tall. 'Bressingham Purple' performs well throughout USDA Hardiness Zones 4 to 9 and AHS Heat Zones 8 to 1. Polemonium generally prefer locations with light shade; full shade in the southern United States. Keep in mind that lower light levels will result in less intense leaf colorations. Jacob's ladder is commonly used for accent, border and mass plantings, and with its compact growing habit, 'Bressingham Purple' performs well in containers and patio pots.

Propagation

'Bressingham Purple' is a patented cultivar and is vegetatively propagated from basal cuttings by licensed propagators. Asexual propagation is prohibited without a license.

The rooting medium in the liner tray should be moistened prior to sticking. The base of the cuttings can be dipped into a rooting hormone, such as a solution of indolebutyric acid (IBA) at rates between 500 and 750 ppm prior to sticking. Polemonium can successfully root without rooting compounds but tends to do so slightly faster and more uniformly when these treatments are provided.

Place the cuttings under a low misting regime for about the first seven to 10 days of propagation. The misting can be greatly reduced during cloudy weather. When possible, it is usually best to propagate them under high humidity levels (90 percent relative humidity) with minimal misting. The misting can gradually be reduced as the cuttings form calluses and root primordia. Polemonium are usually rooted in 14 to 21 days with soil temperatures ranging from 68¡ to 74¡ F. For best results, the air temperature during rooting should be maintained above 60¡ F and below 80¡ F. It is beneficial to begin constant liquid feeding with 150-ppm nitrogen at each irrigation beginning 10 days from sticking. They will usually be fully rooted and ready to transplant within four to six weeks of sticking.

Production

'Bressingham Purple' is well suited for container production and commonly produced in quart to 1-gallon sized containers. Polemonium performs best in a well-drained peat- or bark-based medium with a slightly acidic pH (5.5-6.4). When planting, 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. Jacob's ladder prefers to be grown with an even supply of moisture, as it does not tolerate excessively dry or wet conditions. Overly dry growing conditions lead to leaf scorch, and too much moisture often results in crown and root rots. When irrigation is necessary, water them thoroughly and allow them to dry slightly before applying irrigation again.

They are light to moderate feeders requiring only modest amounts of fertility. When planting polemonium, I recommend incorporating a controlled-release fertilizer into the growing media at a rate equivalent to _ to 1 pound of elemental nitrogen per yard of growing medium. Another method to deliver fertility to this crop would be using a constant liquid fertilizer program, delivering 100-ppm nitrates to the crop at each watering.

With its compact habit, it is usually not necessary to control plant height when producing 'Bressingham Beauty'. The height can usually be controlled by providing adequate space between each plant, which reduces the competition between plants for light and prevents them from growing taller. Under certain circumstances, particularly when producing small containers at high plant densities, it may be necessary to tone them with plant growth regulators. In the Midwest, I recommend toning polemonium by applying paclobutrazol at 30 ppm or uniconazole at 5 ppm; one or two applications are often sufficient.

Insects and Diseases

Occasionally, aphids, leafhoppers, leafminers and slugs may appear, causing only minimal crop injury. Leafhoppers and leafminers are predominantly observed in outside production facilities. Aphids are by far the most troublesome insect pests of polemonium; it may be beneficial to implement preventative treatments to prevent the occurrence of aphids.

The primary diseases growers should watch for are Botrytis, crown/root rots caused by the pathogens Pythium and Thielaviopsis, and powdery mildew. Other diseases that may be seen on polemonium include Erwinia, leaf spots, rust and viruses (CMV and INSV). None of these diseases require preventive control strategies. Growers should have routine scouting programs to detect their presence early and determine whether control strategies are necessary.

Forcing

Polemonium 'Bressingham Pur-ple' is considered a cold-beneficial plant: Providing a cold treatment increases the number of flowers and enhances the uniformity of bloom. It is beneficial to bulk them in small containers for four to six weeks prior to providing the cold treatment. I recommend providing cold to large plugs or small containers for six to nine weeks at 35-40¡ F. Jacob's ladder is an obligate long-day plant. It is recommended to provide at least 14-hour photoperiods or night interruption lighting when the natural photoperiod is less than 14 hours, providing a minimum of 10 foot-candles of light at plant level. The time it takes to reach flowering depends on the growing temperature after the plants are placed under long-day conditions. Plants grown at 64¡ F will flower in about seven weeks, while plants grown at 68¡ F will flower in as little as five weeks.

Availability

Polemonium 'Bressingham Pur- ple' is widely produced and readily available to the industry as rooted liners and finished containers. Many plant brokers will be able to help you locate this variety in its various forms and sizes.