Perennial Solutions: Penstemon Rock Candy Series By Paul Pilon

This exceptional series presents a brilliant display of color on compact plants, perfect for borders, mass plantings and containers.

Star Roses and Plants recently introduced a new line of penstemon marketed as Rock Candy series, which is comprised of five cultivars with bold flower colorations: Blue, Light Pink, Pink, Purple and Ruby. Besides their vibrant colors, the cultivars in this series were selected for their strong basal branching and compact growing habits.

‘Light Pink Rock Candy’

The Rock Candy series of penstemon is hardy throughout USDA Hardiness Zones 5 to 9. The newer introductions (Blue, Light Pink and Ruby) are listed as being hardy in Zones 6 to 9, but I’ve seen these cultivars over winter successfully in Zone 5. Each cultivar has clean, attractive medium-green foliage and remains compact, staying less than 12 inches in height (often between 8 and 10 inches). They produce a brilliant display of color that nearly covers the entire plant in the late spring and into the early summer. Additionally, they rebloom well after the original flush of flowers are removed. Penstemon prefer full sun in northern locations and partial shade in the south.

The Rock Candy series works exceptionally well when used in borders, along walkways, in mass plantings or in containers. They are also great plants for attracting hummingbirds into the landscape. This series is easy to grow, has an extended bloom time and flowers the first year without vernalization; these attributes make Rock Candy Penstemon a great option for spring and early summer programs.


Penstemon Rock Candy are vegetative propagated by tip cuttings. The series is patented; therefore, self propagation of any of the Rock Candy cultivars without a license is prohibited. However, unrooted cuttings are available to purchase. Propagators can easily propagate them when following a few guidelines.

The cuttings should be stuck into a growing media that has been moistened prior to sticking. It is not necessary to use rooting hormones as they will root readily without these compounds. Place the cuttings under a low to moderate misting regimen for the first four to six days of propagation. When possible, it is usually best to propagate under high humidity levels (90 percent relative humidity) with minimal misting.

Maintain soil temperatures of 70 to 74o F during the rooting process. Fertilizers can be applied continuously with the mist water at 50- to 60-ppm nitrogen or every five to seven days applying 100- to 120-ppm nitrogen with each irrigation. Decrease the amount of mist being delivered as the roots begin to develop; misting can generally be removed altogether at nine to 12 days after sticking. Liners take approximately five to six weeks from sticking to become fully rooted and ready for transplanting.

'Ruby Rock Candy'
‘Ruby Rock Candy’


With their compact size, the Rock Candy cultivars are best suited for production in 1-quart to 1-gallon containers. They can be potted and grown as a traditional perennial crop in the late summer the year prior to the intended market date or planted during the same growing season they are going to be sold. Penstemon prefer to be grown in a well-drained growing mix. When transplanting, plant a single liner in the center of the container planted with the original soil line of the plug being even with the surface of the growing medium of the new container.

Penstemon require moderate amounts of nutrients. Fertility can be provided using water-soluble fertilizers by delivering 125- to 150-ppm nitrogen plus micronutrients with each irrigation or 250-ppm nitrogen as needed. Nutrition can also be delivered using controlled-release fertilizers by incorporating the equivalent of 0.9 to 1.1 pounds of elemental nitrogen per cubic yard of growing mix before planting or topdressing using the medium labeled rate. Maintain the growing mix throughout the production cycle with a pH between 5.5 and 6.5.

They require an average amount of irrigation. When irrigation is necessary, water them thoroughly and allow the soil to dry slightly between waterings. To encourage good branching, it is recommended to pinch them approximately two weeks after planting once the roots have reached the edge of the pot. They can also be cycle trimmed as needed to stagger the flowering times or to remove old blooms and promote a flush of new flowers.

They will rebloom in about four to five weeks after they have been trimmed.

With their compact growth habits, it is usually not necessary to manage plant height when growing the Rock Candy series of penstemon. If it becomes necessary to tone the plants during production, one or two spray applications of 2,500-ppm daminozide (B-Nine or Dazide), 5-ppm uniconazole (Concise or Sumagic) or the combination of the two PGRs (2,000-ppm daminozide plus 3-ppm uniconazole) can be applied.


There are only a few insect pests and diseases that are likely to be observed on penstemon. Aphids, thrips and powdery mildew are the most common problems growers are likely to face on occasion. Penstemon are also susceptible to caterpillars, impatiens necrotic leaf spot virus (INSV), root rots, rust and spider mites. The occurrence of these problems can be detected with routine crop monitoring and control strategies implemented if and when necessary.


The Rock Candy series can easily be scheduled and grown to produce flowering plants throughout most of the growing season. As mentioned above, the Rock Candy series can be grown as a traditional perennial and be planted in the late summer or early fall the year before they are to be sold. This approach is advantageous when the plants are needed for early sales. Allow six to ten weeks in the fall for bulking prior to overwintering them. The Rock Candy series of penstemon are first- year-flowering perennials and do not require cold for flowering; therefore, they can also be planted during the same growing season. When spring planting, growers can use fresh or vernalized liners.

They are day-neutral plants, which will flower under any photoperiod and can be forced into bloom under natural day lengths. The length of the photoperiod does not have any effect on the time to flower or the number of blooms produced. The best quality plants with the most flowers is obtained when they are grown under high light intensities (avoid growing them under hanging baskets). It takes approximately ten weeks to flower when they are grown with 24-hour average temperatures between 63 and 68° F.

Paul Pilon

Paul Pilon is a horticultural consultant, owner of Perennial Solutions Consulting (, and author of “Perennial Solutions: A Grower’s Guide to Perennial Production.” He can be reached at 616.366.8588 or

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