Perennial Solutions: Brunnera macrophylla ‘Jack Frost’
Brunnera ‘Jack Frost’ was first introduced by Walters Gardens, Inc. in 2000 and over the past 11 years this hardy perennial has become a reliable and popular perennial for shade gardens across the country. With its ease of production, desirable attributes, and garden performance brunnera ‘Jack Frost’ was named the 2012 Perennial Plant of the Year by the Perennial Plant Association (www.perennial
‘Jack Frost’ was discovered in the greenhouses at Walters Gardens growing as a sport of brunnera ‘Langtrees’ and has become one of Walters Gardens’ best introductions in their 65-year history. The most distinguishing characteristics are its heart-shaped frosty silver leaves. The leaves are intricately detailed with narrow green veins and edges providing a crisp, crackle-like finish. ‘Jack Frost’ forms attractive hosta-like mounds of heart shaped leaves reaching 12 to 15 inches high by 15 inches wide. In the mid-spring, clusters of small, baby blue ‘forget-me-not’ flowers appear above the shimmering foliage.
‘Jack Frost’ is hardy in USDA Hardiness Zones 3 to 8. In the landscape, brunnera require little maintenance and perform best in locations with partial shade and consistently moist, but not wet soil. Brunnera are intolerant of drought conditions and will exhibit leaf scorch when they are grown too dry. In Southern locations, it is best to place them in areas with dense shade and moist soils. Another notable attribute for brunnera is the fuzzy, rigid texture of the leaves are resistant to deer feeding.
‘Jack Frost’ is commonly utilized as a specimen plant or in small groups in the landscape. It is also well suited for use in border, woodland, and container plantings. This cultivar offers outstanding foliage color both at retail and in shade gardens. The foliage alone will sell this plant throughout the season.
Brunnera ‘Jack Frost’ is currently propagated via tissue culture and is a patented cultivar (PP13859). Therefore, propagation without permission of the applicant is illegal.
‘Jack Frost’ is commonly grown in 1-quart to 1-gallon sized containers. Growers should use a growing mix that provides both good water holding ability and adequate drainage. Liners should be transplanted so the soil line of the plug is even with the soil line of the final container. Planting them too deeply will reduce the growth rate of the plant and may lead to crown rot. To reduce the likelihood of crown rot and enhance initial rooting, it is often beneficial to apply a broad-spectrum fungicide drench after planting.
Siberian bugloss is a light feeder and performs best when the pH is kept at 5.8 to 6.3. Growers using water-soluble fertilizers commonly apply 50- to 75-ppm nitrogen with every irrigation or use 125 to 150 ppm as needed.
Controlled-release fertilizers are commonly incorporated into the growing medium prior to planting at a rate equivalent to 0.75 pound of elemental nitrogen per yard of growing medium.
‘Jack Frost’ requires an average amount of irrigation and should be kept consistently moist, but not wet during production. Brunnera that are grown too dry will develop leaf scorch on the margins of the leaves. Conversely, plants that are kept overly wet will often develop crown rot. When irrigation is required, water them thoroughly and allow the growing mix to dry slightly between waterings.
Under high light intensities leaf scorch is likely to occur. Although brunnera can tolerate short durations of direct light (provided they are kept sufficiently moist), it is best to grow them under 35 to 55 percent shade cloth during production (between mid-April and early September).
With its compact habit, it is usually not necessary to control plant height during production. If toning is required, spray applications of 2,500 ppm daminozide (B-Nine or Dazide) or 5-ppm uniconazole (Concise or Sumagic) are effective. One to two applications should provide adequate height control.
Insects and Diseases
Although brunnera can be produced relatively insect free, aphids and slugs often become problematic. There are several plant pathogens including Botrytis, Fusarium, Phytopthora and Thielaviopsis that may occasionally attack Siberian bugloss. The occurrence of these diseases can be reduced by planting them at the proper depth into a well drained growing mix, maintaining good irrigation and fertility practices (EC and pH), and avoiding plant stresses.
With routine scouting, insects and diseases can be detected early; control strategies may not be necessary unless the scouting activities indicate actions should be taken.
Brunnera ‘Jack Frost’ can be produced and marketed as a variegated foliage item throughout the entire growing season. When transplanted from 72-cell liners, ‘Jack Frost’ typically takes seven to nine weeks to finish 1-gallon sized containers when they are grown at 65° F. Brunnera can be forced into bloom by following the guidelines discussed below.
‘Jack Frost’ has an obligate cold requirement for uniform flowering. It can be vernalized as a large plug (20-cell or larger) or in the final container. For early spring sales, I recommend planting brunnera in the late summer or early fall, bulking them up, and vernalizing in the final container. This will result in fuller, more colorful plants when they bloom. A good guideline is to plant them six to eight weeks before the first expected frost. Provide a minimum of nine weeks for vernalization at temperatures less than 40° F before forcing them into flower.
Planting vernalized brunnera liners in the spring will still result in marketable product; however, when small liners are used, they will likely reach peak flowering before the plants have filled out the container. When transplanting in the spring, use large plugs (such as 20-cell liners) to reduce the amount of time required for bulking.
After the cold requirement is achieved, they can be grown at any day length; however, they grow best under long days. ‘Jack Frost’ that has been bulked up in the fall takes approximately four weeks to bloom when they are grown at 65° F. As mentioned above, it may take longer (six to eight weeks) to produce a marketable plant when small plugs are transplanted in the spring. Growing them with cooler temperatures will delay flowering and produce the highest quality finished plants.
Brunnera ‘Jack Frost’ was introduced by Walters Gardens, Inc. (www.waltersgardens.com). Liners are available from Walters Gardens and several reputable perennial propagators and plant brokers.