March 2003

Brunnera macrophylla ‘Jack Frost’ By Paul Pilon

Brunnera ‘Jack Frost’ is a new cultivar of Siberian bugloss discovered at Walters Gardens in Zeeland, Mich. This cultivar was the winner of the Plantarium “Best New Plant Award in 2001.” Its most distinguishing characteristic is its frosty silver leaves with green veins and edges. The foliage forms hosta-like mounds of heart-shaped leaves reaching 12-18 inches high x 18 inches wide. The foliage alone will sell this plant throughout the season. Plants in the genus brunnera are also commonly referred to as the Forget-me-not Anchusa, referring to the small, light-blue flowers that appear above the foliage in the spring.

Jack Frost is a sport of Brunnera macrophylla ‘Langtrees’. In the landscape, Jack Frost is low-maintenance, requiring partial shade, and it has medium water requirements. Brunneras are intolerant of drought conditions and will exhibit leaf scorch when grown under these conditions. In southern portions of the country, or areas with relatively high light levels, planting in areas with dense shade is necessary. This cultivar is commonly utilized as a specimen plant in the landscape. It is also well-suited for use in groups, borders, woodland gardens or as a groundcover. Jack Frost is suitable for production throughout most of the United States with its USDA hardiness from Zones 3-8.

Propagation

Jack Frost is a patented variety (PPAF), and asexual propagation is prohibited. Currently, this is a pricey cultivar, as supplies are limited. Recently, Jack Frost has also become available from tissue culture laboratories. Tissue culture has allowed larger quantities of this variety to reach the market in a relatively short period of time. This patented variety is available as a plug or potted plant from a limited number of licensed producers.

Production

When transplanting into the finishing container, plant even with the soil line of the plug or pot that it was previously produced in. It performs best in a media with both good water-holding characteristics and, more importantly, adequate aeration. A well-drained media is important to prevent the crown from staying wet too long, possibly leading to crown rot. To reduce the likelihood of crown rot and enhance initial rooting, I have found it beneficial to apply a fungicide drench after planting, using the combination of Subdue Maxx and Medallion.

Maintain the media throughout the production cycle with a pH between 5.8 and 6.2. Brunneras are light feeders, requiring nitrate levels of 50 parts per million (ppm) under a constant liquid fertilizer program or a controlled-release fertilizer at a rate equivalent to 3?4 pound of nitrogen incorporated per yard of growing medium. Jack Frost requires an average amount of irrigation, as it does not tolerate really wet conditions or overly dry conditions. Under high light intensities, leaf scorch is likely to occur. I would recommend producing brunneras under 35 percent shade cloth during the summer months. When transplanting from a 72-cell plug, Jack Frost grown at 65° F typically takes eight weeks to finish a 1-gallon-sized pot in the summer or 11 weeks during the winter months.

To produce blooming plants for early spring sales, many growers plant brunnera during the late summer and overwinter it in coldframe structures. Typically, during the late fall, the foliage is cut or mowed down to a couple of inches above the top of the container. The pots are grouped together within the coldframe, where they will remain until active growth resumes in early spring. In early winter, after the temperatures are consistently below freezing, it is recommended to cover plants with a frost blanket. During warm spells, the covers should be removed to let in fresh air and reduce the likelihood of unwanted diseases such as Botrytis. Be sure to provide extra ventilation during these warm spells by opening the doors to the coldframes. The covers should be replaced again when the temperatures drop below freezing.

When spring-like conditions arrive and Jack Frost begins to grow, space plants out in the coldframe. Any time the outdoor temperatures are above 40° F, keep the coldframe doors open to reduce the temperature inside the structure and to prevent excessive plant development and early flowering. In Zone 5, it is not uncommon to have flowering plants ready for sales by mid-April. Keep in mind that every spring and every location will produce flowering brunnera at different times, depending on the conditions.

Controlling plant height during production should not be a concern, provided they are growing with an adequate amount of space. I have not needed to apply chemical growth regulators to brunneras to reduce their height. If chemical height control is necessary, begin with two applications of B-Nine at 2,500 ppm or Sumagic at five ppm. Evaluate their effectiveness and apply as needed.

The primary insect pests of Jack Frost are aphids and slugs. Generally, I would control both of these pests as needed. Routine scouting should be sufficient to determine the presence of both aphids and slugs. Aphids can be detected on or near the fresh new growth, often located on the bottom side of the leaves. When found, they are frequently in surprisingly high Á quantities, especially if they have gone undetected in previous weeks.

There are a number of good insecticides on the market that are both safe and effective at controlling aphids on brunnera. Slugs are often hard to detect, but the plant injury they leave behind will frequently indicate their presence. Plant injury from slugs appears as irregular-shaped holes with smooth edges. Most often a slime trail is present on the surface of the leaves near the feeding injury. To control slugs, spread a commercial slug bait product such as Deadline or Sluggo around the production facility (For more information on slugs, see the January issue of GPN).

Availability

Jack Frost is available as a 3-inch pot from Walters Gardens Inc. This year, plants propagated via tissue culture are available through Terra Nova Nurseries, Tigard, Ore. Finished containers can be purchased from many reputable finished growers or garden centers throughout the country.