March 2004

Ajuga reptans ‘Black Scallop’ By Paul Pilon

Ajuga ‘Black Scallop’ is a stoloniferous perennial groundcover with deeply bronzed, beautifully scalloped leaves. Cultivars of ajuga belong to the mint family (Lamiaceae), which includes many commercially grown perennials and herbs including agastache, basil, lavender, mentha, monarda, nepeta, oregano, perovskia, physostegia, rosemary, salvia, stachys and thyme. Black Scallop performs well as a groundcover in mass plantings, perennial borders, pots and mixed containers.

Black Scallop is hardy in USDA Zones 3-11 and tolerates a large range of growing conditions, from full sun to full shade. The distinctive evergreen-bronzed leaves provide year-round interest on spreading clumps reaching 5 inches tall and up to 24 inches across. It bears showy, upright spikes of blue to pale violet flowers that reach 8-10 inches tall in late spring. The best leaf coloration is achieved when plants are grown under full sun. Ajugas produced under shady conditions will appear to have green and bronze leaves.

Propagation

Black Scallop is most commonly propagated by cuttings. As the ajuga plant grows, it forms stems called stolons, which grow horizontally at or just below the soil surface. These stolons, often referred to as plantlets, can be removed from the mother plant and used as a propagule for propagation. The stolons often have visible roots growing from them and can easily be rooted into a plug flat or larger container.

Stem cuttings with or without pre-formed roots are easiest to root in mid-summer after flowering is finished but can be propagated successfully anytime of the year. Ajugas should be placed under a light misting regime for the first week of propagation; then for the remainder of the rooting time, it is not necessary. I have found it beneficial to apply a fungicide drench such as Subdue Maxx combined with Medallion at the time the cuttings are stuck. The rooting process usually takes about four weeks to become fully rooted and ready for transplanting, provided the soil temperature is maintained at 69-73º F.

Division is another method growers use to propagate Black Á Scallop. The crowns are commonly lifted from a field, or plants grown in containers are divided, or cut into several plants, including the root system, and transplanted into pots for finishing.

Production

Black Scallop is often produced in quart or 1-gal. containers. The most common practice is to transplant a rooted liner, such as a 72-cell plug, into the finished container. To produce a fuller-appearing container and reduce the crop production time, I recommend planting more than one ajuga plug into a 6-inch or larger pot. For example, I have observed growers who planted two plugs into this size container to have ajugas reaching the finished size 3-4 weeks earlier than when only one plug per pot was used, allowing them to turn their production space quicker. After the plug has been transplanted, the growing medium of the pot should be even with the top of the plug. I recommend growers drench ajugas with a broad spectrum fungicide at the time of planting to reduce potential crop damage from plant pathogens such as Pythium or Rhizoctonia.

For plant establishment, it is recommended to maintain average temperatures of at least 65° F. Once they are established, grow ajugas at 55-70° F during the day and between 55 and 60° F at night. At these temperatures a quart-sized pot can be finished from a 128-size plug in 4-6 weeks. Planting vernalized ajuga plugs in the late winter for spring sales may not be desirable, as they will most likely produce flowers before bulking up to fill out the pot. For spring shipping of Black Scallop in bloom, I typically suggest planting two 72-cell plugs into a 1-gal. container, or one per quart pot, during the late summer. This allows plants to become established and fill out the pot prior to winter.

Black Scallop performs best when grown in a moist, well-drained medium with a slightly acidic pH of 6.0-6.5. It is a light feeder and performs best when either a constant liquid fertilization program is used, feeding at rates of 50 ppm nitrate, or a controlled-release fertilizer is incorporated at a rate equivalent to 1 lb. of nitrogen per yard of growing medium. Ajugas prefer to be kept moist, but not consistently wet. Never let ajugas dry out or significant crop losses will be likely.

Pests & diseases

Growers should be aware of the frequent occurrence of crown rots before putting ajugas into crop production. There are growers who seldom or even never experience crown rot diseases. Conversely, there are others who seem plagued with the occurrence of crown rots. Often times, growers can successfully produce ajugas from spring to fall, only to experience difficulty with crown rots towards the end of the overwintering period.

Crown rots become prevalent when environmental conditions are conducive to their development. Overly moist or wet conditions are most often associated with the onset of these diseases. With ajuga, overly dry conditions often cause injury to the crown, which creates an entry point for these opportunistic diseases to begin their often lethal infection. Managing environmental conditions such as irrigation practices, humidity levels and air movement around plants will greatly reduce the occurrence of these plant pathogens. I still find it beneficial to apply broad spectrum fungicides as sprenches or drenches to crops of ajuga at a minimum of monthly intervals.

Growers must also be aware of potential viruses, which may be observed when producing crops of ajuga. Currently, I am not aware of any observations of viruses on Black Scallop, but this may not always be the case. Ajugas are prone to several viruses including cucumber mosaic virus (CMV) and tobacco mosaic virus (TMV). Virus symptoms often appear as dark green and light green mosaic and mottling of the leaves, growth distortion and stunting, or as ring patterns or bumps on the plant foliage. There are no cures for plant viruses, so symptomatic plants should be discarded to prevent further spreading to healthy plants.

The occurrence of insects is not uncommon, but rarely becomes problematic. Whiteflies are the most troublesome insect pest of ajugas, but occasional outbreaks of aphids, slugs and snails may occur. With the likelihood of whiteflies being vectors, potentially moving viruses from plant to plant, I set very low thresholds on these insects and always implement preventative control programs.

Availability

Ajuga Black Scallop is fairly new to the industry and currently is available in limited quantities. To find a source of Black Scallop near you contact Proven Winners.