May 2007 Culture

Perennial Solutions: Euphorbia ‘Blackbird’ By Paul Pilon

There are numerous perennials with huge potential for both commercial growers and landscapers alike that are currently underutilized by the industry. These plants are often easy to produce with relatively few cultural problems and look good in both containers and landscape settings. With its compact, bushy habit and characteristic dark-purple foliage, euphorbia 'Blackbird' ('Nothowlee') is a perennial I consider to have untapped potential in the market.

The most distinguishing characteristic of 'Blackbird' is its dark-purple evergreen foliage, which takes on an even darker coloration when grown in full sun. It remains compact and bushy, forming 18- to 22-inch mounds at maturity. The yellow/green bracts (flowers) provide a nice contrast against the dark foliage in mid spring.

'Blackbird' is a sport of the popular variety euphorbia 'Redwing' selected in the United Kingdom at Notcutts Nursery. It thrives in locations with full sun to partial shade throughout USDA Hardiness Zones 6-9. It is worth noting that the characteristic dark-purple foliage will take on a lighter coloration when plants are produced at lower light levels. Once established, it is heat and drought resistant with few cultural problems.

Euphorbia is commonly used as specimen or border plants or in container plantings, mass plantings and rock gardens. Although it has evergreen foliage, its leaves will decline in cold winter climates.

'Blackbird', like all spurges, produces a white, milky latex sap that can cause irritation to the skin and eyes. Don't let this discourage you from producing this remarkable plant; simply take care when trimming it back.

Propagation

'Blackbird' is vegetatively propagated by tip cuttings. Since a plant patent is being sought, unlicensed propagation of this cultivar is prohibited. Tip cuttings should measure approximately 11?2 inches and contain several nodes. The well-drained rooting media should be moistened prior to sticking. Euphorbia will root well without rooting hormones.

Place the cuttings under low misting regimes for the first 10 days of propagation. When possible, it is usually best to propagate under high humidity levels (90-percent relative humidity) with minimal misting; placing them under plastic tents is highly effective. The misting can gradually be reduced as the cuttings form callus and root primordia. Prolonged exposure to mist may cause the leaves to rot during propagation. They will usually form roots within three weeks of sticking with soil temperatures ranging from 68 to 73¡ F. A liner takes approximately 4-6 weeks to become fully rooted and ready for transplanting.

Production

For container production, euphorbia 'Blackbird' is suitable for 1-gal. containers. For the best performance, plant in a well-drained media, preferably a nursery-type mix (bark based) rather than traditional greenhouse (peat-vermiculite) media. When planting, plugs should be planted so the original soil line of the plug is even with the surface of the growing medium of the new container. Many growers find it useful to pinch them back ç 3-4 weeks after transplanting to promote basal branching.

They are light to moderate feeders. Fertility can be delivered using water-soluble or controlled-release fertilizers. Growers using water-soluble fertilizers either apply 200- to 250-ppm nitrogen as needed or feed with a constant liquid fertilization program using rates of 100- to 150-ppm nitrogen with every irrigation. Growers commonly apply time-release fertilizers as a top dress onto the media surface using the medium rate or incorporate into the growing medium prior to planting at a rate equivalent to 1 lb. of nitrogen per yard of growing medium. Maintain media pH between 6.0 and 6.5.

'Blackbird' requires a below average amount of irrigation. Although plants can tolerate average watering regimes, they generally perform best under slightly dry conditions. When irrigation is required, water thoroughly and allow the substrate to dry between waterings. Keeping them on the dry side will also intensify leaf and stem color.

With its naturally compact growing habit, controlling plant height should not be necessary. However, during periods of low light levels or when grown at high plant densities, excessive internode elongation might occur, requiring some type of height management strategy. Euphorbia height can often be effectively controlled by providing adequate spacing between the plants or by withholding irrigation and nutrients. Several of the commercially available PGRs are effective at controlling plant height when they are applied using the appropriate rates, frequency and timing. Depending on your geographic location, apply foliar applications of Piccolo or Bonzi (paclobut-razol) at 30 ppm, Sumagic (uniconazole) at 5 ppm or Topflor (flurprimidol) at 45 ppm (northern rates specified). It may require multiple applications at 7-day intervals to provide adequate height control.

Insects And Diseases

Euphorbia is susceptible to several insect pests including aphids, caterpillars, mealybugs, spider mites and whiteflies, but rarely do any of these pests become problematic. Although diseases such as anthracnose, Botrytis, leaf spots, Phytopthora, powdery mildew, Rhizoctonia and rust may be observed under certain growing conditions, euphorbia can generally be grown free of these plant pathogens.

None of these insect pests or diseases requires preventative control strategies. Growers should utilize routine scouting programs to detect their presence early and determine if and when control strategies are necessary.

Forcing

With its attractive foliage, euphorbia 'Blackbird' can be marketed as a foliage plant or in full flower. Non-flowering plants can be produced throughout the growing season from spring to fall by planting 72-cell or larger plug materials and allowing 8-10 weeks at 60¡ F to fill out the container. Naturally flowering plants are usually available in the mid spring.

To achieve flowering, euphorbia must be vernalized prior to forcing. If flowering plants are desired, the most common practice is to plant them during the late summer of the year prior to the anticipated spring sales for bulking and vernalizing them over the winter months. They are day-neutral plants, and the time to flower following the cold treatment is a function of temperature. At 55-60¡ F, flowering will occur in approximately 6-7 weeks.

Availability

Euphorbia 'Blackbird' was brought to the market by Plant Haven, Inc. (www.planthaven.com). Rooted liners are available from several licensed propagators.