December 2013 Culture

Perennial Solutions: Astilbe Younique Series By Paul Pilon

Astilbe has long been a perennial favorite in the American landscape. In recent years, there have been improvements to this popular group of plants. Breeding efforts from the Dutch hybridizer Jan Verschoor have brought a breakthrough series of astilbe to the industry marketed as the Younique series.

The Younique series provides many desirable improvements such as an earlier bloom time; a more compact, rounded growing habit; more flower plumes per plant, which are held in attractive bunches; and increased disease resistance. This series currently consists of eight cultivars: ‘Younique Carmine’ (‘Verscarmine’ PP20324), ‘Younique Cerise’ (‘Verscerise’ PP20661), ‘Younique Lilac’ (‘Verslilac’ PP19847), ‘Younique Pink’ (‘Verspink’ PP 19841), ‘Younique Red’ (‘Versred’ PPAF), ‘Younique Salmon’ (‘Verssalmon’ PP19864), ‘Younique Silvery Pink’ (Verssilverypink’ PP20656), and ‘Younique White’ (‘Verswhite’ PP 19845).

The Younique cultivars develop attractive mounds of fern-like, dissected green foliage. In the early summer, they produce showy plumes of flowers atop numerous panicles neatly held in tidy, eye-catching bunches. These cultivars remain compact, reaching 16 to 20 inches when in bloom.

Astilbe are very popular perennials for locations with partial to full shade; however, they can tolerate a fair amount of direct sunlight. They perform well in locations with average to slightly moist, well-drained soils across USDA Hardiness Zones 4 to 9. Once established, false spirea can tolerate a slight amount of drought stress; however, the leaves will scorch under severe conditions.

The Younique series is easy to grow and well suited for mid-spring sales. They can be utilized and marketed as accent plants, small groupings, mass plantings or cut flowers. Additionally, astilbe are resistant to feeding by both deer and rabbits.

Propagation

The Younique series of astilbe is vegetatively propagated from divisions by licensed propagators. All of the cultivars in this series are patented and cannot be propagated without a license.

Production

The Younique series is available to growers as bare root plants or large plug liners and are most commonly produced in 2-quart to 1-gallon containers. Astilbe perform best in growing mixes with both good water holding ability and adequate drainage. Liners should be planted with the soil line of the plug being equal with the soil line of the container they are being planted into. With bare root starting materials, the woody crown should be placed in the center of the container with the crown only slightly covered or just exposed after they are watered in; planting them too deeply will lead to reduced vigor, crop variability and possibly losses.

It is best to keep astilbe evenly moist throughout production. They are particularly susceptible to water stress shortly after planting. In many instances, the initial flush of top growth develops more quickly than the root system. During the warmest times of the day, this leads to high transpiration rates; unfortunately, the plants are using more water than they can take up which results in scorched leaves (brown and crispy leaf margins). Throughout the entire crop, do not let astilbe become overly dry; overlooking the water status of astilbe at any time may lead to injured leaves which reduces their appearance and marketability. However, avoid keeping leaves consistently wet or crown and root rots may develop.

Maintain the media throughout the production cycle with a pH between 5.8 and 6.3. Astilbe require light to moderate amounts of nutrients. Leaf scorch is likely to occur when astilbe are exposed to high salt levels. Routinely monitor the fertility and leach the plants with clear water if the EC rises above 2.0 using the 2:1 extraction method. After transplanting bare root, it is usually not necessary to provide fertilization during the first few weeks of production.

Growers using water soluble fertilizers either feed with a constant liquid fertilization program using rates of 75- to 100-ppm nitrogen with every irrigation or apply 200-ppm nitrogen as needed. Controlled-release fertilizers can also be incorporated into the growing mix prior to planting using the medium recommended rate or the equivalent of providing 0.8 to 1.1 pounds of elemental nitrogen per yard of growing mix.

Height management strategies are often not necessary when growing the Younique series. In most instances, providing adequate spacing between the plants will sufficiently reduce undesirable stem elongation. There are numerous plant growth regulators which can be used effectively to control plant height. Applying spray applications of 2,500-ppm daminozide (B-Nine or Dazide), 5-ppm uniconazole (Concise or Sumagic) or tank mixing these products at 2,000-ppm daminozide plus 3-ppm uniconazole will effectively control elongation. Make one to two applications at seven to 10 day intervals just as the plant canopy is beginning to enclose.

Insects and Diseases

There are only a few problems associated with insect damage or plant pathogens growers may observe on occasion. The primary insect pests of astilbe are aphids, leafhoppers, spider mites and whiteflies. Crown rot caused by Rhizoctonia is the most prevalent plant pathogen; other diseases which may be observed at times include Cercospora leaf spot, Fusarium wilt, Pythium and Thielaviopsis. All of these pests and diseases can be detected with routine crop monitoring; control strategies may not be necessary unless the scouting activities indicate actions should be taken.

Temperature and Scheduling

The Younique series can be grown into flower by planting them in the late summer/early fall and overwintering the established plants or by planting bare root divisions in the spring. Although plugs are available, I frequently observe that spring-planted vernalized liners do not grow and flower as uniformly as bare root starting materials. Therefore, I prefer to use bare root divisions when spring planting astilbe.

Astilbe have an obligate cold requirement for flowering. They can be vernalized in the final container by providing a minimum of 10 weeks at temperatures of less than 40° F before forcing them into flower. Late winter and spring planted bare root do not require additional vernalization as they received the appropriate cold treatment in the field before the plants were harvested. Vernalized liners also do not need to receive additional vernalization.

Although astilbe are classified as long day beneficial plants, they can generally be grown under natural day lengths. Plants grown under long days will flower slightly quicker than those grown under shorter day lengths. Providing photoperiodic lighting is only necessary when flowering plants are desired for early spring sales; otherwise, grow them under natural day lengths.

The time to flower after vernalization is largely dependent on temperature. When spring planting bare root, it is best to grow them with cooler temperatures (55 to 60° F) for the first week or so of production to help them become established. Otherwise, they can be grown with warmer or cooler production temperatures. Flowering plants of the Younique series can be achieved in as little as seven weeks when they are grown with 24-hour average temperatures of 68° F or 10 weeks when they are grown at 55° F.

Availability

Bare root divisions of astilbe Younique series are currently available from Darwin Perennials (www.darwinperennials.com), Pioneer Gardens Inc. (www.pioneergardens.com) and Walters Gardens Inc. (www.waltersgardens.com). Liners are available from Creek Hill Nursery (www.creekhillnursery.com) and Pioneer Gardens.