Plant Evaluation Notes an Evaluation Study of Hardy Amsonia Richard G

Issue 18, 2002 Plant Evaluation Notes An Evaluation Study of Hardy Amsonia Richard G. Hawke, Manager of Plant Evaluation Programs orticulturists and landscape with woody crowns that slowly increase flower production and more compact designers often talk about the in size but are not invasive. Sturdy, habits. Plants that become loose or open, Hmerits of shrubs and trees that upright stems form rounded to especially when grown in too much provide multiple seasons of interest in the vase-shaped shrub-like habits. The dark shade, can be cut back to 10 inches after landscape, but it is uncommon to have green leaves, which are linear-lanceolate flowering to encourage shorter, sturdier that description attributed to herbaceous to ovate in outline, are arranged stems. The slow-growing bluestars do not perennial plants. One of these alternately to whorled on stems that require division for at least 10 years. exceptional perennials is Amsonia,or contain a milky latex sap. Blue, Bluestars can be planted as bluestar, with early-season blue flowers, star-shaped flowers are produced in loose specimens or en masse in perennial handsome summer foliage, a sturdy habit terminal cymes or clusters in late spring to borders, mixed shrub borders or and golden-yellow autumn color. The early summer. The fruits are paired naturalistic gardens. Amsonia addition of low maintenance and disease cylindrical pods borne either upright or tabernaemontana and Amsonia illustris and insect resistance make bluestars pendant within the leaves on the upper are well placed near ponds, streams and first-rate garden plants. ends of the stems. Some degree of yellow waterfalls, while Amsonia ciliata and Amsonia, a temperate member of the autumn foliage color is typical of Amsonia hubrichtii are good accents in predominantly tropical dogbane family bluestars in the late season. drier sites at the tops of stone walls or on (Apocynaceae), is related to common Bluestars are found growing gentle slopes. The blue flowers periwinkle (Vinca minor) and frangipani naturally in a variety of landscapes, complement most colors in the early (Plumeria rubra). There are about 20 including limestone ledges (Amsonia summer garden; the shrubby habits species of Amsonia native to North ciliata), damp grassy slopes and provide structure throughout the season; America, Europe and Asia, although only riverbanks (Amsonia tabernaemontana) and the golden-yellow leaves shine in the three or four species are commonly or gravel bars in prairie streams (Amsonia late autumn landscape. illustris). In the garden, bluestars prefer cultivated. Many cultivated species are The Evaluation Project listed as winter-hardy in USDA Zones 6 partial shade to full sun in moist soils, The Chicago Botanic Garden (USDA to 9, but Amsonia tabernaemontana and although Amsonia ciliata and Amsonia Hardiness Zone 5b, AHS Plant Heat-Zone Amsonia illustris, which are native to hubrichtii tolerate drier sites. Plants 5) conducted an evaluation study of 11 more northerly regions, are listed as hardy should be watered during dry periods and taxa of Amsonia from 1994 to 1999. At to Zone 5. mulched to conserve moisture. Bluestars the outset of the study, only Amsonia Bluestars are herbaceous perennials grown in full sun will have increased tabernaemontana and Amsonia tabernaemontana var. salicifolia were commonly grown in the local area. Cultural adaptability and winter hardiness were important evaluation criteria because Richard Hawke species occur naturally in diverse environments and some species are cited as hardy in warmer climate zones only. In addition, each taxon was evaluated for floral display, habit display, and disease and pest resistance. Three plants of each taxon were planted in a site that received full sun for the majority of the day. Mature trees near the test site provided shade for all plants until midmorning and for the westernmost plants in the late afternoon. All plants were sheltered from wind by the wooden fences that surrounded the garden. The Amsonia tabernaemontana 2 Plant Evaluation Notes Chicago Botanic Garden clay-loam soil was well-drained, with a cymes in May and June. The light blue Amsonia tabernaemontana var. pH of 7.5. Maintenance practices were flowers faded to pale lavender or almost salicifolia and Amsonia illustris received kept to a minimum to simulate home white as the air temperature increased. the highest overall rating based on heavy garden culture. Overhead sprinklers Plants did not produce flowers the first flower production, superior habit and provided water as needed, and no season in the ground, but production good health. Amsonia tabernaemontana fertilizer was applied. Mulch consisting gradually increased each successive year. var. salicifolia had high flower of shredded leaves and wood chips was A second flush of stem growth at the production, between 80% and 100% placed on the soil around the plants for terminals began about seven to 10 days coverage in all years. Pale blue flowers water conservation and weed suppression. before flowering was complete, and the were produced in large clusters up to 5 new leaves obscured the remaining inches long and 3 inches wide. The dark Observations flowers. Consequently, the cylindrical green, willow-like leaves remained Floral, foliar and habit traits were seedpods ripened within the foliage, up to healthy all season and turned yellow in observed and compared for each bluestar 8 inches below the ends of the stems, and the autumn. A robust vase-shaped habit from spring of 1994 through autumn of were deemed not ornamental. was evident by the second season, when 1999. Information was collected on Foliage was clean and healthy on all plants reached 44 inches tall and 60 flower color, size, coverage and bloom taxa throughout the growing season. Plant inches wide. The centers of the plants period; plant size, habit and foliar quality; health and habit were unaffected by opened slightly by late summer, but stems disease and pest resistance; cultural diseases, insects or animals. Some degree remained erect. adaptability to the local environment; and of yellow autumn color was observed on Amsonia illustris was similar to winter hardiness. Plant traits and all taxa, although some species were more Amsonia tabernaemontana in habit, but evaluation specifics for nine of the 11 taxa ornamental than others. Autumn color its pale blue flowers were borne in are shown in Table 1; the summary rating generally developed in October and smaller flower clusters up to 3 inches long is based on flower production, habit November and was reliable up to a hard and wide. The seedpods were produced quality, plant health and winter injury. frost; stems quickly wilted after several within the upper foliage but were pendant Because Amsonia ludoviciana and successive days of freezing temperatures rather than upward-facing like the other Amsonia rigida were not correctly named, and eventually turned a straw color that bluestars. The glossy, dark green leaves they were excluded from the final remained ornamental into the early winter were healthy all season, and autumn color summary. Amsonia ‘Blue Ice’ was months. Autumn foliage color, like flower was usually a fair yellow mixed with received as ‘WFF Select’, and according production, improved as the plants got green. Stems remained upright all to Tony Avent of Plant Delights Nursery, older. The bluestars were adaptable to the summer, creating an attractive was renamed by White Flower Farm in mostly full sun conditions of the test site. vase-shaped plant. November 2000. The nomenclature of The high pH of 7.5 did not adversely Reduced flower production was the Amsonia montana is indeterminate; affect the majority of plants, but Amsonia reason for the slightly lower ratings for hence, it is often cited as a distinct species ciliata and Amsonia montana had Amsonia hubrichtii and Amsonia or a variety of Amsonia tabernaemontana. moderate to severe chlorosis in more than tabernaemontana. All other traits were Several comments on flowers, foliage one year. No winter injury was noted equal in quality to Amsonia and winter hardiness apply to the growth during the evaluation period despite the tabernaemontana var. salicifolia and traits and performance of all the bluestars. fact that several species are listed as hardy Amsonia illustris. The pale blue flowers Flowers were produced in loose terminal to Zone 6 or warmer. of Amsonia hubrichtii were borne in loose Table 1: Plant Characteristics and Performance Summary Ratings Overall Amsonia Bloom Period Flower Color Flower Flower Plant Plant Rating Size Coverage Height Width 5 ★★★★ ‘Blue Ice’ early June-late June deep blue /8 in. 40-60% 20 in. 36 in. 5 ★★★ ciliata mid June-early July pale blue /8 in. 20-40% 36 in. 36 in. 1 ★★★ ciliata var. texana early June-late June pale blue /2 in. 40-60% 32 in. 28 in. 3 ★★★★ elliptica late May-late June lavender-blue /4 in. 40-60% 30 in. 30 in. 3 ★★★★★ hubrichtii early June-early July pale blue /4 in. 60-80% 44 in. 50 in. 3 ★★★★★ illustris early June-early July pale blue /4 in. 80-100% 48 in. 60 in. 1 3 ★★★ montana late May-late June pale blue /2- /4 in. 40-60% 28 in. 36 in. ★★★★★ tabernaemontana early June-early July pale blue 1 in. 60-80% 50 in. 54 in. ★★★★★ tabernaemontana 3 var. salicifolia early June-early July pale blue /4-1in. 80-100% 48 in. 55 in. Overall ratings: ★★★★★ excellent; ★★★★ good; ★★★ fair; ★★ poor; half-star ratings are included in table. Chicago Botanic Garden Plant Evaluation Notes 3 maple tree and received more shade than any of the other bluestars. Consequently, the plants were not as full or robust, and Richard Hawke the stems leaned toward the light. Plants were in good health throughout the evaluation period, but the habit was consistently open with naked lower stems. Although the plants were similar to Amsonia ciliata in most traits, the pale blue flowers opened earlier in June and were more abundantly produced.

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