How many perennials do you know that offer delicate blossoms in spring, outstanding color in autumn, freedom from pests and diseases, and extraordinary tolerance to cold, heat, and drought? And how many of these are easy to grow and propagate, and can go up to 20 years without needing to be divided?
Blue star (Amsonia), so called for its tiny starlike flowers, fits the bill perfectly. There's no doubt that this beautiful and diverse group of herbaceous perennials deserves more attention.
My wife, Melinda, and I garden on 1-1/2 acres in Pennsylvania, and the blue stars add immeasurably to our landscape's multiseason appeal. Their modest demands fit our main purpose: to enjoy the garden and share it with friends.
Nearly 20 species of blue star are native to parts of central, southern, and eastern North America; a few come from southern Europe, Asia Minor, and Japan. They are outstanding perennials in most of this country, especially in hot-summer regions. They can be grown in the Pacific Northwest, but the relative lack of sun and warmth there somewhat diminishes their vigor and autumn color.
Soil and moisture preferences vary by species, but most prefer full sun or partial shade. Nearly all are hardy to USDA Hardiness Zone 4 (many to zone 3), and the majority thrive in areas as warm as zone 9.
Each year, we delight in cutting blue stars for fresh flowers in spring and golden foliage in autumn. Blue stars are related to dogbanes (Apocynum), oleander (Nerium), and periwinkles (Vinca), and produce a similarly milky sap which can be messy. To reduce the mess, you can dip the cut stems into boiling water or sear them in a flame before placing them in arrangements.
All blue stars produce flower clusters at the tops of the stems in spring or summer. Individual flowers are typically 1/2 inch across with sharp-pointed petals. Color ranges from deep periwinkle blue in bud to light sky blue in bloom.
Foliage varies tremendously. Some species have broad, medium-textured leaves, others finer than lawn grasses. The brilliant gold tones of their autumn foliage frequently rival the fall displays of trees and shrubs. Height varies from less than a foot to more than 3 feet, making some species useful as ground covers and others as deciduous hedging.
Though ideal for growing in pots and showy enough to be used as accents, many species are at their best in sweeps and masses. Until recently, only eastern blue star has been widely grown and available in this country, but renewed interest in our floral heritage is bringing out many new choices. The following are among our favorites.
Eastern blue star (A. tabernaemontana) is native to moist, partly sunny habitats in the eastern states. It is hardy to zone 3. Mature plants form upright mounds to 3 feet tall and across. It performs well in soils from arid sands to heavy clays, but it does best in moderately organic soils with average moisture. It withstands full sun but is also at home in light or partial shade, so it can be an effective addition to a deciduous woodland garden. Bloom period varies from late April to late June, depending upon individual seedlings. Some plants have a light, sweet fragrance.
Though the foliage reliably turns a pleasing light yellow in autumn, leaf width and foliage texture vary considerably causing botanists and horticulturists to disagree on whether this species should be divided into varieties. Leaves are typically 3 to 4 inches long and up to 1 inch wide.
Willow blue star (A. t. salicifolia, sometimes listed as A. salicifolia) is much like eastern blue star except its leaves are narrower, longer, and more tapered, giving a willowlike appearance. It blooms in late spring or early summer.
Dwarf blue star (A. montana) grows to 2 feet tall, with smaller leaves. Botanists don't recognize this species, but gardeners will find it listed for sale under this name. Dwarf blue star is probably best considered a horticultural variety of eastern blue star; however, its smaller stature makes this plant distinctive and valuable in the garden.
Threadleaf blue star (A. hubrichtii, sometimes misspelled as A. hubrectii or A. hubrictii), though little known, is the species we would choose above all others. This exceptional plant, native only to Arkansas and Oklahoma, was discovered in 1942 by Leslie Hubricht and named in his honor. Its threadlike leaves, less than 1/16 inch wide and up to 3 inches long, are numerous and closely spaced along the 2- to 3-foot-tall stems. Mature plants have more than 50 stems, forming a spreading billowy mass up to 4 feet wide.
For two to three weeks in May, multitudes of sky blue blossoms top the stems. Summer foliage is a rich medium green. Peak season for this species is autumn, when the stems and leaves turn a vibrant gold that lasts almost through October and November in my zone 6 Pennsylvania garden. By December, the leaves turn a dark honey brown and begin to drop. Late winter is the only time this plant is not showy.
Both Melinda and I place a premium on plants that thrive in our climate and soil (well-drained, unfertilized loam). We've gradually reduced the size of the lawn and replaced it with mixed borders of trees, shrubs, and perennials. Six years ago, we began a new border and chose threadleaf blue star as a medium-height ground cover, to be mixed with hyssop-leaved thoroughwort (Eupatorium hyssopifolium), Sedum 'Matrona', and Viburnum nudum 'Winterthur'. We planted over 100 threadleaf blue stars as year-old seedlings with only one or two stems each, spaced about 2 feet apart.
Today, each plant has 50 or more stems, and the planting forms a dramatic sweep of color, especially in autumn when it is a river of gold flowing around the burgundy tones of the sedum and viburnum. Our only maintenance chore is to cut the stems back annually, which we do in late February or March. Some garden references recommend partial shearing after bloom to reduce plant size and create a tighter, less billowy mass. We prefer the more natural appearance of unsheared plants. Through experimentation, we've learned that threadleaf blue star's fall color is most pronounced when it's planted in full sun. The plants also do well in as much as half a day of shade.
Threadleaf blue star has proved hardy to zone 4, yet is unfazed by the summer extremes of the southern states. In our garden, prolonged summer droughts are a challenge, yet our plantings have required virtually no supplemental watering to thrive. This species is adapted to a wide range of soils from sands to clays, and seems capable of thriving in both acidic and alkaline conditions.
Downy blue star (A. ciliata) is native to sterile sandy soils in the southern states. Older references often list this species as A. angustifolia. The leaves are up to 1/2 inch wide -- not nearly as narrow as those of threadleaf blue star. Stems and leaves have a fringe of fine hairs that are particularly appealing when plants are side- or back-lit by the sun. The light blue flower clusters tend to be held higher above the foliage and are among the showiest of all blue stars. Typical height is 2 to 2-1/2 feet. For best performance, this species requires sharply drained, relatively infertile soil. Grow it in full sun or very light shade. It is hardy to zone 4 with some snow cover, and also has exceptional heat and drought tolerance. It is not well suited to the cool climate of the Pacific Northwest.
Fringed blue star (A. c. tenuifolia, also known as A. c. filifolia) is a lower-growing, finer textured horticultural variety of downy blue star. Some plants reach only 1 foot and have leaves nearly as fine as threadleaf blue star. Until recently, it was virtually unknown except to plant collectors, but it shows great promise for gardeners. Like downy blue star, this one is native to infertile sandy soils. It does best with very good drainage but also performed well in trials at Longwood Gardens when grown in loam.
For years, European gardeners have grown Rhazya orientalis, a native of Greece and Asia Minor that is so similar to Amsonia that some botanists prefer to call it A. orientalis. It is nearly indistinguishable from lower-growing forms of eastern blue star. A. elliptica, native to Japan, China, and Korea, has rich blue flowers but is largely untried here.
Other American species, though little known, have great potential as garden plants. Lustrous blue star (A. illustris), native to the Southeast, is similar to eastern blue star but has exceptionally shiny leathery leaves. Especially glossy plants offered commercially as eastern blue star actually may be lustrous blue star.
Louisiana blue star (A. ludoviciana), similar to eastern blue star, is found in southern Louisiana but has thrived for years in the Gladwyne, Pennsylvania, garden of Mary Henry, now open to the public as the Henry Foundation for Botanical Research. Far ahead of her time, Mary Henry collected native blue stars and grew more than a dozen kinds by the early 1940s.
All blue stars can be grown from seed or from cuttings. Except for very young plants, division is my least-preferred method of propagation. In time, blue stars develop a deep, nearly woody root mass. Though this contributes to the plants' longevity and drought tolerance, it is nearly impossible to cut through without a sharp spade or an ax. Plants left standing through winter may self-sow, resulting in seedlings for new plantings or to give away.
Most blue stars require only annual cutting to the ground in winter or early spring. They rarely require supplemental fertilization and generally thrive on the fertility usually present in most garden soils. In fact, excess fertilizer is likely to cause floppy growth. Most are strictly clump-forming and are among the longest-lived of all perennials.
Rick Darke is a horticultural consultant living Landenberg, Pennsylvania.
Photography by Suzanne DeJohn/National Gardening Association
Article published on June 23, 2008.