The Perennial Plant Association has done it again: named one of my favorite perennials as its Perennial Plant of the Year. Last year it was Baptisia; for 2011, it is Amsonia hubrichtii, commonly called Arkansas blue star or thread-leaf blue star. It used to be hard to find in the trade, but now that its popularity has soared, it is readily available in catalogs and garden centers and well-deserving of a place in your garden.
This is a native plant, but only in the sense of being indigenous to the North American continent. Its native habitat is the Ouachita Mountains of western Arkansas and eastern Oklahoma, where it was discovered in 1942 by naturalist Leslie Hubricht, after whom it is named. Despite its limited natural range, it is hardy in Zones 4 to 9 and adaptable to a variety of growing conditions, although ideally it prefers moist well-drained soil and sun to light shade.
Amsonia is a wonderful addition to a mixed border of shrubs and perennials, providing a long season of interest through form, texture, and color. Its flowers are nice, but its foliage is its glory. The "blue star" in its common name refers to the small, steel-blue, star-shaped flowers that unfurl in spring and early summer atop the stems, which eventually reach a height and spread of about 36 inches and form a nicely rounded mound of billowy foliage.
The alternate leaves are narrow and threadlike, spaced closely on the stems to create the effect of soft, vibrant green bottlebrushes - what one author describes as a "textural dream." In fall, the leaves turn a golden yellow that is outstanding in the garden and particularly beautiful when backlit by the lowering rays of the autumn sun; this is a sight that people remember. The yellow color has been described as banana, pale pumpkin, or apricot colored, and I suppose it could be any or all of these.
Arkansas blue star is a bit slow to establish, so give it a couple of years to settle in. It forms a strong clump with a tough rootstock that is very long-lived. It can be difficult to divide, but should not really need dividing, although pieces of older crowns can sometimes be broken off easily for transplanting in fall. Amsonia is easy to propagate from seed and will sometimes self-seed.
It requires little maintenance, is drought-tolerant once established, and it is very resistant to insect and disease problems. It is a member of the Dogbane plant family, and as such contains a milky latex sap that some people find irritating to the skin, so gloves may be in order when handling it; but that irritating sap is also a strong deterrent to deer browsing. Its flowers are also a favorite nectar source for early butterflies in the spring.
If it is planted in too rich soil or too much shade, Amsonia has a tendency to flop open. Full sun is best for maintaining the nicely rounded form, but you can also shear it back by about 1/3 to ½ after flowering to maintain the form. Old stems can be cut back in fall or left standing over winter to be cut back during spring cleanup.
Its color, form, and texture make Amsonia an excellent companion for many shrubs and perennials, although it is also very effective planted on its own in large masses. It can be used in mixed borders, cottage gardens, meadow plantings, and large containers. Some stunning plant combinations suggested with Amsonia include ‘Black Lace' elderberry; Japanese anemone; ‘Black Jack' sedum; ‘Dallas Blues' switchgrass, ‘Blue Billow' hydrangea, Virginia sweetspire, and smooth witherod viburnum.
Another excellent Amsonia that you might find next to Arkansas blue star in the garden center is Amsonia tabernaemontana, common or willow blue star. Its leaves are wider, more like those of a willow, and not as vibrantly yellow-hued in the fall, and its native range is much larger, but otherwise it is very similar to Arkansas blue star. Both of these fine native perennials are "invaluable workhorses that provide three splendid seasons of ornamental features."
Annette MaCoy is the extension educator for consumer horticulture and a master gardener coordinator for Penn State Cooperative Extension.