Amsonia hubrichtii Arkansas Blue Star
Adding texture to the landscape, the 2011 Perennial Plant of the Year provides tremendous value from spring to fall.
Amsonia hubrichtii is an outstanding perennial that has largely gone under the radar until recently when it was selected as the Perennial Plant Association’s (www.perennialplant.org) 2011 Perennial Plant of the Year.
Amsonia is commonly referred to as an all-season perennial as it provides differing accolades during the various seasons. In the spring, amsonia bears numerous clusters of light blue star-shaped flowers atop its fine textured foliage. During the summer months, it forms attractive clumps of light-green lacy-fine foliage that grows 3 feet in all directions. The leaves are usually less than ½-inch wide and 2 to 2 ½ inches long that line the stems like bottlebrushes. In the autumn, Arkansas blue star is a showstopper as the foliage turns bright golden yellow and retains this coloration for three to four weeks.
The billowy fine-textured foliage forms an attractive shrub-like clump which adds a unique element to the landscape. Although, Arkansas blue star is native to Arkansas and Oklahoma, it can be grown across much of the country and is hardy throughout USDA Hardiness Zones 4 to 8. Arkansas blue star grows best in full sun, but performs well in locations that receive light shade.
Amsonia adds texture to the landscape where it is commonly used in mass plantings, native gardens, cottage gardens, open woodland areas, or as a backdrop against other fall blooming perennials. It is also resistant to deer feeding. With these attributes, this trouble-free plant is a show stopper which provides great landscape interest and value from spring to fall.
Amsonia hubrichtii can be propagated by seed or division. Due to irregular germination, which occurs over several weeks, division is the most reliable method of propagating this plant.
For growers interested in starting amsonia from seed, sow two to three seeds per cell (128- or 72-cell plug trays work well). Move flats to a cold environment (30 to 40° F) for several weeks, keep them moist but not saturated, and maintain cool temperatures (less than 50° F) throughout the germination process. These steps will improve the germination of the seeds, but results tend to vary each time. It is not uncommon for it to take up to 10 weeks for germination to occur. For this reason, it can take 16 to 20 weeks to obtain a transplantable plug.
Jelitto Perennial Seeds offers Amsonia hubrichtii as Gold Nugget seed, which is pretreated to help improve the germination rate and bypasses the need for the cold treatment. Amsonia from Gold Nugget seed can be germinated at 68° F. Cover the seeds lightly at sowing, maintain them at constant moisture levels, and reduce the growing temperatures slightly after germination. The time from sowing to transplant using Gold Nugget seed is seven to nine weeks.
Amsonia is commonly produced in 1-gallon or smaller-sized containers. The proper planting depth varies slightly depending on the type of starting materials being used; liners should be planted even with the top of the soil line of the plug while bareroot should be planted so the crown is slightly below the soil surface. It performs best in a growing mix with good drainage; many bark-based growing mixes work well.
Maintain the media throughout the production cycle with a pH between 5.6 and 6.2. Arkansas blue star requires low to moderate amounts of nutrients. Growers can provide nutrients using water-soluble fertilizers delivering 75- to 100-ppm nitrates with every irrigation or 150 to 200 ppm every other watering. Controlled-release fertilizers can be incorporated at a rate equivalent to 0.75 to 1.0 pounds of nitrogen per cubic yard of growing mix or top-dressed using the low rate on the CRFs label. They can be grown with slightly below average amounts of irrigation.
Due its growing habit, amsonia often grows taller than growers would like when producing them in containers. It would be beneficial to apply plant growth regulators particularly during spring production. Unfortunately, I am not aware of any research regarding the effectiveness of PGRs on this plant. A different species (A. tabernaemontana) is responsive to spray applications of paclobutrazol (Bonzi, Piccolo or Paczol) at 30 ppm. Consider trialing paclobutrazol or other PGRs on amsonia before making applications to the entire crop. Growth regulator applications are most effective when they are applied early in production; begin applications when the plants reach 4 to 6 inches tall.
Do not pinch the plants in the spring or the flower buds will likely be removed. They can be pinched or trimmed back after they have flowered to increase fullness and plant density.
Insects and Diseases
Compared to many perennials, the occurrence of insect pests and diseases is minimal. Amsonia does not have any serious insect pests at this time. Currently, the only pathogens to occasionally infect Arkansas blue star are a Mycosphaerella leaf spot and rust. Routine scouting should be sufficient to determine the presence of any pests or diseases.
Amsonia is typically marketed when it is in bloom in the early spring or as a non-flowering plant during the late spring and summer. When using plugs as the starting materials, it is best to plant them in the summer to allow adequate time for bulking prior to overwintering. Bareroot can be planted in the fall or in the spring. Arkansas blue star requires vernalization for flowering. The cold treatment can be delivered to large plugs or in the final container; bareroot does not require vernalization as it received the cold treatment in the field before the plants were harvested. After vernalization, they can be grown at natural daylengths. Plants that have been vernalized will flower in approximately seven to nine weeks after the start of forcing when they are grown at 55 to 65° F.
Amsonia hubrichtii Gold Nugget seed is available from Jelitto Perennial Seeds (www.jelitto.com); untreated amsonia seeds are available from various seed distributors. Bareroot can be obtained from Walters Gardens, Inc. (www.waltersgardens.com). Plugs and liners can be purchased from numerous reputable perennial propagators throughout the country.