Amsonia

( am - SOH - nee - uh )

At-A-Glance Information

The genus Amsonia includes dependable, low-maintenance species native primarily to North America, with a species in eastern Asia and another in Europe. They are commonly known as Bluestars, and form subshrubs and herbaceous clumping plants. They are at home in moist woodlands, meadows, and thickets and have moderate water needs. Bluestars enjoy full sun to partial shade, and are considered to be easy-care ornamentals once they are established. Use Amsonia in mixed beds or borders in cottage, woodland, native, or informal gardens. Their blue flowers will attract pollinators to the garden, and their toxic milky sap deters rabbits and deer. Take care when planting Amsonia in the garden, as plant parts are toxic to humans and pets, and can also cause skin irritation.

Amsonia Species and Varietals

The genus Amsonia includes 18 species and is a member of the Dogbane plant family, Apocynaceae. The genus is named for American physician John Amson. The best growing zones for Amsonia vary depending on the species. For more information, see the descriptions below. To find your zone, click here: Zone Map.

Amsonia ciliata (Fringed Bluestar, Downy Bluestar, Sandhills Bluestar)
These low-growing plants, native to North America, were first formally described in 1788. Individual plants reaches up to between 15 and 24 inches tall. Leaves are green and about 2 inches long, attached to the main plant stem without a petiole. Foliage has fine hairs. Flowers are pale blue with white centers and 0.5 inches across, held in loose clusters at stem tips. This species does well in USDA hardiness zones 3-9.

Amsonia hubrichtii (Hubricht's Bluestar, Arkansas Bluestar, Thread-Leaf Bluestar, Blue Star Amsonia)
These plants reach between 2 and 3 feet in height at maturity, and form upright stems in bushy clumps. Leaves are narrow, giving plants a somewhat feathery appearance. Leaves are bright green, turning golden in fall. Flowers are blue, and fade to white in warmer temperatures. This species is considered vulnerable to extinction due to habitat loss in its native range (Oklahoma and Arkansas in the south-central United States). It is commonly grown as a garden ornamental. These plants do well in USDA hardiness zones 3-9.

Amsonia jonesii (Colorado Desert Bluestar, Jones' Bluestar)
Native to the Colorado River's surrounding desert area, these drought-tolerant perennial plants have grey-green foliage and produce chalky blue to light purple flowers in springtime. Individual plants reach up to between 8 and 20 inches tall when mature and in bloom. This species grows well in USDA hardiness zones 4-9.

Amsonia kearneyana (Kearney's Bluestar, Kerney's Slimpod)
This rare species is native to southern Arizona in the United States and Sonora in northern Mexico. It is considered endangered due to habitat destruction in its native range. It grows to reach up to 35 inches tall and spreads widely to 6.5 feet across. Foliage is green and flowers are creamy white, opening from light blue buds. Blooms are held in inflorescences at stem tips. The species name honors Thomas Henry Kearney, a botanist who studied plants in the American Southwest.

Amsonia orientalis (European Bluestar, European Amsonia)
This species of perennial flowers is native to Turkey and Greece (though it may now be extinct there). It is critically endangered due to habitat loss and over-harvesting for research. It is the only European member of the genus. Individual plants reach up to 15 inches tall and produce purple-blue flowers.

Amsonia tabernaemontana (Eastern Bluestar, Blue Dogbane, Willow Amsonia, Woodland Bluestar)
Native to central and eastern areas of North America, this species reaches up to 1-3 feet tall. These plants form clumps of upright stems crowded with narrow green leaves that turn a beautiful golden-yellow in autumn. Flowers are blue and tubular, flaring into a star shape at the mouth. The species name honors 16th century German herbalist Jakobus Theodorus Tabernaemontanus. Best in USDA zones 3-9.

Amsonia tharpii (Tharp's Bluestar, Feltleaf Bluestar)
These plants are native to New Mexico and Texas in the United States. They form a woody underground taproot and reach up to 8 inches tall when mature and in bloom. The species forms a tuft of green foliage on upright stems, which are topped with white, pale blue, or greenish flowers in springtime. It is considered to be critically imperiled in its native habitat, threatened by oil and gas development, overgrazing, erosion, and competition with invasive species. Best in USDA hardiness zones 4a-9b.

Amsonia tomentosa (Wooly Bluestar, Gray Amsonia)
These plants are native to the southwestern United States (including Arizona, California, Nevada, New Mexico, Texas, and Utah) and northern Mexico (around Chihuahua). It forms a short, woody plant with erect stems clothed with either smooth green or greenish-grey wooly foliage. Flowers are white with a green or blue tint to them. These plants are best in USDA hardiness zones 6b-10b.

Caring for Amsonia

Amsonia, or Bluestar, can be sown as seeds directly into the garden or purchased as starts from a reputable nursery. They develop into a mature-sized plant in about two to three years when grown from seed. Amsonia can tend to get a little floppy and stems can be decumbent if they don't have enough structure around them. It is recommended to plant Amsonia in groups so that their stems hold each other up, or to plant Amsonia in the middle of other more woody plants for structure. Choose a space in the landscape that receives full sun to partial shade. They will flower and perform best in sun during most of the day, with some shade in the hottest hours of the afternoon. Soil should be well-draining, moderately fertile, and have a pH of between 6.2 and 7.0 (slightly acidic to neutral). If local soils are particularly poor in nutrients or do not drain well, make sure to amend the planting area with plenty of organic compost before installing. Apply a thick layer of mulch around the planting area to suppress weed growth, increase soil moisture retention, and regulate soil temperatures.

Water newly planted Amsonia regularly to help them develop a strong root system in their new home. At maturity, these plants are drought-tolerant, able to get by on rainwater alone with some supplemental waterings in the warmest months of the year, but perform best with regular waterings. Soil should never be allowed to dry out completely. Fertilize yearly in early springtime with a application of 2 inches of compost around the base of plants. Pull mulch back before applying, work the compost gently into the soil a bit, water it in, and then replace mulch. These plants are not heavy feeders, and this should be sufficient to provide the necessary nutrients for the active growing and flowering seasons. These plants are considered low-maintenance, and do not need much care in the way of pruning. They sometimes can get floppy, especially when in bloom or when seed pods are present. They can either be staked with a hoop, or plants can be cut back by a third after flowers fade to encourage a more bushy habit to form. Deadheading is not necessary; the seed pods are quite decorative and many gardeners choose to leave them on plants. Amsonia plants have to potential to last decades in the landscape, possibly up to 90 years in some cases.

Propagation

Amsonia can be propagated either by division or from seed. To ensure that your propagated plant will have the exact same qualities and characteristics as the parent plant, use the division method. If you would like to experiment and see what plant you can grow from seed, just keep in mind that it will not necessarily be the same as the parent plant. To propagate your Amsonia, follow the instructions below:

Propagating Amsonia by Seed

Keep in mind that the plant you get from seed will not necessarily mimic all of the characteristics of the parent plant. To propagate by seed, you will need a paper bag, a pot or tray filled with well-draining compost or other organic material, a spray bottle and a fork. If you don't have any potting medium on hand, you can make some. The best potting medium for seed germination is a light blend of 1/3 pasteurized soil or compost, 1/3 sand, vermiculite or perlite, and 1/3 coconut coir or peat moss.
Harvest your seeds. The seed heads on plants are pretty easy to spot, they're left behind after flowers have faded. Once they have dried out and begun to split open, you can harvest the seeds inside. Cut them off of the plant, placing them in a paper bag. Harvest as many as you can, and place the paper bag in a dry, cool, dark area. They will continue to split open and release the seeds contained inside. Once they have been released, take them out of the bag and put them in a container. Amsonia seeds can stay viable for up to four years if stored correctly.
Seeds will need a period of cold-stratification in order to germinate. To complete this process, first place seeds in a bag or other sealable container filled with moistened sand or peat moss. Then, place the container in the refrigerator (or similarly cool, dark space) for 2-3 months. The ideal temperature for cold-stratification is around 40 degrees Fahrenheit. After this period has passed, remove seeds from the container.
In either late winter or early springtime, get a container prepared. Water the prepared substrate and ensure that good drainage is occurring. Place your seeds about a quarter inch to a half-inch under the soil surface. Cover them with soil and water with a spray bottle to avoid dislodging them. Place your tray of seeds in a warm area that receives bright, indirect light. A greenhouse is the most ideal climate, but a planting tray inside a window will do as well. Germination usually takes 8-14 weeks from sowing. Once seedlings sprout and they are able to be handled, transplant them to their own individual 3-inch pots, filled with the same potting medium as described in Step 1. Loosen the seedlings from the potting medium by digging down with the tines of a fork about an inch and a half, gently working the seedling up until it is out. Then, replant the seedling in its own pot.
Keep the seedling's potting medium moist, but not wet, by spraying the soil with your spray bottle. Keep your new plant in an area with bright light (though not direct sunlight) and temperatures between 65 and 72 degrees Fahrenheit. Ensure that the soil stays moist, but not wet, until the next spring (after the danger of frost has passed), when you can transplant it to its new home.

Propagating Amsonia by Division
This type of propagation is relatively simple; you will need a clean shovel, mulch, fresh soil (any kind will do, though it does need to be well-draining), compost and pots (only necessary if you will be saving divided plants for transporting elsewhere). Division is best done in very early spring (after the plant has entered dormancy, but before it has begun its new spring growth), and needs to be done every few years regardless of whether you have the goal of propagating the plants for best performance. To propagate by division, follow these steps:

Using a shovel, dig up your existing plant. Start from the outside and work your way in to avoid damaging the root(s). Once you have worked it up out of the ground, lay it out.
Start to tease apart the roots with your gloved fingers. Once the roots are untangled, separate your plant into segments, ensuring that each one has at least one thick root section attached.
Prepare your pot(s) or new planting site(s) with amended, well-draining soil. You can either bring in new soil or simply amend your existing soil with compost to complete this task. Then, plant your divisions and water thoroughly.
Apply a thick layer of mulch over the new planting to protect it from cold weather as it acclimates to its new home. Then, enjoy watching it sprout in the next growing season.