How to Grow Blue Star (Amsonia) Flowers

Blue star plants in garden with blue flowers and narrow leaves

The Spruce / Evgeniya Vlasova

The plant species amsonia was given the common name blue star for its soft blue, star-shaped flowers. The individual flowers are small, but they bloom in fluffy clusters and put on a lovely show for several weeks. Blue star flowers are one of the truest blues you will find in flower colors.

The medium-growing plants are native to many regions of North America and grow pretty much problem free. The airy but substantial texture of the foliage provides a great foil for almost any other plant, making them very versatile in garden design. A bonus is the brilliant yellow fall color of their foliage.

Most blue star varieties will grow about 2-3 feet tall, and 2-3 feet wide, but size will depend on the variety you are growing and the growing conditions. They tend to be small, tidy, shrub-like clumps.

Leaves are 3-4 inches, narrow and lance-shaped, with a pronounced mid-rib. Hardiness will vary with species. Most are perennial to at least USDA Hardiness Zones 3-11.

  Botanical Name Amsonia tabernaemontana
  Common Name Blue star, Eastern blue star, Arkansas amsonia, Hubricht's amsonia
  Plant Type Perennial
  Mature Size 2-3 ft. tall, 2-3 ft. wide
  Sun Exposure Full sun, partial shade
  Soil Type Sand, chalk, clay, loam
  Soil pH 6.2-7.0
  Bloom Time March-May
  Flower Color Blue, purple, white
  Hardiness Zones Zone 3-11 (USDA)
  Native Area North America
  Toxicity Toxic to pets and humans

Amsonia Care

The plants bloom sometime in spring; The flowers are followed by interesting seed pods. The soft, billowy foliage of blue star allows it to fit in almost anywhere. It pairs especially well with larger leaved plants, like peonies and hosta. A classic favorite combination is blue star paired with the seed heads of oat grass.

The bright yellow fall foliage is stunning next to tall sedum or purple coneflowers.

Blue star plants will bush out and get fairly wide, up to a couple of feet across, but they do not spread or travel very far or become a nuisance. You can divide the plants if you want to make more plants, but it is rarely necessary.

Blue star plant stem with blue flowers and leaves in garden

The Spruce / Evgeniya Vlasova

Blue star plant with blue star-like flower clusters in sunlight

The Spruce / Evgeniya Vlasova

Blue star plants with blue star-like clusters in garden

The Spruce / Evgeniya Vlasova

Light

You will get more flowers if you plant your blue star in full sun, but the plants can handle partial shade and may even grow better there in hot, dry climates.

Soil

Amsonia plants like a neutral soil pH, between 6.2-7.0, but will grow just about anywhere, even in poor soil. They do not like prolonged dry conditions but once established, Blue star plants can handle brief periods of drought.

Water

Amsonia needs regular watering, the soil they're planted in should never dry out.

Temperature and Humidity

Blue star thrives in a hot and humid climate.

Fertilizer

When new leaves emerge, feed with a water-soluble fertilizer, making sure it's slow-released.

Is Amsonia Toxic?

Both the leaves and stems of amsonia exude a milky white latex sap that is toxic to humans and pets.

Symptoms of Poisoning

In humans and pets, the sap can cause contact dermatitis. Call your physician or veterinarian immediately to explain the situation and see if you need to rush you or your pets into treatment.

Amsonia Varieties

  • 'Arkansas amsonia,' 'Hubricht's blue star,' 'Narrow leaf blue star' (Amsonia hubrichtii): Narrow, needle-like leaves and bright blue flowers
  • 'Ozark blue star' (Amsonia illustris): Taller plant (4-feet) with glossy leaves and larger flowers
  • 'Blue dogbane,' 'Eastern blue star,' 'Willow amsonia,' 'Woodland blue star' (Amsonia tabernaemontana): Wider leaves and pale blue flowers

Pruning

Blue star requires very little maintenance. They can get floppy when they are heavy with flowers and when the seed pods form. To prevent this, you can either stake them with a hoop or trim them back by one third, after flowering. Trimming will mean losing the attractive seed pods.

Since deadheading is not necessary, framing your blue star with sturdy plants on either side is a third choice for helping to support the plants and keep them upright, while allowing the seed pods to remain on the plant.

How to Grow Amsonia From Seed

Blue star can be grown from seeds harvested when the pods dry. You can start seeds in the fall and overwinter them in a cold frame or protected area, then transplant in the spring. Simply cover them lightly with soil and keep the soil moist, until the plants germinate.

If you don't want to start seeds, blue star plants are becoming more commonly available in garden centers and develop into good sized plants within two to three years.

Overwintering

Amsonias may be overwintered by covering them in a heavy frost blanket, or by bringing them inside into an unheated garage that stays just above freezing.

Common Pests/Diseases


Blue star plants are virtually problem free. No pests or diseases regularly bother them.