Ornamental · Apocynaceae
Blue Star
Amsonia tabernaemontana
Give it full sun and a shear after bloom, and Blue Star turns from spring flowers to gold fall color with almost no fuss.
Full sun
Every 7–14 days
USDA 3–9
2–3 ft · 0.6–0.9 m
Overview
Blue Star is a tough native perennial grown for its clusters of soft, steel-blue star-shaped flowers in late spring. It belongs to the dogbane family and forms a slow-growing, shrubby clump of feathery green foliage that holds its shape all season. Hardy from zone 3 to zone 9, it shrugs off cold, heat, clay, and drought once its roots take hold.
The flowers are only half the story. Come autumn the narrow leaves turn a clear golden yellow that lights up a border, and the bitter, milky sap running through the stems keeps deer and rabbits from browsing it. Plant it in full sun and it stands upright on its own, a reliable backbone plant that asks for almost nothing in return.
Care
The short version: full sun, average well-drained soil, and a hard shear after the flowers fade. It needs little water once established and almost no feeding, so the main job is cutting it back to keep it from flopping.
Light
Blue Star does best in full sun, where it grows dense and stands up without staking. It will take partial shade or partial sun, but the plants stretch, lean, and flop the less light they get.
Water
Keep the soil evenly moist while the plant settles in, watering every 7 to 14 days. Once the roots are established it handles drought well and even puts up with the odd wet spell, so you can ease off the hose.
Soil
It isn't fussy about soil and grows in loam, clay, or sand as long as it drains. A pH anywhere from 6.0 to 8.0 suits it, which covers most garden ground.
Temperature
This is one of the hardiest perennials you can plant, taking winters down to -40°F (-40°C) and summer heat up to 100°F (38°C). That range is what makes it dependable from zone 3 all the way to zone 9.
Humidity
Ordinary outdoor air is all it asks for, so there's no number to watch. Good airflow matters more than humidity, since crowded, still conditions are what invite rust onto the leaves.
Feeding
Blue Star barely needs feeding. A light topdressing of compost in spring is plenty, and rich soil or heavy fertilizer only makes the stems soft and prone to flopping.
Pruning
The one job that matters is cutting the stems back by one-third to one-half right after the flowers fade. That keeps the clump rounded and self-supporting, and deadheading at the same time stops it from seeding around the garden.
Repotting
Established clumps rarely need lifting and resent being disturbed. Dig and divide only when the center starts to die out or flowering drops off, which can take many years.
Size & spacing
A mature plant reaches 2 to 3 feet (61 to 91 cm) tall and spreads about the same. Give each one roughly 2 to 3 feet (61 to 91 cm) of room so air moves freely and rust stays away.
When to plant
Blue Star is a hardy perennial that comes back on its own each year. Set new plants out in spring after the frost or in fall, watch for its blue flowers in late spring, then cut the whole clump back hard once the bloom is done.
Northern hemisphere
Southern hemisphere
Typical months for a temperate climate: start earlier in warm zones, later where spring frosts linger.
- Sow: Late winter to early spring; seed germinates readily but plants typically flower in their second year.
- Plant: Spring after frost, or fall.
- Harvest: Late spring (April to May), into early summer.
Hardiness
USDA hardiness zones: 3–9
Temperature: down to -40°C
Troubleshooting
Blue Star has almost no serious problems, which is part of why gardeners lean on it. The two things you might run into are floppy stems and the occasional patch of rust. Match what you see below and the fix is usually quick.
Stems that splay open and flop outward
Flopping comes down to too much shade or soil that's too rich, and it gets worse when the plant isn't cut back after it blooms. Shear the stems by a third to a half once the flowers fade and you get a tighter, self-supporting mound. If the clump sits in shade, move it somewhere sunnier and it will stand on its own.
Fix:
- Cut stems back by one-third to one-half after flowering to encourage a denser, self-supporting habit
- Relocate to a sunnier position if grown in shade
Orange, rusty bumps on the leaves
Powdery orange-to-brown spots on the undersides of the leaves point to rust, the one disease this plant tends to pick up. Pull off the marked foliage and bin it. Give the clump more room and keep water off the leaves, since rust spreads in still, damp air.
Fix:
- Remove and dispose of affected foliage
- Improve air circulation and avoid overhead watering
Toxicity
Blue Star is a low-risk plant in the garden. Its bitter, milky sap puts off deer, rabbits, and most curious pets, and it isn't on the ASPCA toxic list. The main thing to know is that the latex from cut stems can irritate skin and eyes, so wash your hands after pruning.
Toxic to
Cats · Dogs · Horses · Livestock · Birds · People
Safe for
Rabbits
Cats Caution
There's no specific report for cats, but the bitter milky sap runs through the same family as other irritant plants. A curious cat that chews a leaf might drool or have a brief stomach upset, though the taste usually puts them off after one bite.
Dogs Mild
One extension source calls the milky sap slightly toxic to dogs, so a chewed stem can leave a dog with mild mouth or stomach irritation. The bitter latex tends to stop most dogs before they swallow much.
Horses Caution
Nothing specific is recorded for horses, but the bitter sap and the plant's dogbane background make it one to keep them away from. Most horses leave it alone on their own.
Livestock Caution
Grazing animals turn away from the bitter foliage, so trouble is unlikely. Still, the milky latex is best kept out of pasture where hungry stock have little else to eat.
Birds Caution
There's no record of harm to birds, and they aren't drawn to the foliage. Keep an eye out only if a pet bird has a habit of nibbling houseplants and cut greenery.
People Caution
No part of this plant is meant to be eaten. The milky latex from cut stems can irritate skin or eyes, so wash your hands after pruning and keep small children from chewing the foliage.
Rabbits Safe
Rabbits give this plant a wide berth. The bitter taste deters them so reliably that gardeners plant it where rabbits are a problem, and browsing damage is rare.
The milky latex released when stems or leaves are cut or torn can be a mild skin or eye irritant; wash hands after pruning.
Helpline numbers
United States
ASPCA Animal Poison Control: (888) 426-4435 (fee applies)
Poison Control: (800) 222-1222
Canada
Poison Centres: 1-844-764-7669
Varieties
A few named selections improve on the wild plant, mostly for richer flower color or more striking foliage. These are the ones you'll find at most nurseries.
Storm Cloud
Its stems emerge nearly black in spring against dark green leaves marked with bright silver veins.
Blue Ice
A compact selection that carries deeper, more violet-blue flowers than the species.
var. salicifolia
This form has narrow, willow-like leaves only about a quarter inch wide.
How to propagate
Blue Star is slow to spread on its own, so most gardeners speed things along by splitting clumps, saving seed, or rooting cuttings.
DivisionEasy
Spring or fall while dormant · One growing season
Lift and split the clump while it's dormant in spring or fall, making sure each piece keeps at least one eye, or growth bud. The divisions settle in over a single growing season.
SeedEasy
Late winter to spring · Flowers usually in the second year
Sow seed from late winter into spring and it sprouts without fuss, though the young plants usually wait until their second year to bloom. Snip off the seedheads you don't want, or the plant sows itself around.
Stem cuttingsModerate
Late spring to early summer · Several weeks to root
Take root or stem cuttings from non-flowering shoots in late spring or early summer. They need several weeks to root, which makes this the slower and trickier route compared with division.
Myths
Blue Star is dangerously poisonous to people and pets and should be kept out of the garden.
Only one extension source flags the milky sap as slightly toxic to dogs, and the bitter taste keeps most animals from eating it at all. It is widely grown as a safe, low-maintenance native ornamental and doesn't appear on the ASPCA toxic plant list.