Smooth Aster 

It’s fall, the time for Goldenrods and Asters. For this month’s Plant of the Month, I will be covering Smooth Aster – a beautiful purply-blue, prolifically blossoming fall staple in the garden. One of the earlier asters to bloom in my garden, it signals the coming of autumn with its cooler temperatures and fall colours. As usual, the Plant Description and In the Garden sections are courtesy of Shaun Booth from In Our Nature. 

Symphyotrichum laeve (Smooth Aster) and Solidago nemoralis (Gray Goldenrod) – the colours of autumn

Common Name: Smooth Aster 

Scientific Name: Symphyotrichum laeve 

Family: Asteraceae (Aster Family) 

Alternate Common Names: Glaucous Aster, Purple Aster, Smooth Blue American Aster, Smooth Blue Aster, Smooth-leaved Aster 

Plant Description: Smooth Aster features one to a few erect, hairless stems that are usually green but can be a reddish colour. Leaves clasp the stems in an alternate pattern and measure about 10 cm long and 4 cm wide. They are smooth (almost waxy), shiny, toothless, and greenish blue on top and light green underneath. The basal leaves are toothed with winged petioles and are oblanceolate. Open, branching flower clusters are found at the top of the plant. Flowers can also arise from upper leaf axils (where the leaves meet the stem). Individual flowers measure up to 2.5 cm across and feature 15 to 30 oblong ray florets (petals) surrounding yellow centres that turn purplish red with age. Petal colour can vary from light blue to light purple. Four to six layers of bracts surround the base of each flower. They are smooth, appressed (flattened), and light green to bluish green and have diamond-shaped ends with a dark tip. Flowers give way to dry, brown, narrowly cone-shaped seeds, each with a tuft of light brown hairs that allow them to be carried by the wind. 

In the Garden: Smooth Aster is valued by gardeners for its copious blue blooms and non-aggressive growth habit. The flowers are frost hardy and bloom late into fall while the tough stems will persist through the winter months. Smooth Aster is easy to grow but doesn’t like being shaded by taller plants. A favourite food of rabbits, possibly due to the smooth leaves. 

Skill Level: Beginner 

Lifespan: Perennial 

Exposure: Full sun 

Soil Type: Any well-drained soil 

Moisture: Dry 

Height: 20–70 cm (occasionally to 120 cm) 

Spread: 30–60 cm 

Bloom Period: Sep, Oct (to frost) 

Colour: Blue 

Fragrant (Y/N):

Showy Fruit (Y/N):

Cut Flower (Y/N):

Pests: No serious insect or disease problems, though powdery mildew can affect the plant in some years 

Natural Habitat: Fields, open woods, and roadsides 

Wildlife Value: The nectar and pollen of the flower heads attract many species of native bees, butterflies, and other insects and Ruffed Grouse (Bonasa umbellus) and Wild Turkey (Meleagris gallopavo) feed on both the leaves and seeds of asters; the seeds are also eaten by mice and American Tree Sparrows (Spizelloides arborea

Butterfly Larva Host Plant For: Silvery Checkerspot (Chlosyne nycteis), Tawny Crescent (Phyciodes batesii), Northern Crescent (Phyciodes cocyta), Pearl Crescent (Phyciodes tharos), Painted Lady (Vanessa cardui

Moth Larva Host Plant For: At least 40 species of moths, including members of the tiger moths, ribbed cocoon-maker moths, case-bearer moths, twirler moths, geometer moths, leaf-blotch miner moths, slug caterpillar moths, owlet moths, clearwing moths, flower moths, trumpet leafminer moths, and tortrix moths. 

USDA Hardiness Zones: 3–9 

Propagation: Direct sow the seeds in late fall or early spring. No pretreatment is necessary even when starting indoors, but seeds need light to germinate. Germination is said to be slow. Transplanted seedlings will likely bloom in their first year. You can also multiply plants from root cuttings. 

Additional Info: Symphyotrichum laeve will tolerate short durations of seasonal flooding. It also self-sows strongly in open areas that are burned and mowed and is walnut (juglone) tolerant. 

Native Range: