Blue wood aster

Sun

Partial shade or full sun; flowers best with several hours of sun. Avoid hot, exposed positions in dry climates.

Water

Moderate while establishing; established plants tolerate some dry soil, especially in partial shade. Water during prolonged drought.

Soil

Well-drained soil, from sandy loam to clay, with dry to moderately moist conditions. Tolerates a range of soil pH; avoid waterlogged ground.

Height

2-5 ft (0.6-1.5 m)

Spread

1-2 ft (0.3-0.6 m), gradually forming wider colonies

Flowers

Late summer to autumn; small pale blue to lavender flowers, occasionally nearly white, with yellow centres that age to reddish purple.

Foliage

Green, toothed leaves; lower leaves are heart-shaped on stalks, while upper leaves are smaller and narrower. Foliage dies back for winter.

Growth

Moderate

Care needed

Low

• Pollinators
• Drought tolerant
• Cold hardy
• Wildlife
• Low allergen

Northern hemisphere months. In Australia, shift them by six.

Difficulty

Easy

Toughness

Very tough

Pick your zone, country, light and whether you have pets.

1. Planting day

   1 ft (0.3 m)

2. After 1 year

   1.8 ft (0.55 m)

3. After 2 years

   2.6 ft (0.8 m)

4. After 3 years

   3 ft (1.1 m)

   full size

About 3 years to full size.

From a typical nursery size, at an average rate for this kind of plant. A rule of thumb, not a forecast.

Cats and dogs

Safe

Native to

the US and Canada

In the garden

Spreads vigorously: plan to contain it.

Growth rate

Moderate

• Water newly planted clumps regularly through their first growing season, allowing the soil surface to dry slightly between waterings.
• Apply a thin mulch of leaf mould or compost in spring, keeping it away from the crown.
• Avoid heavy feeding, which encourages tall, floppy growth.
• Use twiggy supports in spring if plants lean in shade or rich soil.
• Divide crowded clumps in spring every three to four years, or when flowering declines.
• Remove spent flower heads before seeds mature if you do not want seedlings.

Pruning. For shorter, bushier plants, shorten stems by about one-third in late spring or early summer, before flower buds develop. Leave stems standing over winter for wildlife, then cut them to near ground level in late winter or early spring.

Late winter

Cut down last year's stems before new shoots lengthen.

Spring

Plant or divide clumps, water them in and add a light mulch.

Late spring

Install discreet supports or shorten stems if a more compact plant is needed.

Summer

Water during prolonged drought and check lower leaves for mildew.

Autumn

Enjoy flowering; remove fading flower heads if self-seeding is unwanted.

Late autumn

Leave healthy stems and some seed heads standing for winter shelter and food.

• Plant in loose groups along a woodland edge for a haze of autumn colour beneath deciduous trees.
• Mix with goldenrods and ornamental grasses in an informal border for contrasting flower shapes and colours.
• Use behind lower shade plants such as coral bells, allowing the airy flower sprays to rise above their foliage.
• Include in a North American native planting to extend the season of flowers available to pollinators.

Garden styles it suits: Woodland, Wildlife garden, Pollinator garden, Cottage garden, New perennial (prairie) and Autumn colour.

Plant it with: Zigzag goldenrod, White wood aster, Pennsylvania sedge, Christmas fern, Wild geranium and Coral bells.

Wildlife: Flowers attract bees, butterflies and other pollinating insects. In its native range, the foliage supports several moth and butterfly larvae; seed heads can feed small birds, and standing stems provide shelter for overwintering invertebrates.

• Powdery mildew can coat leaves, particularly in crowded or stressed plants; improve air circulation, water during drought and remove badly affected foliage.
• Leaf spots may develop in damp conditions; water at soil level and clear diseased fallen leaves.
• Tall stems may flop in deep shade or rich soil; provide more light, avoid nitrogen-rich fertiliser and shorten stems in late spring.
• Self-sown seedlings and spreading clumps can crowd neighbours; remove unwanted seedlings and divide or reduce clumps in spring.

Not known to be toxic to people, cats, dogs or horses; no particular part is recognised as poisonous. It is not a food plant, and eating substantial amounts of any garden plant may cause digestive upset.

• White wood aster (Eurybia divaricata)

  Also has heart-shaped lower leaves, but usually bears white flowers in flatter, more open clusters on dark, often zigzagging stems.

• Short's aster (Symphyotrichum shortii)

  Has similar blue flowers and heart-shaped leaves, but its leaf margins are usually smooth or only slightly toothed; blue wood aster has more clearly toothed lower leaves.

• New England aster (Symphyotrichum novae-angliae)

  Has larger, usually deeper purple flowers and narrow, stalkless leaves that clasp the hairy stems, rather than stalked, heart-shaped lower leaves.

The epithet cordifolium means heart-leaved, referring to the shape of the lower leaves, while blue wood aster describes its flower colour and woodland habitat.

Propagation. Divide established clumps in spring. Sow seed outdoors in autumn, or give spring-sown seed a period of cold, moist treatment; seedlings may vary where other asters grow nearby. Use division to preserve the characteristics of a named selection.

Accepted botanical name

Symphyotrichum cordifolium

In the US and Canada

Native in Canada and the lower 48 states.

Native range

Alabama, Arkansas, Connecticut, District of Columbia, Georgia, Illinois, Indiana, Iowa, Kentucky, Maine, Manitoba and Maryland.

Measured

Average height 136 cm.

Sources: GBIF (CC BY 4.0), USDA PLANTS and Trefle (CC BY 4.0). The care and design notes are LandscapeGPT's own.