Blue wood aster

Blue wood aster: how do you recognise it?

Habit

A clump of upright stems, much branched towards the top, about 1 m tall and 60 cm wide.

Leaves

The lower leaves are heart-shaped, toothed and long-stalked; higher up they become smaller and narrower.

Flowers

Small, starry flower heads about 1.5 cm across in loose sprays, in September–October.

Colour

Pale blue to lilac, sometimes almost white, with a yellow centre that later turns reddish purple.

Roots

Fibrous roots and a short rootstock; the clump widens gradually.

Flowering: September–October · Plants: March–May and September–October · Pruning: March and May

Why grow it?

• Late flowers in September and October, with countless small blooms per plant.
• Tolerates partial shade, where most asters flower less well.
• An important source of nectar for bees, bumblebees and butterflies in autumn.
• Loose, airy clouds of flowers that weave beautifully through other perennials and grasses.
• The heart-shaped leaves form a fresh clump early in the year.
• Fully hardy and easy to look after.

Blue wood aster: where does it grow best?

• Sun or partial shade; in light shade the flowers stay fresh for longer.
• Loam or clay, rich in humus, acid to neutral.
• Normally moist; water during long dry spells.
• Plant 50–60 cm apart, for example on a woodland edge, among shrubs or in a natural-style border.

Light

Sun and partial shade

Soil type

Loam or clay

Acidity

Acid or neutral

Moisture

Normal

Size

1 m tall, 0.6 m wide

Hardiness

Hardy (to -30 °C)

Goes well with

• Japanese anemone: Flowers at the same time and also likes partial shade.
• Blue-stemmed goldenrod: The yellow garlands of Solidago caesia beside the pale blue clouds, on the same woodland edge.
• Ferns: A fresh green underplanting between the asters.
• Smooth hydrangea: Its flower heads turning green in autumn go beautifully with the small blue flowers.
• Japanese forest grass: A cascading green or golden edging at the front in partial shade.

Blue wood aster: when and how to plant it?

Plant in March–May and September–October.

1. Plant from March to May or in September–October, when the soil is neither frozen nor waterlogged.
2. Loosen the soil and work in a few spadefuls of compost.
3. Dig a hole well larger than the pot and set the plant at the same depth as it was in its pot.
4. Leave 50–60 cm between plants.
5. Firm the soil and water in thoroughly straight away.
6. Spread a layer of compost or leaf mould as a mulch around the base.

Care

Watering

Water during prolonged drought, especially in sun and in the first year.

Feeding

A little compost in spring; too much fertiliser gives long, floppy stems.

Support

In rich soil or shade the stems may flop: support them with twiggy sticks or place the plant among sturdy neighbours.

Mulching

A layer of compost or leaf mould in spring keeps the soil moist.

Dividing

Dividing every 3–4 years keeps the plant young and free-flowering.

Tip: If your aster grows in shade or rich soil, cut the stems back by a third in late May: it will stay more compact and will not need staking.

Blue wood aster: when to prune it?

Pruning months: March and May.

When: Asters need little pruning. Cut the old stems down in February–March; for a shorter, sturdier plant, cut the stems back in late May.

How:

• In late winter, cut the dead stems back to just above the ground.
• In late May, cut the stems back by a third to a half: the plant stays shorter, branches more and flowers a little later.
• In autumn, feel free to leave the faded stems standing; they look lovely with frost.

Caution: Do not cut back after the end of June, or the plant will flower less.

Pruning: when and how · Pruning calendar and basic rules

Propagation

Division

In March–April, cut pieces with shoots from the edge of the clump and replant them straight away; the old centre can be discarded.

Cuttings

In April–May, take young shoots of 5–8 cm from the base and root them in moist cutting compost under plastic.

Seedlings

The plant sometimes self-seeds; seedlings can be moved in spring. They do not always resemble the parent.

Blue wood aster: how does it get through the winter?

Hardy to −30 °C.

• Fully hardy, to about −30 °C: no protection needed.
• Leave the stems standing until late winter: they give shelter to insects.
• In a pot: stand it in a sheltered spot and do not let the compost become sodden.

Diseases and pests

• Powdery mildew: White powder on the leaves in dry late summers, mainly on the lower leaves. Water in dry spells, leave space and divide regularly.
• Bare base: The lower leaves turn brown and drop, especially in drought. Water and plant shorter plants in front.
• Slugs: They eat the young shoots in spring. Pick them off in the evening.
• Flopping stems: Caused by too much shade or fertiliser. Cut back in late May or give support.
• Seedlings: In loose soil it can self-seed. Weed out unwanted seedlings while small or cut the stems after flowering.

Through the year

• March: Cut down old stems. Add compost. Plant and divide.
• April: Plant. Take cuttings. Watch for slugs.
• May: Plant. In late May, cut the stems back if you wish.
• June: Put in supports if needed. Water in dry spells.
• July: Water in dry spells.
• August: Buds appear. Water in dry spells.
• September: Flowering begins. Plant and divide.
• October: Full flower, busy with bees and butterflies. Plant.
• November: Leave the faded stems standing for winter.

Good for wildlife

Bees

Nectar and pollen for honeybees and wild bees.

Butterflies

Nectar for butterflies.

Did you know?

• Following DNA studies, most American asters were moved to a genus of their own, Symphyotrichum; the name Aster now mainly covers European and Asian species.
• Aster comes from the Greek word for ‘star’, after the shape of the flower heads.
• Cordifolium means ‘with heart-shaped leaves’, after the lower leaves.