How to Grow Astilbe in Australia

Astilbe (Astilbe hybrids), also called false spiraea, is a clump-forming perennial in the saxifrage family, Saxifragaceae. It is grown for feathery plumes of tiny flowers in pink, red and white above fern-like leaves. It lights up damp, shady corners where few other flowers bloom, and it is happiest in cool gardens such as Hobart, Ballarat and Katoomba. The one thing most gardeners get wrong is letting the soil dry out.

Three mistakes that matter

1. Letting the soil dry out. Dry soil shows fast as brown, crisp leaf edges, and a drought can send the whole clump dormant early. Keep the root zone moist from spring to autumn, as set out under Care by growth stage.
2. Planting in hot sun. In full sun the foliage scorches in summer and the soil dries faster. Choose dappled or part shade, with shelter from hot afternoon sun and drying winds, such as a hot northerly in Melbourne.
3. Leaving a clump for too long. An old, crowded clump keeps its healthy leaves but flowers less and less. Lift and divide it about every four years as shoots appear, as shown under Planting Crowns.

When to plant

Plant bare-root crowns while they are dormant, from winter to early spring, before the new shoots unfurl. The roots then settle in cool, moist soil and have a full season to grow before the first summer heat. Astilbe needs a cool winter and a mild summer. It suits the cool and alpine zone and cooler temperate districts best, and struggles in hot, dry or humid climates.

A crown planted late, once shoots are showing, needs watering two or three times a week through its first summer. Where summers are long and dry, a pot in a shaded courtyard is easier to keep moist than a garden bed.

Site, soil and setup

Astilbes come from damp woodland and moist grassland in East Asia and North America, and the garden spot should copy that.

A clump stays in one spot for years, so crop rotation does not apply. On sandy soil, the extra organic matter matters even more, because it holds the water astilbe needs.

Weed status, state by state

Astilbe is not a declared weed in any Australian state or territory.

Spent plumes can self-sow, so cut them before they shed seed if you want only the colour you planted.

Container growing

Compact astilbes up to about 90 cm tall suit pots, and a potted plant can come indoors while it flowers. Use a pot at least 30 cm wide, about 15 L, with drainage holes. Fill it with premium potting mix and add a few handfuls of compost to help it hold moisture.

• Watering. Pots dry out fast, so water whenever the mix is dry 2.5 cm down, which can be daily in summer. See watering pots in summer.
• Feeding. Water in fish and seaweed blend at 25 mL per 9 L every fortnight from first shoots to late summer.
• Position. Bright shade, out of afternoon sun and hot wind. In summer only, stand the pot on a saucer filled with sand, so the roots stay cool without sitting in water.

Planting Crowns

Astilbe is started from a crown: a bare-root clump of buds and fibrous roots, or a division lifted from an established plant.

Crown planting

Unwrap bare-root crowns as soon as they arrive and plant them the same week. A healthy crown feels firm, with plump reddish buds on top and moist roots below. Clumps over about 7.5 cm across can go straight into the bed. Smaller pieces are safer potted up and grown on in a sheltered spot, then planted out once they fill the pot.

Planting a crown

1. Prepare the bed as under Site, soil and setup.
2. Dig a hole just deep and wide enough to take the roots spread out.
3. Bury only the roots, with the buds at or just above the soil surface.
4. Space crowns 45 to 60 cm apart.
5. Firm the soil and water in well with seaweed solution at 30 mL per 9 L, even if the ground is moist.
6. Mulch 5 to 7 cm deep with compost, leaving a 10 cm gap around the crown.

Dividing a clump

Lift the clump with a fork in late winter or early spring, just as the shoots appear. Slice it into pieces with a sharp spade or knife, each with three to five healthy shoots and plenty of roots. Discard the old, woody centre and replant the young outer pieces at their original depth straight away. Water them in and mulch them as for a new crown. See dividing perennials for the method.

From seed

Seed from garden hybrids does not come true, so seedlings vary in colour and height. They take about three years to reach flowering size, which is why named colours are grown from divisions.

Care by growth stage

Water decides almost everything with astilbe, especially in the first year. In rich, moist soil an established clump finds most of the nutrients it needs.

When you water, give a long, deep soak rather than a daily sprinkle, and use tank water where you have it. If growth stalls in poor soil, a light dose of all-purpose controlled release granular (NPK around 14-5-10) scratched in as shoots appear can help.

Cutting Back

Astilbe dies down in autumn and shoots again in spring, so the main job is clearing old growth once a year. The faded plumes turn buff and brown and look good through winter, and they shelter small creatures, so there is no rush.

• Leave spent plumes standing over winter, or cut them for the vase.
• In mid to late winter, cut all old stems and leaves down to ground level.
• Finish before the new shoots push through, so you do not snap the buds.
• Pick off badly mildewed leaves during the season to slow the spread.
• Bin diseased leaves rather than composting them.

Companion planting

Good partners like the same moist shade and contrast with astilbe's fine, ferny texture.

Grows well with

• Hostas, whose broad, bold leaves set off the feathery plumes.
• Ferns, which share the damp shade and fill gaps between clumps.
• Hellebores, which flower in winter and early spring before astilbe begins.
• Primulas and rodgersias, other moisture lovers for bog gardens and pond edges.

Keep apart

• Shallow-rooted, thirsty trees, whose roots steal the moisture astilbe needs.
• Dry-climate plants such as lavender, whose lean, dry conditions would starve astilbe.

Harvesting Blooms

Each variety flowers for about two to three weeks. Cut a stem when half to three-quarters of its flowers are open and the top buds are swollen and coloured.

• Cut in the cool of early morning, low on the stem.
• Stand stems in water straight away, because even a few hours dry shortens vase life.
• Add a cut-flower preservative to the vase water.
• Keep the vase away from ripening fruit, because the flowers are sensitive to ethylene.

Plumes also dry well for dried arrangements. Florists hold fresh stems in water at about 1 to 4°C for up to 7 to 10 days. Bare-root crowns and divisions do not store, so plant or pot them up straight away.

Common problems

Brown, crispy leaf edges. Leaf scorch from heat, dry soil or wind; see Three mistakes that matter, and move the plant to moister shade.

Healthy leaves but no flowers. A crowded clump that needs dividing (see Three mistakes that matter), or shade so deep it cuts flowering. A new crown may not flower well until its second year.

Plant vanishing in autumn. This is normal: astilbe dies down to its crown for winter and reshoots in spring.

Crown failing over winter. Heavy clay holding stagnant water around a dormant crown; improve the soil with organic matter before replanting.

Plants failing in warm, humid districts. Hot summers and mild winters wear astilbe out; grow it in a shaded pot or choose another shade plant.

Pests and diseases

Given moist shade, astilbe is largely trouble free, and most problems follow heat or dry soil.

Black vine weevil (Otiorhynchus sulcatus). A weevil recorded in Tasmania, Victoria, South Australia, New South Wales and the ACT. Adults notch leaf edges at night, and the white, C-shaped grubs eat roots, causing wilting. Pick off adults by torchlight and check the roots of potted plants.

Powdery mildew (Erysiphe species). White, powdery patches spread over the leaves, worst in warm, humid weather. Remove badly infected leaves and spray 1 part full-cream milk to 10 parts water. Read more

Root-knot nematode (Meloidogyne species). Microscopic soil worms that cause lumpy galls on the roots and stunted plants, mostly in warm, sandy soils. Remove affected plants and do not replant astilbe there. Read more

Wilt (Rhizoctonia and Fusarium species). Plants stunt or wilt, with sunken brown patches on the stems near soil level. There is no cure: bin affected plants.

Prevention first

• Plant in moist, humus-rich soil and part shade, so clumps stay vigorous.
• Water the soil, not the leaves, and water in the morning.
• Divide crowded clumps to keep air moving through the foliage.
• Check bought crowns for grubs or soft, rotten roots before planting.

Tips

1. Plant several crowns of one colour together, because astilbe looks best in a drift rather than dotted singly.
2. Watch the new spring leaves: the red- and pink-flowered plants often open with bronze or copper tints.
3. Use a soaker hose under the mulch to keep a shady bed evenly moist through summer.

Due to quarantine restrictions and our shipping policy, we cannot send astilbe plants to Western Australia or Tasmania.