How to Grow Asters in Australia

China aster (Callistephus chinensis) is an annual in the daisy family, Asteraceae, grown for cut flowers, bedding and pots. It flowers in every Australian climate zone, but grown in the same bed each year it often falls to a soil-borne wilt. It is a different plant from the perennial Michaelmas or Easter daisy (Symphyotrichum species).

Three mistakes that matter

1. Sowing into cold soil or planting out too early. Seed sits in cold soil and comes up slowly and patchily, and annual asters are not frost tolerant. Wait for the soil temperature in When to plant, or raise seedlings in trays under cover.
2. Growing asters in the same bed every year. Fusarium wilt (Fusarium oxysporum f. sp. callistephi) builds up in soil where asters keep growing, persists for years and kills infected plants. Move asters to a new bed each year.
3. Watering over the leaves. Wet foliage in damp or humid weather invites the leaf diseases described in Pests and diseases. Water at the base of each plant, and only in the morning.

When to plant

Seed germinates best at 22 to 24°C, so sow direct only once the soil holds 20°C or more. Where spring is cool, as in Canberra or Hobart, start seed in trays under cover about six weeks before the last frost. For a longer run of flowers, sow a second batch two weeks after the first.

Site, soil and setup

Give asters at least six hours of full sun a day. They need free-draining soil rich in organic matter, with a pH between 5.5 and 7.5.

A bed that grew asters can take vegetables or other flowers next year, as the strain of wilt that attacks asters usually spares other plants.

Container growing

The dwarf types, 'Colour Carpet Mix' and 'Milady Mix', suit pots best, along with the branching 'Perfection Mix'. Give each plant a pot of at least 10 L and at least 20 cm across, filled with fresh potting mix.

• Watering: keep the mix evenly moist, watering whenever the surface feels dry.
• Feeding: water in fish and seaweed blend at 25 mL per 9 L fortnightly after planting. Once the first flowers open, switch to liquid potash at the label rate, fortnightly.
• Position: the sunniest spot available, such as a north-facing step or courtyard.

Sowing Seeds

Raise seedlings in trays and plant them out, or sow direct where they will flower once the soil is warm.

Starting under cover

• Sow seed 6 mm deep in punnets or cells of seed raising mix, then firm it down gently.
• Keep the mix moist but never wet, in a warm, bright spot; seedlings appear in 10 to 20 days.
• Plant out after 4 to 6 weeks, once seedlings have true leaves and stand 5 to 10 cm tall.
• Harden off seedlings first (a week or two of longer spells outdoors each day, so they toughen up to sun and wind).

Sowing direct

• Sow a few seeds per spot, 6 mm deep and 30 to 40 cm apart, then water gently.
• Keep the surface moist until seedlings are up, then thin to the strongest one or two at each spot.

Varieties fall into three height groups. 'King Branching Mix', with large doubles, and 'Single Rainbow Mixed', with yellow-centred singles, are cutting types too.

Care by growth stage

Steady moisture and a light hand with feeding decide how well asters flower. Weeks count from sowing in trays.

If plants stay pale or stall before budding, a light dose of all-purpose controlled release granular (NPK around 14-5-10), scratched in, can help.

Deadheading

Removing faded flowers keeps plants tidy and flowering for longer. Pinching out the growing tips is not needed.

• Dwarf bedding types: snip each faded flower off just above a set of leaves, using sharp, clean secateurs or snips.
• Branching cut types: cutting long stems for the vase does the same job, and the more you cut, the more flowers follow.
• All types: clear spent flowers and fallen leaves from the bed, as leaf-disease fungi can survive on old plant debris.

Companion planting

Few companion pairings are proven, so the ones below are chosen for the insects they bring and the diseases they share.

Grows well with

• Sweet alyssum (Lobularia maritima): a low edging in front of asters that draws hoverflies and other predatory insects.
• French marigold (Tagetes patula): like alyssum, it brings beneficial insects into the bed.

Keep apart

• Tomatoes, capsicum and lettuce: all three suffer heavy losses to tomato spotted wilt virus (Orthotospovirus tomatomaculae), which thrips carry between plants.
• Weedy corners: sowthistle (Sonchus oleraceus), capeweed (Arctotheca calendula) and cobbler's pegs (Bidens pilosa) host both the virus and its thrips.

Harvesting Blooms

Cut stems for the vase when a flower is about half open.

• Use sharp secateurs or snips, and cut the longest stems you can.
• Strip any leaves that would sit below the waterline.
• Stand stems in a clean bucket of water as soon as they are cut.

In a clean vase, stems usually last 7 to 10 days. For dried flowers, leave blooms on the plant until they are fully open, then cut them.

Common problems

Seedlings collapse at soil level. Damping off (Pythium species and Rhizoctonia solani) rots young stems and roots, most often in wet seed raising mix or soil. Read more

Stems stay short. Heat and shortening days push plants into flower early, so late sowings often bloom on short stems.

Tall stems lean or snap. Tall types carrying a full load of flowers can fall over in wind or heavy rain unless staked early.

Pests and diseases

Snails threaten seedlings first, then sap-sucking insects and soil or leaf diseases take over as plants grow.

Common garden snail (Cornu aspersum). Snails and slugs graze tender leaves and shoots at night and can strip newly planted seedlings. Read more

Aphids (family Aphididae). These small sap-suckers cluster on new shoots and flower stems, where leaves can wilt or discolour and sticky honeydew draws ants. Read more

Tomato spotted wilt virus (Orthotospovirus tomatomaculae). Western flower thrips (Frankliniella occidentalis) and other thrips spread it, leaving ringspots, mottling and line patterns on leaves. Infected plants cannot be cured. Read more

Fusarium wilt (Fusarium oxysporum f. sp. callistephi). This soil fungus blocks the water-carrying vessels, so plants yellow, wilt and die, often with blackened, rotting stem bases. Rest an affected bed from asters for four to five years, or try the Fusarium-tolerant 'Matsumoto Blue' there.

Powdery mildew (Golovinomyces species). White powdery spots spread over the leaves, mostly on older leaves in damp or humid weather, and heavy infections yellow the leaves. Read more

Prevention first

• Hunt snails and slugs by torchlight while plants are small.
• Check shoot tips weekly, and hose aphids off with a firm jet of water.
• Pull out and bin, rather than compost, any plant that wilts suddenly or shows ringspots.
• Raise your own seedlings or check bought punnets closely, as infected seedlings can bring the virus in.

Tips

1. Set dwarf types at the front of a bed, medium types in the middle and tall types at the back, so no plant hides another.
2. Grow a separate row just for cutting, so picking for the vase does not thin the garden display.
3. Leave the last flowers of the season standing, as asters draw bees, butterflies and other pollinators.