How to Grow Cornflowers in Australia

Cornflower (Centaurea cyanus) is a hardy annual in the daisy family, Asteraceae. It is grown for borders, cut and dried flowers, edible petals and the bees it draws in. It suits most Australian gardens in the cool months; the usual mistake is sowing it into warm weather, where it flowers briefly and fades.

Three mistakes that matter

1. Sowing into the heat. Cornflowers do best in cool weather, so late spring sowings in warm zones flower briefly before heat and humidity finish them. Sow in autumn in tropical, subtropical and inland gardens, at the times in When to plant.
2. Leaving faded flowers on the plant. Heads left to set seed slow the supply of fresh buds, and the display can end early. Cut spent flowers off every few days, as described in Deadheading, and keep picking for the vase.
3. Planting in shade or wet ground. In shade the stems grow weak and flop, and soil that stays wet can rot the roots. Give plants full sun and light, free-draining soil, as set out in Site, soil and setup.

When to plant

Seed germinates best in soil at about 15 to 20°C, and plants grow best in cool weather, so autumn is the main sowing season everywhere. This hardy annual tolerates frost, so temperate, Mediterranean and cool gardens can add a spring sowing that flowers into early summer. Sow a second batch a few weeks after the first to spread the picking.

At the warm start of a window, raise seedlings in trays in a shaded spot and plant them out as the nights cool. In temperate and Mediterranean gardens, Anzac Day falls in the middle of the autumn window; in cool gardens it is the last call.

Site, soil and setup

Choose a spot in full sun with at least six hours of direct light a day. Cornflowers want light, free-draining soil and tolerate poor ground, so the bed needs no manure. They grow in acid, neutral or alkaline soil, and pH rarely needs correcting.

Clear old plant debris before sowing and move to a fresh bed when you can, as damping-off fungi survive between crops on it.

Container growing

'Tom Pouce Mixed', at about 30 cm, is the one to grow in pots. The semi-dwarf 'Dwarf Polka Dot Mix' and the tall Ball types do better in beds. Use a pot holding at least 20 L of fresh potting mix, with drainage holes so excess water runs away from the roots. A 40 cm pot holds about 28 L, room for three plants.

• Watering: water deeply at the base, early in the morning, whenever the top 5 cm of mix is dry.
• Feeding: potted plants need more feeding than beds. Water in fish and seaweed blend at 25 mL per 9 L fortnightly from the first buds.
• Position: full sun, as set out in Site, soil and setup. A pot also makes self-sown seedlings easier to control.

Sowing Seeds

Cornflowers are sown straight into the bed, which suits them best, or raised in trays and planted out young.

Sowing direct

• Sow seed 3 to 5 mm deep in short rows, or scatter it thinly over loosened soil and rake it in lightly.
• Keep the surface moist but never wet until seedlings appear, usually in 7 to 14 days at the temperature in When to plant.
• Once seedlings have their first true leaves, thin them to 20 to 30 cm apart, keeping the strongest.

Starting in trays

• Sow thinly into punnets or cell trays of seed raising mix, 3 to 5 mm deep, and cover lightly.
• Water with a fine mist so the seed is not washed out, and keep the trays in bright light.
• Plant out about 3 to 6 weeks after sowing, once seedlings have true leaves and stand 5 to 10 cm tall.
• Harden off the seedlings first (stand them outdoors for a little longer each day so they adjust to sun and wind).
• Space them 20 to 30 cm apart, or 20 cm for the dwarf mixes, no deeper than they sat in the punnet.
• Water in with seaweed solution at half strength (15 mL per 9 L) to ease transplant shock.

All eight varieties are forms of the one annual species, grown the same way and usually flowering 10 to 14 weeks from sowing.

Care by growth stage

Sun, sharp drainage and steady picking decide how long cornflowers flower. Weeks count from sowing; autumn sowings take longer, as seedlings sit through winter before growing on in spring.

If growth stalls in very poor soil, scratch in a light dose of all-purpose controlled release granular (NPK around 14-5-10) at the label rate.

Deadheading

Start when the first flowers fade and keep going until the planting finishes.

• Spent flowers: cut each one off just above a set of leaves with sharp secateurs or snips.
• Picking: stems cut for the vase count as deadheading, so pick freely.
• Self-sowing: remove every seed head if you do not want seedlings next year, and pull unwanted seedlings while small.
• Seed for birds or next year: late in the season, leave a few heads to ripen on the plant.
• End of season: once flowering slows in the heat, pull plants out and follow them with summer flowers such as zinnias.

Companion planting

Cornflowers share the cool-season timing of many flowers and vegetables, and they bring bees wherever they grow.

Grows well with

• Vegetable beds: a few cornflowers among the crops draw in bees and beneficial insects.
• Calendula: sown in the same months, and another edible flower for a mixed picking bed.
• Iceland poppies and larkspur: sown in the same autumn months for a mixed cutting bed.
• Cosmos, flax and dill, for a spring sowing: sown with cornflowers in a City of Melbourne summer meadow, where the dill fed beneficial insects.

Keep apart

• Tall sunflowers or corn on the north side: their shade leaves cornflower stems weak and leaning.
• Rock edgings and dense groundcover hard against seedlings: snails and slugs shelter there by day and feed at night.

Harvesting Blooms

Cut stems for the vase when the flowers are a quarter to half open, early in the morning once the dew has dried.

• Cut the longest stems you can with clean, sharp snips, just above a set of leaves.
• Strip the lower leaves and stand the stems in a clean bucket of water straight away.
• For drying, cut flowers almost fully open, before they begin to set seed.
• For eating, pull petals from fully open flowers, and only from plants that have not been sprayed.

To dry, band a few stems together and hang them upside down somewhere dry, airy and dark. They are ready in about a month, when stems and flowers feel rigid; the flowers shrink a little as they dry. Box dried stems in tissue paper until needed. Scatter fresh petals over salads and cakes or add them to tea; they have a mild, slightly spicy, clove-like flavour.

Common problems

Seed does not come up. Seed buried too deep, soil that dried out, or soil far outside the temperature in When to plant are the usual causes. Read more

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

Seedlings grow tall and pale. Too little light and too much warmth stretch them, so move trays into full light in a cooler spot. Read more

Flowering stops early. Check the two causes in Three mistakes that matter: seed heads left on the plant, and sowing too late into warm weather.

Pests and diseases

Cornflowers have few serious pests, and most trouble comes from sap-suckers, snails and mildew in crowded, damp plantings.

Aphids (family Aphididae). Small sap-suckers cluster on fresh shoots and under the leaves, causing wilting and discoloured leaves, and leave sticky honeydew that draws ants. Hose them off or spray insecticidal soap at the label rate. Read more

Garden snail (Cornu aspersum) and slugs. They feed after dark, chewing holes in leaves and eating young seedlings, and leave slime trails. Collect them by torchlight, set beer traps, or run copper tape around plastic or wooden pots and beds. Read more

Powdery mildew (order Erysiphales). White patches spread across the leaves into a grey-white film, often starting on older, lower ones. Remove the worst leaves, water in the morning at the roots, and spray 1 part full-cream milk to 10 parts water. Read more

Prevention first

• Space plants as set out in Sowing Seeds, so air moves freely between them.
• Keep the leaves dry by watering as set out in Care by growth stage.
• Start trays with fresh seed raising mix in clean punnets.
• Check shoot tips and fresh transplants for aphids and snails on each visit.

Tips

1. Use the petals of the blue varieties to colour icing or drinks, as the flowers hold an edible blue dye.
2. For florist-style bunches, try 'Black Ball': a flower farmer near Mount Macedon in Central Victoria found dark purple cornflowers a favourite with florists.
3. Check with your local council before letting cornflowers self-sow near bushland or paddocks, as the plant has escaped gardens near Cooma in New South Wales.