How to Grow Cosmos in Australia

Cosmos (Cosmos bipinnatus and Cosmos sulphureus) is a frost-tender annual in the daisy family, Asteraceae, grown for cut flowers and for bees. It comes from Mexico and thrives on heat, sun and ordinary soil, which most Australian gardens have in good supply. What most gardeners get wrong is kindness: rich soil and regular feeding give tall, leafy plants with few flowers.

Three mistakes that matter

1. Feeding it like a vegetable. Manure, blood and bone and nitrogen feeds push soft, top-heavy growth that flops and flowers late. Sow into ordinary soil, skip the manure, and feed only as set out in Care by growth stage.
2. Sowing into cold soil. Cosmos is frost tender, so seed sown into cold ground sits idle, and a late frost kills the seedlings that do come up. Wait until the soil has warmed and the last frost has passed.
3. Letting spent flowers set seed. A plant busy making seed slows its flowering, and the dropped seed comes up all over the bed next season. Cut flowers for the house and deadhead the rest, as set out in Deadheading.

When to plant

Seed comes up fastest in soil of about 20 to 27°C. Sow once the soil holds at least 16°C and the last frost has passed. Cosmos flowers best while days are 14 hours or shorter, which is why northern gardens sow through the dry, mild months. A second sowing 2 to 4 weeks after the first keeps the vase full into autumn.

Where frost lingers into spring, sow in cells under cover 5 to 7 weeks before the last frost and plant out once it passes. In Canberra and on the tablelands, that is traditionally around Melbourne Cup weekend.

Site, soil and setup

Give cosmos full sun, at least 6 hours a day, in a spot sheltered from strong wind so the tall stems stay upright. Any free-draining soil suits it. Garden cosmos grows in neutral to alkaline soil and sulphur cosmos in acid to neutral soil, so most gardens need no pH correction.

Cosmos sits outside vegetable rotations, but avoid beds where seedlings collapsed last season: the organisms behind damping off live in the soil.

Container growing

Choose a pot of at least 20 L with drainage holes, such as a 40 cm pot, which holds about 28 L. The 30 cm 'Carpet Formula Mix' and the bushy 70 cm 'Polidor Mix' suit pots best. Bagged potting mix can carry Legionella bacteria: dampen it first, wear a mask and gloves, and wash your hands afterwards.

• Watering: water deeply when the mix is dry about 5 cm down, and never leave the pot standing in water.
• Feeding: from the first buds, give fish and seaweed blend at 25 mL per 9 L fortnightly. A fruiting and flowering soluble (NPK around 15-4-26) at the label rate does the same job.
• Position: full sun, out of strong wind.

Sowing Seeds

Sow cosmos where it is to flower, or start garden cosmos in cells. Sulphur cosmos dislikes being moved, so sow it direct.

Sowing direct

• Rake the soil fine and sow the long, thin seed thinly, 5 to 6 mm deep.
• Firm the soil over the seed and water gently. Keep it moist, never wet or dry.
• Seedlings usually appear in 1 to 3 weeks.
• Thin to 40 to 50 cm apart once the first true leaves show.

Starting in cells

• Fill cell trays with seed raising mix and sow two seeds per cell, 5 to 6 mm deep.
• Keep the tray warm and moist, then move it into full light as soon as seedlings emerge.
• Snip the weaker seedling in each cell at soil level.
• Harden off the seedlings (stand them outdoors for a few hours, then longer each day, until they take a full day of sun and wind).
• Plant out after the last frost, once seedlings are 5 to 10 cm tall with true leaves. Water in with seaweed solution at half strength, 15 mL per 9 L.

The seven varieties belong to two species grown the same way. Garden cosmos has feathery leaves and cool colours; sulphur cosmos has coarser leaves, hot colours and more heat tolerance.

Chocolate cosmos (Cosmos atrosanguineus) is a perennial raised from cuttings, not seed, and is not covered here.

Care by growth stage

Water and restraint decide how well cosmos flowers, not fertiliser. Established plants tolerate dry spells, so water deeply and less often.

Deadheading

A plant left to set seed slows down, so pinch early and remove spent flowers to keep cosmos branching and flowering through the warm months.

• Young plants: at about 20 cm tall, pinch out the top few centimetres just above a set of leaves, and stockier, branching stems follow.
• Through flowering: snip faded flowers regularly, just above a set of leaves.
• Tired sulphur cosmos: cut the whole plant back to 30 to 45 cm, and it usually flowers again in about a month.
• End of the season: leave a few heads to ripen only where self-sown seedlings are welcome, then pull the plants.

Companion planting

Cosmos pairs best with other sun-loving flowers and with crops that gain from the insects it draws in.

Grows well with

• Zinnias: sown in the same warm months, they make a matching summer cutting bed.
• Marigolds: a lower, warm-season edging in front of tall cosmos.
• Cornflowers: sown from autumn, they flower through spring and hand the bed over to cosmos.
• Tomatoes: sulphur cosmos flowers shelter praying mantis and assassin bugs, which hunt grasshoppers and caterpillars.

Keep apart

• Violas and other low bedding on the south side: tall cosmos shades them out.
• Pumpkins: their runners smother cosmos seedlings, and the rich soil they need makes cosmos leafy.

Harvesting Blooms

Cut for the vase when the first flower on a stem is just opening, before its petals flatten out.

• Pick often, even daily, which also does most of the deadheading.
• Cut long stems just above a set of leaves, and the plant branches below the cut.
• Buds already showing colour can be cut too, and may last longer in the vase.
• Strip the lower leaves and stand the stems in a clean bucket of water straight away.

Cosmos lasts about 4 to 6 days in a vase, a little longer with a floral preservative. It keeps poorly out of water, so cut it as you need it. The flowers of sulphur cosmos also give a yellow natural dye.

Common problems

Seed does not come up. Cold soil, dry soil or seed buried too deep are the usual causes. Read more

Seedlings collapse at soil level. Damping off (a seedling rot, chiefly from Pythium species and Rhizoctonia solani) strikes in wet, cold soil or mix. Read more

Seedlings grow tall and pale. They are reaching for light, so move trays into full sun the day seedlings emerge. Read more

Lots of leaves, few flowers. Rich soil is the usual cause, the first point in Three mistakes that matter; young plants also flower more slowly in long midsummer days.

Plants flop over. Overfed, unpinched plants grow tall and top-heavy, so grow them lean, pinch them young and shelter tall types from wind.

Pests and diseases

Cosmos has few serious problems: sap-suckers in warm weather, mildew late in the season and snails on seedlings.

Aphids (superfamily Aphidoidea). They cluster on new shoots and buds, leaving sticky honeydew that turns black with sooty mould. Hose them off or snip the worst tips, and spray insecticidal soap if they return. Read more

Plague thrips (Thrips imaginis). These insects, only 1 to 2 mm long, feed in flowers and on leaves, leaving silvery speckles and streaks, and they peak in spring. Rinse the leaves and spray insecticidal soap weekly for 4 to 6 weeks. Read more

Powdery mildew (fungi of the order Erysiphales). White powdery patches spread over the leaves in warm, humid weather, most often as plants age. Remove the worst leaves and spray 1 part full-cream milk to 10 parts water. Read more

Garden snail (Cornu aspersum) and grey field slug (Deroceras reticulatum). They chew seedlings to stalks overnight. Hand-pick them by torchlight and set beer traps or an iron-based bait. Read more

Grey mould (Botrytis cinerea). Ageing petals turn soft and brown under grey fur when cool weather brings heavy dew. Remove spent flowers promptly and water at soil level.

Prevention first

• Keep to the spacing in Sowing Seeds, so the foliage dries quickly after rain.
• Water the soil, not the leaves, early in the day.
• Remove fading flowers before they rot on the plant.
• Leave ladybirds, lacewings and hoverflies to eat aphids, and spray only the pests you can see.

Tips

1. Sow 'Versailles Mixed' beside a taller Sensation mix: Versailles flowers earlier, so the patch starts sooner.
2. Press a few flowers of 'Carpet Formula Mix', which keep their colour once dried.
3. Both species self-seed, and sulphur cosmos has gone wild in the east Kimberley in Western Australia. Near bushland, deadhead before seed drops and check your state or territory weed advice.