How to Grow Celosia in Australia

Celosia (Celosia argentea) is a warm-season annual in the amaranth family, Amaranthaceae, grown for garden colour and for flowers to cut fresh or dry. Three forms are sown: plumed (Plumosa group), crested or cockscomb (Cristata group) and wheat or spiked (Spicata group). It thrives in heat, and the usual mistake is starting it in cold soil.

Three mistakes that matter

1. Sowing into cold soil. Temperatures below about 15°C stunt celosia and push it into flowering early, on short stems with small heads. Wait for the soil warmth set out in When to plant, and plant out only once nights stay warm.
2. Leaving seedlings in punnets. Celosia roots dislike disturbance, and seedlings held until they are root-bound stay stunted and flower too soon. Sow into individual cells and plant out young, as described in Sowing Seeds.
3. Treating it as a drought plant. Celosia handles heat, but dry spells stunt the plants and bring on early, undersized flowers. Water steadily and mulch early, as set out in Care by growth stage.

When to plant

Seed germinates best when the soil holds 21 to 27°C, and the plants are frost tender. Sow once that warmth arrives and frost has passed. A seedling heat mat lets you start trays under cover up to 6 weeks before the last frost.

Sow again every 3 to 4 weeks for a steady run of flowers.

Site, soil and setup

Choose full sun, at least 6 hours a day, because plants in too little sun grow leggy and flower poorly. Celosia prefers fertile, well-drained soil rich in organic matter, at pH 6.0 to 7.0. Tall crested types can topple under the weight of their heads, so give them a spot out of strong wind.

Rotate beds, and avoid following tomatoes, capsicum or other root-knot nematode hosts, because celosia is a host too.

Container growing

The dwarf types suit pots and window boxes. Grow one plant in a pot at least 20 cm wide and deep, which holds about 5 L of fresh potting mix. Potting and seed raising mixes can carry Legionella bacteria, so wet them down, wear gloves and a P2 mask, and wash your hands afterwards. Good choices are 'Lilliput Mixed', 'Lilliput Kewpie Mix' and 'Lilliput Golden Feather'.

• Watering: water whenever the top of the mix feels dry, which in summer heat can mean daily.
• Feeding: fish and seaweed blend at 25 mL per 9 L fortnightly, then liquid potash at the label rate fortnightly from first flowers. A fruiting and flowering soluble (NPK around 15-4-26) at the label rate fortnightly suits too.
• Position: as for beds, see Site, soil and setup.

Sowing Seeds

Start celosia in cell trays for planting out, or sow it direct where summers are long and warm.

Starting under cover

• Fill a cell tray or small pots with fresh seed raising mix, one cell per plant.
• Sow a few seeds per cell and cover them very lightly, no more than 3 mm deep. Snip off all but the strongest seedling later.
• Keep the mix moist but never wet, misting or watering from below so the fine seed stays put.
• Keep the tray at the soil temperature in When to plant; seedlings usually appear in 1 to 3 weeks.
• Move seedlings into bright light as soon as they show, and grow them on at about 17 to 20°C.

Sowing direct

Rake the bed fine, sow thinly, 2 to 3 mm deep, and keep the surface moist until seedlings appear. Thin them to the spacing in the variety table below. Weed seed germinates at the same time, so hand weed early.

Transplanting

• Plant out at the two to four leaf stage, often about 4 weeks after sowing, before roots circle the cell.
• Harden off the seedlings first (stand them outdoors for a little longer each day so they adjust to sun and wind).
• Plant out after the last frost, once nights stay above about 15°C.
• Space plants as the variety table below sets out, and firm the soil gently around each one.
• Water in with seaweed solution at half strength (15 mL per 9 L) to ease transplant shock.

Care by growth stage

Warmth and steady moisture decide how big celosia grows before it flowers, and so how long its stems are. Weeks count from sowing.

Deadheading

Pinching gives more stems and removing spent heads keeps new ones coming, in ways that depend on the form.

• Plumed types: pinch out the growing tip when plants are 15 to 30 cm tall, just above a pair of leaves.
• Crested types: leave them unpinched for one large comb, or pinch at 20 to 30 cm for several smaller ones.
• Wheat types: these branch freely by themselves and rarely need pinching.
• Spent flowers: cut faded heads back to a side shoot or leaf joint to prolong flowering.
• Seed: remove heads before seed ripens unless you want self-sown seedlings (see Tips).

Companion planting

Good partners want the same heat, full sun and steady water.

Grows well with

• Zinnia: sown at the same time for summer colour, it shares the sun and the cutting bed.
• French marigold: planted thickly, its roots can cut root-knot nematode numbers, as larvae die inside them.

Keep apart

• Tomato, capsicum and other root-knot hosts: they build up the same nematodes in the soil.
• Tall sunflowers on the north side: their shade makes celosia leggy, with poor flowers.

Harvesting Blooms

Cut plumed and wheat types when about half the flowers on the spike are open. Cut crested types once the centre of the comb has expanded, before the outer edge discolours.

• Check that the stem feels firm just below the flower head.
• Cut the longest stem you can with sharp secateurs or snips.
• Strip the lower leaves and stand stems straight in a clean bucket of water.
• For drying, cut at the same stage and strip every leaf from the stem.
• Hang bunches in a dark, well-ventilated place to dry quickly, which keeps the colour brightest.
• Handle dried wheat types gently, as the spikes tend to shatter.

Fresh stems usually last 5 to 14 days in a vase. Crested and plumed heads dry well for everlasting arrangements, and 'Flamingo Feather' dries to a snowy white.

Common problems

Seed does not come up. Cold mix, seed buried too deep and a surface left to dry out are the usual causes; see Sowing Seeds. Read more

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

Seedlings grow tall and pale. They stretch when they do not get bright light straight after germinating. Read more

Plants stay small and flower early. Cold, root-bound punnets and dry spells all cause it; see Three mistakes.

Crested heads turn brown and rot. Crowding, rain and high humidity rot flower heads, so space plants well and water at soil level.

Dried flowers drop seed indoors. Heads cut late have already set seed, which shatters as they dry, so cut at the stages in Harvesting Blooms.

Pests and diseases

Celosia has few serious pests; most trouble comes from sap-suckers in hot weather and from soil nematodes.

Aphids (family Aphididae). Green peach aphid (Myzus persicae) and cotton aphid (Aphis gossypii) cluster on growing tips, and heavy colonies stunt plants and deform flower heads. Hose them off or spray insecticidal soap at the label rate; ladybirds and hoverfly larvae eat them too. Read more

Two-spotted mite (Tetranychus urticae). These mites, under 0.5 mm long, feed under the leaves and leave yellow, stippled patches, and they multiply fast in hot, dry weather. Hose the leaf undersides and avoid broad-spectrum sprays, which kill the predatory mites and ladybirds that control them. Read more

Western flower thrips (Frankliniella occidentalis). These slender insects, 1.5 to 2 mm long, leave silvery, flecked leaves and distorted flowers. Tap flowers over white paper to spot them, then spray insecticidal soap at the label rate into buds and leaf undersides. Read more

Root-knot nematode (Meloidogyne species). Plants stay stunted, yellow and wilt in heat, and the roots carry lumps 1 to 10 mm across. Pull and bin affected plants, dig in compost, and rotate the bed. Read more

Garden snail (Cornu aspersum) and slugs. They feed at night and can eat newly planted seedlings to the ground. Hunt them by torchlight about two hours after sunset, set beer traps, or run copper strips around pots. Read more

Prevention first

• Raise seedlings in fresh seed raising mix, in clean trays kept off the ground.
• Space plants as set out in Sowing Seeds, so air moves between them.
• Water at soil level in the morning, keeping the leaves and flower heads dry.
• Check shoot tips and leaf undersides on each visit in hot, dry weather.

Tips

1. Celosia is naturalised and listed as an environmental weed in Queensland, and has naturalised in the Kimberley in Western Australia. In the north, cut heads before seed ripens, and never let plants self-sow near bushland.
2. For a massed bed that flowers into autumn, pair plumed celosia with marigolds and globe amaranth.
3. Grow 'Flamingo Feather' for the vase: its tall stems carry little foliage, so there is less to strip.