How to Grow Amaranth Flowers in Australia

Ornamental amaranths (Amaranthus species) belong to the amaranth family, Amaranthaceae, and are grown for drooping tassels, upright plumes and leaves that colour like a bonfire. They thrive in Australian summer heat and ask for little once established. The usual mistake is to feed them like a hungry vegetable, which gives soft, top-heavy plants and duller leaves.

Three mistakes that matter

1. Sowing before the soil warms. Amaranth seedlings are very sensitive to frost, and seed in cold soil comes up slowly and patchily. Wait for the soil temperature in When to plant, or start seed in cells under cover.
2. Feeding like a vegetable. Rich, nitrogen-heavy soil gives soft, lanky stems that lean or collapse, and Joseph's coat (Amaranthus tricolor) leaves lose their bright colour. Dig in compost and keep later feeds light, as Care by growth stage sets out.
3. Letting the tassels drop their seed. Ripe heads shed seed readily, and several amaranth species have naturalised in Australia. Cut tassels and plumes for the vase or for drying before the seed falls, as Pinching and Deadheading explains.

When to plant

Sow once frosts are over and the soil holds at least 18°C. Seedlings usually appear in one to two weeks at 21 to 26°C. Plants take roughly 100 to 110 days to reach full display, so count back from your first cold nights. For a longer show, sow a second batch about a month after the first.

Where spring is slow to warm, start seed under cover four to five weeks before your last frost.

Site, soil and setup

Give amaranths full sun and shelter from strong wind, because tall, soft stems lean or snap in a gale. They want free-draining soil of average fertility and grow well at a pH of about 6.5 to 7.5.

Avoid following beetroot or silverbeet, which share the beet webworm, and move away from any bed where root-knot nematodes troubled the last crop.

Container growing

Compact 'Pygmy Torch' and the Joseph's coat lines 'Early Splendour' and 'Illumination' suit pots. Love-lies-bleeding (Amaranthus caudatus) can tumble over the rim of a large tub. Use a pot at least 30 cm across, about 13.5 L, filled with fresh premium potting mix. Potting mix can carry Legionella bacteria, so wet it before handling, wear gloves and a mask, and wash your hands afterwards.

• Watering: pots dry out fast in summer, so check them daily and water whenever the top few centimetres feel dry.
• Feeding: water in fish and seaweed blend at 25 mL per 9 L fortnightly.
• Position: full sun, out of strong wind, in a pot heavy enough not to tip once the plant is tall.

Sowing Seeds

Sow amaranths direct where they will grow, and start them in cells only where the warm season is short or slow to arrive.

Sowing direct

• Sow the tiny seed 2 to 5 mm deep into fine, firm soil; mixing it with dry sand helps spread it evenly.
• Keep the surface damp but never soggy until seedlings appear, usually in 7 to 14 days.
• Thin in two or three stages to the spacing in the table below, keeping the sturdiest seedlings.
• Mulch once seedlings stand about 5 cm tall.

Starting under cover

• Sow two or three seeds per cell of seed raising mix, just covered, and keep the tray warm; a seedling heat mat helps.
• Give seedlings strong light from the day they emerge, as weak light makes them thin and spindly.
• Thin each cell to the strongest seedling and plant out after four to five weeks, before roots circle the cell.
• Harden off seedlings over about a week first (moving them outdoors for longer each day so they adjust to sun and wind).

The nine lines fall into three groups; heights and spacings are those on each listing.

Amaranth grown for eating has its own Amaranth guide. Globe amaranth (Gomphrena) and Celosia are separate genera with guides of their own.

Care by growth stage

Steady water while young and a light hand with feed decide how amaranths turn out. Weeks count from sowing.

Set a stake beside each tall or hanging plant at about knee height. Tie the main stem loosely at two points. 'Red Cathedral', love-lies-bleeding and the Joseph's coat lines need this most in windy spots.

If growth stalls in poor soil, a light dose of all-purpose controlled release granular (NPK around 14-5-10) scratched into the soil can help.

Pinching and Deadheading

One early pinch gives bushier plants with more, slimmer stems for cutting, and taking heads off before they shed keeps self-sown seedlings in check.

• Pinch once: when a young plant has six to eight sets of leaves, snip out the growing tip just above a leaf.
• Cut as you go: every stem cut for the vase or for drying is one less head to drop seed in the bed.
• Before seed falls: cut faded tassels and plumes as their colour dulls, and bag them for the bin.
• End of season: pull spent plants before the last heads ripen, and compost only those without seed.

None of the species sold is on a state or territory declared weed list, but several are recorded as naturalised. The yellow-green line's listing also names slim amaranth (Amaranthus hybridus). It is naturalised everywhere except Tasmania.

Companion planting

Amaranths want the same hot, sunny bed as other summer flowers, and tall types make a backdrop for shorter plants in front.

Grows well with

• Marigolds and zinnias: they share full sun and heat, and their flowers sit well against bold amaranth leaves.
• Capsicum, chilli and eggplant: good neighbours that want the same warm soil and long summer.

Keep apart

• Beetroot and silverbeet: beet webworm feeds on both as well as on amaranth.
• Tomatoes, beans and cucumbers where root-knot nematodes are present: all are hosts and keep numbers high.
• Short sun-lovers on the shady side of tall types: plants up to 1.6 m tall shade them for much of the day.

Cutting and Drying Flowers

Cut tassels and plumes when about three-quarters of the tiny flowers along them have opened.

• Cut long stems with clean secateurs, back to a leaf joint.
• Strip leaves that would sit below the waterline and stand stems in clean water straight away.
• For drying, strip all the leaves, tie small bunches and hang them upside down in a warm, dry, airy shed for 2 to 3 weeks.
• Cut Joseph's coat stems for their coloured tops, and expect them to last only about 5 days in a vase.

Fresh tassels and plumes usually last about a week in a vase. Dried stems keep their shape, which is how the genus earned its name, from the Greek for unfading.

Hanging tassels spill over the rim of a tall vase, while upright plumes add height and texture to a dried arrangement.

Common problems

Patchy rows. Poor germination is common in amaranth, and the tiny seed fails if sown too deep or left to dry out.

Seedlings collapse at soil level. Damping off (Pythium species and Rhizoctonia solani) rots young stems, most often in wet soil or mix when temperatures do not suit the seedling. Read more

Thin, floppy seedlings. Low light and too much nitrogen make amaranth seedlings weak and spindly before they reach the garden. Read more

Joseph's coat leaves stay dull. Too much fertiliser, very rich soil or too little sun can leave the top leaves green instead of red and gold.

Pests and diseases

Amaranths are seldom badly troubled, and most damage comes from sap-suckers and leaf-eaters in warm weather.

Aphids (family Aphididae). Small sap-suckers cluster on shoot tips and under leaves, leaving growth distorted and sticky with honeydew. Read more

Beet webworm (Spoladea recurvalis). Brownish-green caterpillars with sparse white hairs live together in silky webs of leaves; pick off and bin webbed leaves when first seen.

Common garden snail (Cornu aspersum). Snails chew seedlings and young leaves overnight, most after rain, so check by torchlight and hand-pick. Read more

Root-knot nematodes (Meloidogyne species). These soil pests swell roots into galls and leave plants stunted, wilting and yellow, worst in warm climates and sandy soils. Read more

Melon thrips (Thrips palmi). In northern Australia these tiny insects rasp leaves until they turn silvery and crinkled, especially along the veins. Read more

Prevention first

• Sow into warm, free-draining soil so seedlings grow away quickly.
• Hose aphids off shoot tips and leaf undersides with a firm jet.
• In the north, check the undersides of new leaves weekly for thrips from planting.
• Rotate beds, and pull and bin plants with galled roots rather than composting them.

Tips

1. In a short cool-zone summer, the compact 'Pygmy Torch' fits a smaller bed and may need no stake.
2. Birds relish ripe amaranth seed. Hang a few cut heads in a paper bag for an aviary or feeder, so the seed stays out of the bed.
3. Run a line of love-lies-bleeding along the edge of a raised bed or low wall, where the tassels can hang free.