How to Grow Rosella in Australia

Rosella (Hibiscus sabdariffa) is a frost-tender annual in the mallow family, Malvaceae. It is grown for the fleshy red calyces that swell around each seed pod after the flower falls. Queensland once made rosella jam in factories for export, and the tart calyces still set a jam without added pectin. The mistake most gardeners make is sowing too late.

Three mistakes that matter

1. Sowing late or into cold soil. Rosella flowers as the days shorten after midsummer, whatever the sowing date. A late sowing reaches that point as a small plant and, in southern gardens, meets the cold before its calyces ripen. Sow at the start of your window.
2. Picking calyces too late. Calyces left on the bush turn tough and stringy, and a plant left to ripen its seed stops flowering sooner. Pick them plump, while they still snap off by hand, as Harvesting Calyces describes.
3. Planting into heavy, wet soil. Root rot strikes in poorly drained ground: plants grow well, then suddenly wilt as if dry, and nothing saves them. Plant into a raised bed or build up the soil with compost first.

When to plant

Soil warmth sets the date. Seed needs soil of at least 20°C and comes up best above 25°C. The plant then needs five to six warm, frost-free months to ripen a crop. Sow once: a single planting keeps cropping for months, until the cold arrives.

At the early edge of a window, raise seed indoors on a seedling heat mat and plant out once the soil warms. Past the end of it, wait for the next season.

Site, soil and setup

Rotate beds so rosella does not follow the root-knot nematode hosts named under Companion planting, because that pest is its main enemy.

Container growing

Rosella grows in a pot but needs a large one, because it reaches 1.5 to 2 m in a season. Grow one plant per pot of at least 45 L and about 40 cm across, with drainage holes. Fill it with a premium potting mix that meets the Australian Standard.

• Water: push a finger into the mix daily in hot weather and water when the top few centimetres are dry. See watering pots in summer.
• Feed: mix a controlled release granular for containers into the potting mix at planting, at the label rate. From first flowers, give fish and seaweed blend at 25 mL per 9 L fortnightly.
• Position: full sun, in the warmest spot you have.

Sowing Seeds

Sow rosella direct where the soil is already warm, or raise it in pots under cover and plant out once the soil has warmed.

Starting in pots

• Sow seed 10 to 12 mm deep in seed raising mix, in punnets or 10 cm pots.
• Keep the mix moist and warm; seedlings usually appear in 5 to 21 days.
• Plant out at 5 to 10 cm tall, usually 4 to 6 weeks after sowing.
• Harden off (accustom seedlings to sun and wind outdoors) for about a week first.
• Space plants 50 to 60 cm apart, in rows about 1 m apart, then water in and mulch.

Sowing direct

Once the soil holds 20°C, sow two or three seeds 10 to 12 mm deep at each spot, 50 to 60 cm apart. Keep the soil damp until seedlings appear, then thin each spot to the strongest one.

Care by growth stage

Regular deep water and a light hand with nitrogen decide the crop, because heavy nitrogen builds leaf at the expense of flowers. Weeks count from sowing and shift with zone and day length.

Companion planting

Rosella shares root-knot nematode with many summer vegetables, so what grows beside it, and after it, matters.

Grows well with

• French marigold, planted through the bed, whose roots stop root-knot nematode completing its life cycle.
• Onions, cabbage or sweet corn as the next crop in the bed, because they resist root-knot nematode.

Keep apart

• Tomato, capsicum and eggplant, which host root-knot nematode and build it up in shared ground.
• Beans, cucumber and pumpkin, other common root-knot nematode hosts.
• Ornamental hibiscus and okra, mallow family relatives that host cotton harlequin bug.
• Low, sun-loving crops on its southern side, which a 2 m rosella shades.

Harvesting Calyces

The first calyces are usually ready 3 to 6 months after sowing, about 3 weeks after each flower. Pick them when they are plump and 2 to 3 cm across.

• Pick while calyces still snap off easily by hand, which is easiest in the morning.
• Use secateurs on lower calyces whose stems have toughened.
• Work up from the bottom of each branch, where calyces ripen first.
• Pick regularly, starting with the first small flush: harvesting prompts new flower buds and lengthens the season.
• Leave a few lower calyces to mature if you want seed, as Tips describes.

To prepare them, cut around the tough base of each calyx and push out the green seed capsule. Fine hairs on the capsule can end up in jam or cordial, so strain anything cooked with the capsules through fine cloth.

Freeze calyces for later jam and sauces. To dry them, spread them on trays somewhere warm and out of direct sun, or use a low oven or fruit dryer. Keep dried calyces in a clean, dry glass jar in a dark cupboard. Cook them into jam, jelly, cordial and sauces, boil dried calyces for tea, and cook the tart young leaves like spinach.

Common problems

Seed slow to come up. Cold soil is the usual cause, so move pots somewhere warmer or wait for the soil warmth set out in When to plant. Read more

A big green bush with no flowers. Rosella is a short-day plant that holds its flowers until the days shorten, often from about January around Brisbane. A bush that stays leafy well into autumn has usually had too much nitrogen, so switch to the potash feeds in Care by growth stage.

Flowers but no ripe calyces. In cool southern gardens, cold nights can arrive before the calyces fill, as Three mistakes explains. Treat the young leaves as the harvest if the calyces fail.

Seedlings come up everywhere. Rosella is naturalised across northern Australia and is listed as an environmental weed in Western Australia and the Northern Territory. Pull plants out once picking ends and gather every seed pod, so no seed reaches bushland or creeks.

Pests and diseases

Rosella has few serious pests, and before any spray, check that its label names the crop or situation.

Root-knot nematode (Meloidogyne species). These microscopic soil worms cause lumpy galls on the roots, and plants yellow, stunt and wilt, often at flowering. Do not plant where the pest is known, and bag and bin affected plants rather than composting them. Read more

Mealybugs (family Pseudococcidae). These small sap-suckers, covered in white powder, cluster along stems and leaf veins and leave honeydew that grows sooty mould. Wipe them off or spray insecticidal soap, and keep ants off the plants. Read more

Aphids (family Aphididae). Soft green or yellow sap-suckers crowd onto new shoots and flower buds and distort the growth. Squash small colonies or hose them off with a strong jet of water. Read more

Cotton harlequin bug (Tectocoris diophthalmus). Found in eastern Australia, this shield bug feeds on young shoots of the mallow family and can make flower buds drop early. Pick adults and young bugs off by hand.

Leaf-eating beetles (Nisotra breweri and others). Several beetles in Australia chew the leaves of rosella. Light damage rarely affects the calyx harvest, so tolerate a few holes.

Powdery mildew (Oidium species). White, powdery patches spread over the leaves where air is still. Water at the roots early in the day, and spray 1 part full-cream milk to 10 parts water in the morning. Read more

Prevention first

• Check shoot tips, buds and leaf undersides weekly for sap-suckers and bugs.
• Keep ants off plants, because they guard aphids and mealybugs from predators.
• Rotate beds and keep the soil rich in compost and mulch, which root-knot nematode dislikes.
• Spare the bees: if you must use pyrethrum, spray late in the day, away from open flowers.

Tips

1. Pinch out the tips of young plants for bushier growth and more flowering shoots. In the subtropics, keep tip pruning until January.
2. Plant three or four in a row as a summer screen, which shelters the patch from hot northerly or westerly winds.
3. For seed, let a few lower calyces dry on the bush until the pods turn brown and split. Freeze the seed for 24 hours before storing it, which kills weevils.
4. Pick a few flowers as well: each lasts less than a day, and dried flowers make a tea like the calyces.