How to Grow Marigolds in Australia

Marigold (Tagetes species) is a frost-tender annual of the daisy family (Asteraceae), grown for bedding, cut flowers, edible petals and companion planting. Most garden types are French (Tagetes patula), African (Tagetes erecta) or signet (Tagetes tenuifolia), and none are calendula, the pot marigold with its own guide. Marigolds thrive in Australian summer warmth, and a common mistake is sowing before the soil has warmed.

Three mistakes that matter

1. Sowing into cold soil. Seed and seedlings in cold, wet soil are the most likely to rot or damp off, and a late frost kills young plants outright. Hold off until the soil has warmed and frost has passed, using the cues in When to plant.
2. Counting on a few plants to stop nematodes. A border of marigolds dotted among tomatoes does little to protect them from root-knot nematodes (Meloidogyne species). The effect comes from a solid bed of French marigolds dug into the soil, as set out in Tips.
3. Watering over crowded plants. Wet, packed foliage and petals invite grey mould (Botrytis cinerea), and the weight of sprinkler water can snap tall African stems. Space plants to suit their type and water the soil, not the flowers.

When to plant

Marigold seed germinates best once the soil holds 21°C to 27°C, and the plants cannot take frost. One sowing usually flowers from late spring until the cold arrives, as long as spent flowers are removed.

In the tropics, heavy wet-season rain rots flowers, so sowings made then need a spot sheltered from rain.

Site, soil and setup

Give marigolds a warm spot in full sun, in light, well-drained soil of average fertility. A pH of about 6 to 7 suits them, and compact types such as 'Calando Mix' also manage part shade.

In a vegetable rotation, a summer of French marigolds gives a bed that has grown tomatoes a useful break (see Tips).

Container growing

Give each plant a pot at least 20 cm wide and 20 cm deep, about 5 L, filled with fresh potting mix. Compact French types suit pots best. Try the extra dwarf 'Petite Series Harmony', the dwarf 'Bolero', 'Sparky Mixed' at 30 to 35 cm or 'Bonita Mixed' at about 30 cm.

• Watering: water when the top of the mix feels dry, and never leave the pot sitting in a saucer of water.
• Feeding: from the first true leaves, water in worm casting liquid diluted to the colour of weak tea, fortnightly.
• Position: full sun, such as a north-facing courtyard, step or balcony.

Sowing Seeds

Sow marigolds direct where they are to flower, or raise seedlings in punnets for an earlier start where spring is cool.

Starting under cover

• Sow seed 2 to 3 mm deep in punnets of seed raising mix and keep it moist.
• Keep the punnets somewhere warm; seedlings usually appear in 5 to 10 days.
• Plant out 4 to 6 weeks after sowing, once seedlings reach about 5 cm and frost has passed.
• Harden off seedlings first (a week or so of longer spells outdoors each day, so they toughen up to sun and wind).

Sowing direct

• Sow 2 to 3 mm deep once the soil has warmed, at the spacing for the type in the table below.
• Water gently and keep the surface moist until the seedlings are up.
• Thin to the strongest seedling at each spot once true leaves show.

Care by growth stage

Sun, warmth and a light hand with feeding decide how well marigolds flower. Weeks count from sowing.

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.

Deadheading

Removing spent flowers stops plants setting seed and keeps new buds coming, and the method depends on the type.

• Young plants: pinch out the growing tip to make them branch.
• French types: pinch or snip off each head as it fades.
• African types: cut spent heads back to a leaf, or pick long stems for the vase, which does the same job.
• Signet types: if flowering slows in midsummer, shear the plants back by a third.
• Wet weather: take off any browning or mouldy flowers at once and put them in the bin.
• Late in the season: leave a few heads to ripen if you want seed.

Companion planting

French marigolds have a measured effect on two tomato pests.

Grows well with

• Tomatoes: in a glasshouse trial, French marigolds planted among tomatoes from the start slowed the build-up of greenhouse whitefly (Trialeurodes vaporariorum).
• Any summer vegie bed: the flowers feed bees and beneficial insects through the warm months.

Keep apart

• Potatoes, sweet corn and cabbages where African black beetle (Heteronychus arator) is a problem: all are hosts of the same pest.
• Stinking Roger (Tagetes minuta): a weed of disturbed ground, listed as pest vegetation in Brisbane, so pull it out rather than grow it.

Harvesting Flowers

Pick flowers once they are fully open but the centres are still tight.

• Cut the longest stem you can with clean secateurs.
• Choose pompon-flowered heads for the vase, as they last longest once cut.
• For the kitchen, pull the petals free from the picked heads.

To dry flowers, hang small bunches upside down in a dry, dark cupboard and check them regularly for mould. Scatter fresh petals over salads for colour. Signet petals have a spicy, tarragon-like flavour, and French petals taste herbal and slightly bitter.

Marigolds belong to the daisy family, whose pollen is a common cause of hay fever, asthma and dermatitis. Anyone with those allergies needs to take care handling the flowers or eating the petals.

Common problems

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

Flowering stalls in a heatwave. High temperatures can pause growth and flowering for a while, so keep watering and deadheading until plants pick up again.

Bronze speckles on the leaves. In soil below about pH 5.5, marigold leaves can speckle bronze from manganese or iron toxicity, so lime the bed before the next sowing.

Pests and diseases

Use only sprays labelled for the pest and the plant, and keep to any withholding period before eating the petals.

Common garden snail (Cornu aspersum). Snails and slugs favour young marigold seedlings, and slugs also eat leaves and flowers in damp spots. Read more

Two-spotted mite (Tetranychus urticae). These tiny sap-suckers stipple leaves pale from the underside and spin fine webbing, and numbers climb fast in hot, dry weather. Leafhoppers (various species) leave similar stippling but hop sideways when disturbed. Read more

Marigold thrips (Neohydatothrips samayunkur). Recorded in New South Wales and Queensland, these tiny insects breed in the flowers and on the leaves and can badly distort leaves. Read more

African black beetle (Heteronychus arator). A key pest in New South Wales and Western Australia. Its C-shaped curl grubs, up to 25 mm long, eat roots, and the shiny black adults chew seedling stems at ground level at night.

Grey mould (Botrytis cinerea). Petals turn water-soaked and brown, then carry a grey mass of spores, most often in cool weather with heavy dew.

Prevention first

• Hunt snails and slugs by torchlight while seedlings are small.
• In hot, dry spells, check leaf undersides weekly; if mites appear, spray insecticidal soap over both leaf surfaces, never in the heat of the day.
• Dig around any wilting or loose seedling to look for curl grubs at the roots.
• Water deeply through dry spells, as water-stressed plants are more open to leafhopper attack.

Tips

1. To treat a bed for root-knot nematodes, sow it solid with French marigolds and grow them on. Dig the plants into the topsoil, then keep the soil damp for three weeks before planting tomatoes.
2. Save seed from open-pollinated types such as 'Crackerjack', because seed of hybrids may not come true. Pick heads once the petals are dry and the base is turning brown. Dry them on paper indoors for about a week, then store the seed sealed somewhere cool and dry.
3. Avoid broad-spectrum sprays on marigolds, as they kill the ladybirds and predatory mites that usually keep two-spotted mite in check.
4. Check with your local council about the weed potential of marigolds if your garden borders bushland.