How to Grow Basil in Australia

Basil (Ocimum basilicum) is a frost-tender herb in the mint family, Lamiaceae, grown for its scented leaves. Australian summers suit it: once nights are warm it can be picked for months, and in the tropics it grows year-round. The usual mistake is planting out too early, into nights cold enough to damage the leaves.

Three mistakes that matter

1. Planting out into cold nights. Nights below about 10°C can make basil leaves wilt and darken. Keep seedlings in a warm, bright spot and plant out only once nights stay above 10°C.
2. Letting plants run to flower. Once flowers and seed form, stems turn woody, new leaves slow and the foliage can taste bitter. Pinch stems back to a leaf pair about every two weeks, and cut any budding stem at least four leaf pairs below the spike.
3. Watering the leaves late in the day. Foliage that stays damp overnight, especially on crowded plants, invites basil downy mildew, which can strip plants bare (see Pests and diseases). Water the soil in the morning and space plants 20 to 30 cm apart.

When to plant

Basil seed germinates once the soil holds at least 18°C, and fastest at about 20 to 30°C. Where spring warms slowly, sow indoors 4 to 6 weeks before planting out, on a seedling heat mat in a cool room.

In the cool zone, even summer nights can dip below 10°C, so plant out once nights average above 10°C, into the warmest spot.

Site, soil and setup

Choose a spot with at least 6 hours of direct sun, and allow light afternoon shade in hot inland and tropical summers. Basil wants rich, free-draining soil with a pH of 6.0 to 7.0, and it sulks in clay that stays wet.

Container growing

One plant needs a pot at least 20 cm wide and deep, about 4 to 6 L, filled with premium potting mix. In a trough, space plants 15 to 20 cm apart. Compact types suit pots: bush basil 'Fine Leaf' stays low and bushy, and 'Medinette' is compact and slow to run to flower.

• Water: push a finger into the mix each morning and water when it comes out clean and dry.
• Feed: fish and seaweed blend at half strength, about 12 mL per 9 L, each fortnight.
• Position: full sun, sheltered from strong wind.

Sowing Seeds

Start seed in trays while nights are cold, or sow direct into warm soil.

Starting under cover

• Sow 2 to 6 mm deep in punnets or trays of fresh seed-raising mix.
• Keep the mix moist and warm (see When to plant); seedlings emerge in 5 to 14 days.
• Moist seed forms a jelly-like coat, which is normal.
• If nights are still cold once several true leaves show, pot seedlings on into 7.5 cm pots.

Sowing direct

Once the soil has warmed (see When to plant), sow a few seeds at the same depth every 20 to 30 cm. Keep the surface damp, not soggy, until seedlings appear, then thin each spot to the strongest once its first true leaves open.

Transplanting

• Plant out seedlings about 10 cm tall, usually 4 to 6 weeks after sowing.
• Harden them off first for at least a week, ideally two (longer spells outdoors each day, so they adjust to sun and wind).
• Set plants at the same spacing and water them in with seaweed solution at half strength (15 mL per 9 L).

Care by growth stage

Weeks count from planting out. Feed lightly, because over-feeding can reduce the essential oils behind basil's flavour and aroma. Water by the finger test, not the calendar.

Spread 5 cm of compost or pea straw around plants, clear of the stems, and top it up as it breaks down. If growth stalls in poor soil, a light dose of all-purpose controlled release granular (NPK around 14-5-10) scratched in can replace the blend. Before cold nights arrive, move pots to a bright, warm windowsill indoors, where they can keep producing for weeks longer.

Pinching Out

Pinching out the growing tip once plants reach 15 to 25 cm makes the stem branch into a bushy plant. A bud sits at each leaf joint, so a cut above a leaf pair leaves two buds to grow on, usually within a week.

• First pinch: nip the main stem just above the second leaf pair from the base.
• Flower spikes: nipping off the spike alone can prompt new ones lower down, so cut the stem at least four leaf pairs below it.
• Bush basil: skip the height rule and snip shoot tips all over once the plant fills out.

Companion planting

Good partners share basil's planting months and sunny beds.

Grows well with

• Tomato, which goes in over the same spring months.
• Capsicum and chilli, which start at the same soil warmth.

Keep apart

• Mint, whose spreading underground stems crowd basil out of a shared bed or pot.
• Rosemary, which prefers dry, lean soil and suffers under the steady watering basil needs.

Harvesting Leaves

The first pinch (see Pinching Out) is also the first pick, usually 6 to 12 weeks after sowing.

• Take whole stem tips rather than single large leaves from the base.
• After the first cut, leave at least half the plant at each picking.
• Pick in the late afternoon; in shelf-life trials, evening-cut basil kept longer than dawn-cut.
• Before autumn nights drop below about 10°C, cut whole plants for pesto or the freezer.

Stand cut stems in a glass of water on the bench, change it daily, and use them within a few days. The fridge blackens basil, whose leaves are damaged below about 12°C. For longer storage, freeze whole leaves in a bag, or chopped leaves under water in ice cube trays.

Use the leaves raw or cooked in Italian and South-East Asian dishes, or steep them for tea.

Common problems

Leggy seedlings. Tall, thin seedlings that flop over are stretching for light, often on a windowsill. Read more

Damping off. Seedlings that collapse at soil level have been attacked by organisms such as Pythium, which thrive in overwatered, poorly ventilated mix. Read more

Flower spikes on young plants. Heat, dry soil and high-potassium fruiting feeds can all push young plants to flower; see Three mistakes that matter for cutting them back.

Wilting in damp soil. A plant that wilts in wet soil may have roots rotting from waterlogging, or Fusarium wilt (see Pests and diseases). Read more

Pests and diseases

Snails and caterpillars chew leaves, aphids and mites suck sap, and two diseases can kill plants.

Common garden snail (Cornu aspersum). Ragged holes and slime trails on seedlings show snails, or grey field slugs (Deroceras reticulatum), feeding at night. Collect them by torchlight after rain and set beer traps. Read more

Looper caterpillars (Chrysodeixis spp.). Green caterpillars that move with a looping action chew large, ragged holes in leaves. Pick them off, or spray a Bt (Bacillus thuringiensis) product labelled for herbs while they are small.

Green peach aphid (Myzus persicae). Small, usually green insects cluster under leaves and on shoot tips, distorting new growth. Hose them off in the morning, or wet leaf undersides thoroughly with insecticidal soap. Read more

Two-spotted mite (Tetranychus urticae). Fine pale speckling, then bronzed leaves, show mites feeding underneath, especially in hot, dusty weather or on water-stressed plants. Hose leaf undersides forcefully every few mornings. Read more

Basil downy mildew (Peronospora belbahrii). Yellow patches bounded by leaf veins, with grey-purple fuzz beneath, mark this disease, which thrives in humid weather and spreads mainly on the wind. Pull and bin affected plants. Thai, lemon and lime types usually show fewer symptoms. 'Prospera' was bred to resist it, though a newer strain overseas has overcome that resistance. Read more

Fusarium wilt (Fusarium oxysporum f. sp. basilici). Plants that stunt and wilt at 20 to 30 cm, with brown streaks on the lower stem, often have this soil-borne fungus. It is recorded in Australia and can arrive on seed. In soil it survives 8 to 12 years, so rotation alone will not clear a bed. Bin affected plants, then grow basil in fresh potting mix or choose 'Prospera', bred to resist this wilt.

Prevention first

• Turn a few leaves over each week to catch aphids, mites and silverleaf whitefly (Bemisia tabaci) early.
• Leave ladybirds, lacewings and hoverflies to eat aphids by avoiding broad-spectrum sprays.
• Do not save seed from plants that had Fusarium wilt or downy mildew, since both diseases can travel on seed.
• Pull nearby weeds, which can host aphids between crops.

Tips

1. Root the tips you pinch off: a non-flowering shoot in a glass of water on a bright windowsill roots within a few weeks.
2. Sow seed thickly in a tray for microgreens, ready to cut in 10 to 14 days.
3. Seed saved from F1 hybrids such as 'Prospera' will not come true. For true seed, grow a non-hybrid variety and let no other basil flower alongside it, since varieties can cross.