How to Grow Fennel in Australia

Florence fennel (Foeniculum vulgare var. azoricum) is a member of the carrot family, Apiaceae, grown for the crisp bulb formed by its swollen leaf bases. Herb fennel is the same species grown for its leaves and seed, and it makes no bulb; dill is a different plant altogether. Grown through the cooler months, it gives sweet, aniseed bulbs from Darwin to Perth. The usual failure is sowing into warming weather, where it flowers instead.

Three mistakes that matter

1. Sowing into lengthening, warming days. Heat and long days push fennel to bolt (send up a flower stem early), and a bolting plant forms a thin, woody bulb or none. Sow in the cooler windows shown for your zone in When to plant.
2. Disturbing the roots. Fennel bolts readily when its roots are disturbed, so a clumsy transplant can end the crop before it starts. Sow direct, or raise one seedling per cell and plant it out with the root ball whole.
3. Letting the soil dry out. A dry spell checks growth, and a checked plant often runs to seed instead of swelling. Water deeply and regularly, mulch the bed, and keep the moisture even from sowing to harvest, as set out in Care by growth stage.

When to plant

Sow when the soil sits between 10°C and 25°C, and aim for the bulb to swell in mild weather. Florence fennel grows best in cool conditions, and hot weather makes it woody and quick to bolt. Sow a short row every three to four weeks through your zone's window for a steady supply.

In the tropics, start once the wet season ends, and shade young plants on very hot days at the warm end of a window. In frosty gardens, wait for warm soil, because cold snaps also cause bolting.

Site, soil and setup

Fennel wants a sunny, sheltered bed with rich soil that holds moisture yet drains freely. Prepare the bed a month ahead so manure and lime have time to settle.

Keep feeding moderate. Rich soil grows a good bulb, but heavy feeding and uneven moisture encourage bolting and split bulbs.

Container growing

Florence fennel grows well in a large pot. Two or three plants fit a pot 30 cm across, which holds about 13.5 L. A 40 cm pot of about 28 L gives three plants more room. Fill it with premium potting mix and a few handfuls of compost. Both our Florence fennel lines suit pots.

Potting mix can carry Legionella bacteria. Wet it first, wear gloves and a face mask to handle it, and wash your hands afterwards.

• Water: pots dry out fast, and a dry pot sends fennel to seed, so check the mix daily in warm weather.
• Feed: fish and seaweed blend at 25 mL per 9 L, fortnightly from the first true leaves.
• Position: full sun in the cool months; move the pot into afternoon shade on hot days.

Sowing Seeds

Sow Florence fennel direct where it is to grow, because it bolts if its roots are disturbed.

Choosing the right fennel

Our fennel seed is one bulbing type, Florence fennel, in conventional and organic lines. Check the packet says Florence or finocchio before you sow. Fennel is a declared weed in Victoria and Tasmania, so under those states' weed laws and our shipping policy we cannot send fennel seed there.

Sowing direct

• Sow 5 to 10 mm deep in a shallow drill, rows about 40 cm apart.
• Keep the surface moist with a fine spray; seedlings usually show in 7 to 14 days.
• Thin first to 10 to 15 cm, then to a final 20 to 30 cm, and eat the thinnings.
• Mulch with straw once the seedlings stand a few centimetres tall.

Starting in cells

Where space is short, sow one seed per cell and plant out while the seedlings are small, before the roots fill the cell. Harden them off first (a week of gradually longer spells outdoors, so they adjust to sun and wind). Then plant out at the same depth without breaking the root ball.

Care by growth stage

Steady moisture and steady growth decide the bulb. Weeks count from sowing, and cool weather stretches every stage.

Once the base swells past golf-ball size, blanch it (keep light off so it stays white and mild). Mound soil around it, or set a cardboard collar and mound soil against that. Bulbs are usually ready about four to five weeks later.

Companion planting

Fennel does best beside plants that share neither its pests nor its rot.

Grows well with

• Lettuce and chicory, low leaf crops that share the cool season without shading the fennel.
• Peas, which climb a trellis and sit outside the carrot family and its aphids.

Keep apart

• Carrots, celery, dill and coriander, which host the same carrot-family aphids.
• Beans and tomatoes, both hosts of Sclerotinia rot, set out in Pests and diseases.

Harvesting Bulbs

Start cutting once a bulb is firm and about tennis-ball size. Finish before it lengthens and sends up a flower stem.

• Pull away the collar and mounded soil.
• Cut at ground level with a sharp knife.
• Leave the root in: it often sends up small shoots for salads, though never a second bulb.
• Bulbs can grow on to 10 to 15 cm across while they stay firm.

Store whole bulbs in a bag or container in the fridge's produce drawer, where they usually keep one to two weeks. Store the fronds separately; they keep about a week.

Slice the bulb raw into salads, or roast or braise it, and use the fronds and young stalks like a herb.

Common problems

A flower stem and no bulb. The plant bolted from heat, long days, a cold snap, root disturbance or dry soil, as in Three mistakes. Pull it and sow again in your zone's window.

Stringy, tough bulbs. Bulbs left too long turn stringy and tough, so cut them while they are firm.

Split bulbs. Uneven watering and heavy feeding split the swelling bulb; keep moisture even, as in Care by growth stage.

Patchy or no germination. Seed sown deeper than 10 mm, a surface left to dry, or soil outside 10°C to 25°C. Read more

Seedlings cut off at the soil. Cutworms (Agrotis species), the caterpillars of night-flying moths, chew through young stems at or below the soil surface. Read more

Seedlings collapsing. Damping off (a soil-borne rot of young seedlings) attacks at the one to two leaf stage, leaving water-soaked patches on stem and roots. Read more

Pests and diseases

Australian fennel growers rank these as the troubles that matter, and home plantings meet the same ones.

Snails and slugs (Gastropoda). They graze seedlings and snack on the swelling stems, and they feed after dusk. Hunt them by torchlight, or scatter an iron-based snail bait. Read more

Green peach aphid (Myzus persicae). Yellow, green, orange or pink aphids suck sap, distorting growth and leaving sticky honeydew where sooty mould grows. Hose small colonies off, or spray insecticidal soap, following its label and withholding period. Read more

Thrips (Thysanoptera). Several species pierce and rasp the leaves, and they are hard to tell apart in the garden. Keep plants well watered and weeds down, and spray insecticidal soap if they build up. Read more

Rutherglen bug (Nysius vinitor). Grey-brown native bugs about 4 mm long, most active in spring and early summer, which breed on nearby weeds and suck sap. Clear weeds around the bed and check plants often. Read more

Sclerotinia rot (Sclerotinia species). Water-soaked rot covered in white, fluffy fungal growth, with hard black resting bodies forming later. Bin affected plants with their soil, and space plants for air movement.

Septoria leaf spot (Septoria species). A leaf spot favoured by cool, wet weather, surviving on old leaves, weeds and seed. Water at the base rather than over the leaves, and clear crop debris after harvest.

Prevention first

• Check seedlings at dusk for snails, slugs and cut stems.
• Turn a few leaves each week for aphids and thrips.
• Pull weeds around the bed, which harbour Rutherglen bug and leaf spot.
• Rotate beds as in Site, soil and setup.

Tips

1. Never let fennel set seed. Wild fennel smothers native vegetation along roads and waterways, so cut any flower stem before seed forms.
2. Grow baby fennel in a spare corner at 10 to 15 cm apart, and cut the bulbs at about 5 cm across.
3. Pick young leaves and stalks as needed, but leave enough foliage to keep the plant growing.