How to Grow Radishes in Australia

Radish (Raphanus sativus) is a fast-growing member of the cabbage family, Brassicaceae, grown as an annual for its crisp, peppery root. Small salad radishes can be ready about four weeks from sowing, and the larger Asian and winter radishes give roots to cook and store.

Three mistakes that matter

1. Sowing thickly and skipping the thinning. Crowded seedlings put their effort into leaves, and the roots stay small or grow misshapen. Sow thinly, then thin as soon as the first true leaves show, to the spacing in Choosing a type.
2. Letting the soil dry out. Roots that go short of water stall and turn hot and woody, and a soaking after a dry spell splits them. Keep the bed evenly moist, which can mean watering every day in warm weather.
3. Leaving roots in the ground. Salad radishes stay crisp for only a short time after reaching size, then turn pithy, hot and woody. Pull them at about 2.5 cm across, and sow a short row every two weeks instead of one long one.

When to plant

Radish seed germinates in soil from 8°C to 30°C, so planners list it for most months in most of Australia. The roots grow best in mild weather, though, and heat pushes plants to flower before the root swells. Winter and Asian types form their roots as days shorten and nights cool, so outside the tropics sow them from late summer into autumn.

Summer sowings need the afternoon shade in Site, soil and setup. In the coldest months growth slows, so crops take longer than the packet says.

Site, soil and setup

Radishes need loose, fine soil that lets the root swell evenly, and steady moisture. Long daikon and 'Mino Early Long White' roots need soft soil as deep as the root grows, which can be more than 30 cm.

Coarse, unrotted material can fork and distort the roots, so dig organic matter in weeks ahead. Rotation matters because two brassica diseases in Pests and diseases survive in the soil and on old brassica debris.

Container growing

A pot at least 20 cm across and 15 cm deep, holding about 5 L, suits a short crop of round radishes. A 30 cm pot of about 13 L gives long salad types more depth. Fill it with premium potting mix blended with compost. Round types such as 'Cherry Belle', 'Champion', 'Golden' and 'Easter Egg Mix' suit pots best; daikon grows too long for most.

• Water: pots dry out fast, so check them daily in warm weather and water before the mix dries.
• Feed and position: as for the bed, in Care by growth stage and Site, soil and setup.

Sowing Seeds

Sow radish seed direct where it is to grow, because the root does not transplant well.

Choosing a type

'Heirloom Mix' holds salad and winter types together, so give it winter-type spacing.

Sowing direct

• Sow salad types 10 to 15 mm deep and winter types about 20 mm deep.
• Space seed 2.5 to 5 cm apart in rows 15 to 20 cm apart.
• Sow winter types two seeds to a station, 15 to 20 cm apart, or 40 to 50 cm each way for big daikon.
• Keep the surface moist with a fine spray; seedlings usually show in 4 to 8 days.

Some radish seed is sold treated with thiram, a fungicide, and the packet says so. Never eat or sprout treated seed, keep it away from children and animals, and wear gloves to sow it.

Care by growth stage

Speed decides the quality of the root, so the aim at every stage is growth without a check. Weeks count from sowing.

Mulch lightly between the rows, keeping it clear of the plants, to hold in moisture and keep weeds down. 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.

Thinning

Tiny radish seed tends to land in clumps, and a clump of seedlings grows tops at the expense of roots. Where seedlings stand closer than the spacing in Choosing a type, thin them when the first true leaves show. Pull out the smaller, weaker seedlings first.

• Winter and Asian types: keep one seedling per station.
• Thinnings: rinse and eat them as microgreens, but only those grown from untreated seed.

Companion planting

Radish is quick enough to fill the gaps between slower crops.

Grows well with

• Carrot and parsnip, whose slow rows the radish marks and then vacates before they need the space.
• Peas and potatoes, with radish sown between their rows.
• Lettuce and spinach, with radish sown in the gaps between plants.

Keep apart

Grow radish away from cabbage, broccoli, turnip and other brassicas, which carry the same pests and diseases, set out in Pests and diseases.

Harvesting Roots

Pull salad radishes as soon as they reach the size in Three mistakes. Start lifting winter types from about 8 weeks. Daikon is ready once the shoulder is about 5 cm across, and 'Black Spanish Round' at 8 to 10 cm. They grow much larger than salad types and stay crisp.

• Harvest in the cool of the morning, because radishes wilt quickly.
• Grip the tops firmly near the base of the leaves and pull.
• Loosen long types with a garden fork first; daikon roots are brittle and snap.
• Cut the tops back to about 1 cm before storing.
• In cool weather, leave winter types in the ground and dig them as needed.

Salad radishes keep one to three weeks in a plastic bag in the fridge crisper. Winter radishes keep about two weeks in the fridge, and for several months packed in a box of damp sand somewhere cold.

Eat salad radishes raw, and grate daikon and other winter radishes into salads or cook them in soups and stir-fries.

Common problems

Leaves but no root. Nitrogen-rich feed or fresh manure sends growth into the tops, and so does crowding, as in Three mistakes.

Flowering before the root forms. Heat and dry soil push plants to bolt (flower and set seed early), and so do long spring days on winter types. Sow by the calendar, and pull bolted plants.

Forked or misshapen roots. Stones, lumps, compacted soil or unrotted material in the bed deflect the root; see Site, soil and setup.

Black patches on the root. A soil-borne disease blackens radish roots in dark, irregular patches, and long types suffer most. Good drainage and the rotation in Site, soil and setup control it.

Gaps in the row. Seed sown too deep, a surface left to dry before emergence, or snails grazing the seedlings. Read more

Pests and diseases

Radish shares its pests and diseases with cabbage and cauliflower, although its short season lets it escape many of them.

Cabbage white butterfly (Pieris rapae) and diamondback moth (Plutella xylostella). The green caterpillars of both chew the leaves. Diamondback caterpillars grow to about 10 mm and graze small windows that tear into holes. Cover rows with fine insect mesh from sowing, and squash eggs and caterpillars under the leaves. Read more

Cabbage aphid (Brevicoryne brassicae). Dull grey-green aphids up to 3 mm, in colonies coated with whitish powder on leaf undersides. They also spread turnip mosaic virus. Hose small colonies off, or spray insecticidal soap, following its label and any withholding period. Read more

White rust (Albugo candida). Smooth, shiny white blisters under the leaves that burst into white powder, worst in cool weather with heavy dews. Remove affected leaves, water early in the day, and bin crop debris.

Clubroot (Plasmodiophora brassicae). A soil-borne disease that swells and distorts roots and stunts plants, which wilt on warm days. It favours wet, acid soil and survives there for years. Bin infected plants with their soil, improve drainage and lime acid soil.

Prevention first

• Turn a few leaves each week for eggs, caterpillars and aphid colonies, under netting too.
• Rotate brassica beds as in Site, soil and setup.
• Pull wild radish and other brassica weeds, which carry aphids and white rust.
• Water the soil in the morning so the leaves dry quickly.

Tips

1. Eat the whole plant. Young leaves go raw into salads or cook like silverbeet. A plant left to flower gives young seed pods to pick before they turn stringy.
2. For radish sprouts, use seed sold for sprouting from the radish sprout seed range, never treated garden seed. Grow them with strict hygiene, which lowers but does not remove the risk of E. coli and Salmonella. Young children, pregnant women, older people and anyone with low immunity should avoid raw sprouts.
3. In cool and alpine gardens, a cloche or fleece over autumn rows on frosty nights keeps winter types growing and lengthens the season.