How to Grow Tillage Radish in Australia

Tillage radish (Raphanus sativus, cabbage family, Brassicaceae) is a daikon-type radish grown as a cover crop. Its long taproot pushes down into hard, compacted soil and is left there to rot. It covers bare ground fast, smothers winter weeds and draws up nutrients that would otherwise wash out of reach.

Three mistakes that matter

1. Letting it flower. Once a flower stalk rises, the plant spends soil water on making seed. Any seed that falls can come up for several seasons. Cut the crop as the first flower stalk appears, as Ending the Crop describes.
2. Sowing into summer heat. A radish sown into hot weather, or late in spring, bolts (runs to flower early) before its root has grown. Sow as the heat goes out of late summer and into autumn, in the windows When to plant gives.
3. Digging the roots out. The channels a rotting root leaves behind are what let water and the next crop's roots through hard ground. Pulling the roots, or digging the bed over, throws that work away. Cut the tops and leave the roots in place.

When to plant

Sow once the worst heat has passed and the soil sits between about 10°C and 20°C. In the cool zone and the far south, a late summer sowing makes the most leaf and root before winter. In warmer zones, wait for the months the calendar gives. Autumn sowings also root well, while spring sowings grow fast but flower quickly. One sowing covers a bed for a season.

At the hot edge of a window, water the bed a day before sowing and give the seedlings afternoon shade. At the cold edge, sow early, because growth slows sharply in the coldest weeks of winter.

Site, soil and setup

Choose the bed that most needs opening up: a heavy patch, a path turned into garden, or ground packed down by machinery. Breaking the hard layer is the crop's job, so deep digging beforehand is not needed.

Sowing Seeds

Sow Tillage Radish seeds straight into the bed where the crop will stand, because the taproot is the whole point and it resents being moved. The seed is sold under a variety licence, so read its terms before opening the bag, and never save seed from the crop.

Sowing direct

• Rake the bed to a fine, even surface and water it if the soil is dry.
• Scatter the seed at 100 g per 50 square metres, which is 2 g per square metre.
• Rake it in 10 to 20 mm deep, a little deeper in dry soil, and firm the surface with the back of the rake.
• Water in gently and keep the surface moist until the seedlings show.
• Expect seedlings in 4 to 8 days, and sooner in warm soil.

There is no need to thin the seedlings. Sowing thicker than this rate gives smaller, weaker roots, so resist adding extra seed to a hard bed.

Care by growth stage

Water in the first month decides this crop. Once the taproot is down it reaches moisture well below the surface, and it needs almost no feeding.

Nitrogen shortage can hold plants back on sandy or unmanured soil, most of all straight after corn. If growth stalls there, scratch in all-purpose controlled release granular (NPK around 14-5-10) once, at half the label rate.

Companion planting

Tillage radish pairs well with a grass or a legume sown in the same bed, and badly with its own family.

Grows well with

• Oats, a grass that covers the soil as fast as the radish and leaves a longer-lasting mulch.
• Field peas or vetch, legumes that add nitrogen. In a mix, sow a quarter to half the radish rate; a quarter still smothered weeds in trials.
• Corn, as the crop before it: radish fits well into the bed once the corn comes out.

Keep apart

• Cabbage, broccoli and other brassica vegetables, which share its caterpillars, aphids and clubroot.
• Radishes grown to eat, for the same reasons.
• Biofumigant mustard, another brassica cover crop; grow one or the other in a bed, not both in turn.

Ending the Crop

Cut the crop down when the first flower stalk starts to rise from the centre, or sooner if the bed is needed. That is often 6 to 12 weeks after sowing, though autumn sowings in the south usually hold until late winter or spring.

• Slash or mow the tops close to the ground.
• Slice through the crown just below soil level with a sharp spade, because a plant with its growing point left intact can regrow.
• Leave the roots in the soil to rot, unless they are swollen with clubroot (see Pests and diseases).
• Spread the cut tops over the bed as mulch.
• Plant seedlings or sow seed straight away, pulling the mulch back from seed rows.
• Pull any plant that regrows before it flowers.

A hard frost can finish the crop in the coldest gardens, but it takes several nights colder than minus 5°C. Most Australian winters only slow it down, so plan to cut it rather than wait for frost.

Weed lists, state by state

Tillage radish is the same species as garden radish. Its weedy relative, wild radish (Raphanus raphanistrum), grows wild in every state and the ACT and is a major weed of winter crops. Neither appears on a state or territory declared weed list, and cutting before flowering keeps seed out of the soil.

Common problems

Flowering within weeks. Heat, drought or a late spring sowing makes the crop bolt early (see Three mistakes that matter). Cut it at once and resow in the windows When to plant gives.

Patchy seedlings. Seed left on a dry surface, raked in too deep or sown into a crusted bed often fails to come up. Resow the gaps while the soil is still above 10°C, and check for cutworms (see Pests and diseases).

Short, thin roots. Very dry soil or a waterlogged patch holds the taproot back. Water deeply through the first month, and choose a bed that drains, as Site, soil and setup describes.

A smell of rotten eggs. Cut tops and roots can smell of rotten eggs as they rot, most noticeably in a warm spell. The smell fades as they break down, so leave them in place.

Pests and diseases

As a brassica, tillage radish draws the same pests as cabbages, and they are worst in warm autumn and spring weather.

Cabbage white butterfly (Pieris rapae). White butterflies lay eggs under the leaves, and velvety green caterpillars chew ragged holes. A cover crop can carry some damage, so pick off caterpillars on seedlings and leave older plants be. Read more

Diamondback moth (Plutella xylostella). Small pale green caterpillars, up to 10 mm long, wriggle hard when touched and leave windows in the leaves. The moth resists many sprays and small parasitic wasps keep it in check, so avoid broad insecticides. Wild radish nearby carries it between crops. Read more

Cabbage aphid (Brevicoryne brassicae). Grey-green aphids up to 3 mm long form colonies coated in white powder, mostly under the leaves. Hose them off with a firm jet of water. Read more

Cutworms (Agrotis species). Fat, smooth caterpillars hide in the soil by day and cut seedlings off at ground level at night. Dig gently around a felled seedling to find and remove them. Read more

White rust (Albugo candida). Shiny white blisters form under the leaves and burst into powder, and badly hit leaves twist and die. Pull and bin the worst plants rather than leaving them on the bed as mulch.

Clubroot (Plasmodiophora brassicae). Roots swell into firm white galls, and badly hit plants stall. There is no cure, and rotting galls release spores that last in soil for many years. Dig affected plants out with the soil on their roots, then bag and bin them.

Prevention first

• Clear wild radish, wild turnip and other brassica weeds from around the bed before sowing.
• Check seedlings twice a week for chewed leaves, aphid colonies and cut stems.
• Rotate so no brassica has grown in the bed for the past three years.
• Do not overwater, because clubroot thrives in warm, wet, acid soil.

Tips

1. Before cutting, lift one plant with a fork to see how far the root has gone. The thick top of the root often sits several centimetres proud of the soil.
2. Sow it straight after a summer crop comes out, so the bed is less likely to sit bare over winter.
3. Plant the next crop's seedlings along the old radish rows, where their roots can follow the channels down.