When to sow radishes

Radishes are one of the first things you can sow in spring. From March to September they go straight into the bed, in Germany, Austria and Switzerland as well as the UK. If you can't wait, sow in February under fleece, a cloche or in a cold frame, where the soil warms up sooner.

The seed germinates from about 41°F, but slowly: in cold March soil it can take a few weeks to come up, while at 59–68°F you see seedlings within a week.

Radishes are at their best for only a short time, so don't sow a whole bed at once. A short row every two to three weeks gives you a steady supply instead of a glut that turns woody before you can eat it.

Midsummer is the tricky part. Long days and heat make many spring varieties bolt or go pithy in June and July. Either choose varieties sold for summer sowing, give them a spot with some afternoon shade, or take a break and start again in August. Late summer and early autumn sowings are often the best of the year.

Pick an open, sunny spot; light shade is fine, and welcome in summer. Radishes like loose, humus-rich soil that isn't too heavy. Fork it through, break up clods and take out stones, because a radish that hits an obstacle grows misshapen.

Draw a shallow drill and sow 0.2–0.4 in deep. Deeper sowing tends to give long, woody roots. Drop the seeds 1.2–2 in apart, or sow thinly and thin later, and leave 4–6 in between rows. Water the drill gently and keep it moist until the seedlings are up.

Thin crowded seedlings as soon as you can handle them. Radishes that stand too close make plenty of leaves and no roots, which is the most common reason for a disappointing row.

In a 120 × 80 cm raised bed you could fit five rows of about 30 radishes, around 150 in total. In practice it is better to sow one or two rows at a time and fill gaps between slower crops as you go.

Water is what makes a good radish. Keep the soil evenly moist from sowing to harvest and water in every dry spell, little and often rather than a flood now and then. Dry spells make radishes hot, tough and pithy, and a heavy soaking after drought makes the roots split.

Radishes are light feeders and in reasonable garden soil need no fertiliser at all. Don't use fresh manure: it pushes leaf growth and makes the plants more prone to disease. Too much water and nitrogen give the same result, big lush tops over small roots.

Weed by hand around the seedlings so you don't disturb the roots, and keep an eye on the leaves for small holes, the sign of flea beetles.

In summer radishes are ready about three to four weeks after sowing; in the cool of early spring or autumn it can take up to eight. Start checking once the red shoulders push out of the soil.

Pull them when the roots are about 0.8–1.2 in across. Don't wait for bigger ones: radishes left in the ground quickly turn woody, pithy and very hot, or split. It is better to harvest a few days early than a week late.

Radishes don't keep long. Twist off the leaves, which otherwise draw moisture out of the roots, and store them in a closed box or bag in the fridge. They stay crisp for one to three weeks that way; with the leaves on, only a few days.

Seed catalogues split small radishes into spring varieties, which suit sowing from March to May and again in autumn, and summer varieties, which cope better with long days and warmth. Round red ones are the classic; there are also long, white, yellow and violet types.

Winter and oriental radishes, such as daikon, are larger relatives that grow much more slowly. Sow them in July or August, thin to 4–8 in apart, and lift them from autumn onwards after ten weeks or more. Sown in spring they often run straight to flower.

Flea beetles are the pest you will almost certainly meet. These tiny jumping beetles pepper the leaves with small round holes, worst on seedlings in warm, dry weather. Well-watered radishes usually grow away from the damage; on bad sites, cover the row with fleece or fine insect mesh right after sowing.

Cabbage root fly larvae tunnel into the roots and leave brown channels. The same fine mesh keeps the adult flies from laying their eggs, and is worth using from spring onwards if your cabbages and turnips suffer too. Slugs go for the seedlings in wet spells.

Radishes belong to the cabbage family (Brassicaceae) with cabbage, kale, turnips, rocket and mustard. Although they are only in the ground for a few weeks, they can host clubroot, which survives many years in the soil. Leave three to four years before any cabbage-family crop grows in the same spot. On soils with a pH above 7, a soil-borne fungus can cause dark patches on the roots; a rotation helps there too.

Planting tables pair radishes with lettuce, beans, peas, tomatoes and cucumbers. We found no trials showing that any of these pairings protect against pests or improve the crop; it is gardening tradition.

What does work is a practical kind of mixed planting. Radishes are ready in a few weeks, so you can sow them between lettuce, along bed edges or in the gaps between slow crops, and pull them before their neighbours need the space. Just don't count them as a break from brassicas in your rotation.