Why Are My Radishes Extremely Hot and Peppery?

A radish that is so fiery and sharp that it is unpleasant to eat raw has been stressed in some way during its development. The heat in radish comes from isothiocyanate compounds — the same family of chemicals that give wasabi and horseradish their heat — and the plant produces more of them when it is under environmental stress. The perfect crisp, mild radish is the product of cool temperatures, consistent moisture and prompt harvesting.

Drought stress during root development

Radishes grown in dry conditions produce significantly more heat compounds than those grown with consistent moisture. The stress response involves producing more glucosinolates, the precursors to the hot isothiocyanates. Consistent watering — keeping the soil evenly moist but not waterlogged throughout the three to four weeks from sowing to harvest — is the single most effective way to reduce heat. Do not allow the soil surface to dry out completely between waterings.

High temperatures

Hot weather during root development increases heat levels. Radishes grown in spring or autumn when temperatures are in the 12–20°C range are reliably milder than summer-grown ones. If you must grow in summer, use a daikon or mooli variety, which have been bred for lower heat levels even in warmer conditions, and keep the soil consistently moist.

Left in the ground past the harvest window

As a radish begins to mature past its peak and transition toward bolting, heat levels increase rapidly. A radish that was mild three days ago may be unacceptably hot today. Harvest spring radishes as soon as they reach size — three to four weeks after sowing — and check daily in warm weather. Do not leave them in the ground "until needed" in a warm spell; harvest and refrigerate instead.

Variety choice for milder flavour

Some varieties are bred for lower heat. French Breakfast and Cherry Belle are among the milder spring varieties. Watermelon radish and daikon types tend toward a milder, sweeter flavour, particularly when cooked. If heat is always a problem in your garden, experiment with variety selection alongside the cultural changes above.