Radishes are a short season crop and should be kept growing fast for best flavor. Prepare your soil well with organic fertilizer, compost or well-aged manure. Soil should be finely worked to a depth of at least eight inches. Ideally, radishes prefer a sandy soil as the roots can grow more easily once they germinate. They also do well in containers.
Planting in Place
Radishes don’t take well to transplanting. The tiny seedlings can easily suffer damage in the process. Since they germinate and grow so quickly, it’s best to sow them in the soil where they will grow. Plant ¼ to one inch deep. Deeper planting (for cooler soil) is better as the weather begins to warm. Soil that is too hot can slow germination.
Germination and Temperature
Air and soil temperature affect radish seed germination. Air temperatures of 50 to 65 °F (10 to 18°C) will give the best results. The optimum range for soil temperature is 45 to 85 °F (7 to 29°C). Once soil temperature gets above 95 °F (35°C) it is difficult if not impossible to get radish seeds to germinate.
Germination and Moisture
For best germination, radish seeds need to be moist. Getting them too wet, however, can cause them to rot. Too little water is equally as bad. Seeds that are too dry won’t germinate. If the soil dries out too much, the germination process may simply stop. Water soil when it dries to a depth of ¼ inch.
Radishes germinate quickly as a rule. Fast-maturing varieties may even germinate within two days. Here are some popular radish varieties:
Cherry Belle – 22 days
Early Scarlet Globe – 23 days
French Breakfast – 23 days
Easter Egg – 25 days
Snow Belle – 30 days
China Rose – 52 days
Round Black Spanish – 55 days
Despite normally short germination times, scientists have experimented with radish seeds to speed up the process. Very short bursts of microwave energy and applications of various chemicals have been tried in these experiments but gain only a day at most. These seem unnecessary when germinating and growing radishes in the home garden.
Tinkering With the Season
Radishes are quite cold hardy, but the soil can’t be too cold. If you’re trying to grow radishes extra early, warm the soil by covering with black plastic for three days before sowing your seeds. Lift the plastic, sow your seeds and replace the plastic. Check in three or four days to see if seedlings have emerged – remove plastic as soon as sprouts appear.