Know What to Grow
Radishes are classified as 'spring', 'summer', or 'winter' radishes. Their classifications are related to the number of daylight hours required to produce a bulbous root. When days are longer in early to mid-summer, 'spring' radishes will simply bolt (send up a flower stalk) instead of producing a big root.
You may be able to grow 'spring' radishes in summer by shading the plants from the sun in mid-afternoon, so they get no more than 8 hours of sunlight. However, for greater success, try growing a 'summer' radish type like 'White Icicle'. It will more easily produce a root in mid-summer, but it might also benefit from afternoon shade in very hot climates.
Once it is late summer or early fall, switch to growing 'winter' radishes (like Watermelon Mantang Hong or Daikon) that grow best during the shortening days, and cooler weather, of fall.
Although radishes are usually very easy to grow, there are some situations that can complicate their growth:
Inadequate Spacing: Following the spacing and thinning recommendations when sowing is important. Plants that grow too close together will compete for nutrients, space, and water resulting in stunted root growth.
Not Enough Sunlight: If the plants aren't getting enough sunlight, their leaves can't produce enough energy for good root development. Make sure your planting area gets a minimum of 6 hours of sunlight each day.
Water: If the plants aren't getting enough water, root development will be poor. It's best to water deeply and less frequently for big roots.
Soil Nutrients: You might consider testing your soil for its nutrient balance. Soil that is too high in nitrogen will cause good leaf growth at the expense of the roots. Radishes also need adequate potassium for good root development.
Excessive Soil Acidity: Root crops prefer a pH between 6 and 6.5. If your soil tests lower than this range, adding lime can help raise the pH.
Temperature: Hot weather will stunt the growth of spring radishes. If you plant in the spring and aren't getting mature radishes in 3-5 weeks, the warming weather may prevent further development. When it gets hot, radishes stop sending energy to the roots and send up a flower stalk, instead.