Question: I keep trying to grow radishes every year, but I never manage to grow anything but the greens. The roots never form and the greens that do develop have lots of little holes in them. What am I doing wrong?
Answer: Radishes are a cool-season crop, meaning that they prefer to grow in the cooler temperatures of spring and fall. They grow best in temperatures between 50 and 70 degrees. Radishes will bolt (or flower) when the days begin to lengthen in late spring and the temperatures get warmer. If the plants begin to flower before the root has formed, no root will develop, or if the plant is gearing up to flower, it puts its energy into making that flower instead of fattening up the root. So, what this means is the earlier you plant your radishes, the better.
Radishes tolerate frosts quite well, so you should sow your seeds in late March (about four to six weeks before our last expected frost in mid-May) right into the garden. Harvests begin anywhere from 20 to 40 days after planting when the roots are quarter-sized. Waiting too long to harvest will result in woody, pithy roots. To stagger your harvest, sow a handful of seeds every week or two through the spring until early June.
Another issue you may have is a nutritional one. Root vegetables need phosphorus to develop good roots. So you may need to add some if you have a deficiency. I suggest that each fall you work some bone meal or rock phosphate into the area where you'll be sowing next year's radish crop (or carrots, beets, parsnips, turnips and other root crops). It will be readily available for the plants come spring. Having too much nitrogen in the soil is another no-no. Nitrogen makes lots of green growth which is not what you want, so skip the nitrogen fertilizers in the vegetable garden and turn to compost, aged manures and leaf compost, instead, to feed your garden a more-balanced diet.
The little holes you are finding in the foliage are likely the result of flea beetles, a common pest on radishes. Flea beetles are best deterred by covering your radish crop with floating row covers. These lightweight, white fabric covers rest on the plant tops and create a physical barrier to all kinds of pests. Place the row covers over the area right after the seeds are planted, and leave it in place until harvest. An added plus: Row covers can be used for many years -- I've had the same ones for almost 10 years and use them regularly to protect potatoes from Colorado potato beetles, cabbage and broccoli from cabbage worms and all sorts of root crops from both root maggots and flea beetles.
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