How to Grow Root Crops

By Charlie Nardozzi from Vegetable Gardening For Dummies, 2nd Edition

Root crops, such as potatoes, onions, carrots, beets, and turnips, are easy to grow if you have good soil, water, and proper spacing. The keys to growing great root crops are preparing the soil bed well and giving the plants room to grow. You also need to keep the crops clear of weeds and make sure they have enough water.

Here are further details on each of these important points:

All root crops like well-drained, loose, fertile soil: And with the exception of potatoes, which grow best in hills, root crops grow best in raised beds. They also can grow if you have a gardening spot that gets only 4 to 6 hours of direct sun a day. Try some carrots and onions in that patch.

To prepare the soil, add a 3- to 4-inch layer of compost or manure at least 2 to 3 months before you're ready to plant. If you wait until just before planting to add fresh compost or manure, you're likely to get poor growth because too much nitrogen fertilizer on carrots and potatoes in spring promotes foliage growth but not good tuber and root formation. Instead, root crops enjoy phosphorous, which promotes root growth, so perform a soil test, and based on the results, add bone meal or rock phosphate fertilizer before planting to keep your roots happy.

Onions in particular like lots of fertilizer, and they can stand some extra nitrogen, which promotes leaf growth. Add extra fertilizer when the transplants are 6 inches tall and the bulbs begin to swell. Then add a complete organic fertilizer, such as 5-5-5, at 1 pound per 10 feet.
Root crops, especially carrots and onions, require proper spacing to grow at their best: Thin out the young seedlings when they're 3 to 4 weeks old by pulling them out or snipping them until they're properly spaced. Onions should be 4 inches apart, scallions 2 inches apart, and carrots 3 inches apart. Potatoes don't need thinning and should be planted 8 to 10 inches apart when planted.

Thinning your hearty crops sounds cruel, but if you don't do it, the roots won't have enough room to expand, causing you to get lots of plants but few roots — and fewer roots means fewer carrots and onions.
You'll be rewarded with lots of crisp roots in no time if you regularly weed your root crop patch: After a good thinning, hand-weed beds of carrots and onions; potatoes can be weeded with a hoe. Mulch the bed with hay or straw. You don't have to mulch in between individual onion and carrot plants. Simply mulch around the beds, and keep them well watered.

Carrots, onions, and potatoes, like many root crops, prefer cool temperatures. They grow best and have the best flavor when temperatures stay below 80 degrees Fahrenheit.