How to Grow Onions From Seedlings

Harden off the onion seedlings when you bring them home from the nursery by leaving them outside for one hour the first two to three days and then extending the amount of time they spend outdoors over the course of two weeks.

Test the soil for nutrient content and pH. Onions require a pH of 5.5 to 6.5.

Use a rototiller, shovel, spade, pitchfork or garden fork to loosen the soil in the onion bed so that it's light and fluffy. Add 3 lbs. of 10-10-10 fertilizer for every 100 square feet of soil and use a garden fork to mix it in.

Plant the onion seedlings in holes that are the same depth as those in which they are growing in the nursery pot. Space them 4 inches apart.

Water the onion bed immediately after planting the seedlings and provide an inch of water a week during periods without rain.

Fertilize the onions when they reach 6 inches in height. Use 1 cup of ammonium sulfate or ammonium nitrate for each 20-foot-long row of onions. Reapply every three weeks. Toward the middle of the season, squeeze the neck of the onions – where the stalks join the top of the onion. If it feels soft, stop fertilizing.

Check the onion plants periodically for thrips, a pest common on onion crops. They are difficult to see with the naked eye, but if the foliage appears gray or silver, suspect thrips. Use an insecticide with spinetoram as the active ingredient, and follow label directions.

Harvest the onions when the top half of the foliage falls over. This generally occurs within three to four months of planting. If you prefer small, green onions, harvest at anytime while the plant is young and actively growing.