How to Grow Onion Sets Indoors as Scallions

Fill a large pot or planting tray with potting soil. The amount of surface area determines how many onion sets you can plant. The soil should be at least 6 inches deep. You will harvest the plants while they are immature, so this depth gives them plenty of room to grow.

Place the pot or tray in a well-lighted area, such as a window that receives six hours or more of bright sunlight per day. If that is not possible, arrange artificial light for the plants. A fluorescent light placed about 3 to 4 inches above the tops of the scallions gives adequate light if left on for 16 hours per day. Be sure to move the light up as the plants grow.

Plant the onion sets in the soil with their roots pointing downward. Place them so that they are touching each other, in holes about 1 1/2 inches deep. The depth may vary by variety, so check the specific directions for the sets you are using. Cover them with soil.

Water the sets on a regular basis. Scallions need approximately an inch of water each week. The best way to provide this is in three equal waterings.

Harvest the scallions after the green tops are at least 6 inches tall. If you want some of them to develop into larger, thicker scallions, pull up every other one, giving the remaining ones more room to grow. If you leave them too long they will develop bulbs and no longer will be usable as scallions.