Seedlings are probably the most common way to grow onions in our area. Seedlings arrive at local nurseries starting in December. They are generally available through late February, although it is best to have them in the ground by the end of December. I have occasionally planted them as late as mid-February and had a good crop, but I prefer to have them in earlier.
Seedlings come in bundles, usually of 100 seedlings. If you want to grow more than one kind of onion, and don’t want 100 of each variety, you can either co-op with a friend, or plant the excess onions closer together to be harvested early as scallions or spring onions. People may be surprised to know that many of the green onions we buy at the market are not a specialty scallion variety; they are simply immature bulbing varieties.
Make sure the varieties you select are short day onions and that the bundle is healthy. The seedlings will be slightly dried out, which is fine, but there should be no sign of rot or decay.
Snip the bands or twine holding the bundle together. If the roots were not trimmed by the grower, or if you grew them from seed yourself, trim the roots to 1/2″ long. Trim the tops so that the seedlings are all 4″ – 6″ tall. This will help keep them from being top heavy when they are planted. While some growers do not recommend pre-soaking, I have found that it gets them off to a quick start. Place the seedlings in a small bowl with just enough water to cover the roots, no more. You can do this the night before planting. Do not leave onions soaking for more than a day.
Plant the base of the seedling 1″ – 1.1/2″ deep into prepared soil. Make sure the youngest leaf is not buried. Onions should be spaced 4″ – 6″ apart for full sized bulbs. You can plant them as close as 2″ apart if every other one is harvested as a spring onion, leaving the remainder to bulb fully.
If the seedlings are of good size and hardened off, and the soil has been prepared with compost, they can often just be pressed into the soil without using a dibble or making a furrow. If they are too tender for this, you can make a furrow, space the seedlings, and pull the soil back over them in the row. If planting in wide beds or square foot beds, use a dibble to make holes, set the seedling in the hole and press the soil around the roots. A wooden pencil makes a great dibble.
Water well after planting. Keep the soil evenly moist until the onions are making new growth.
For more information see the Growing Guide for Bulbing Onions.