Onions are a summer garden favorite, as they add flavor to essentially any prepared dish. When planting onions, it is very common to plant onion sets, which are immature bulbs. The other alternative is to plant onion plants, also called transplants or onion seedlings. Transplants can be purchased from nurseries or garden centers, or started from seed in your own home. However you come by the transplants, putting them in the ground is a bit different than planting the onion sets.
Plan to put your onion plants in the ground as soon as the soil can be worked, and immediately after your last expected frost date. The cool temperatures, and the fact that the soil is still moist, is actually beneficial to the development of onion bulbs.
Harden off your seedlings before putting them in your garden. To do this, leave them in their current pots and set them outside for a few hours at a time, gradually increasing exposure to outdoor weather and temperatures over the course of two weeks. If your onion plants are not currently in soil, set them out by laying them down on a screen, and plan to get them in the ground as soon as possible.
Prepare the garden row for your onion plants by digging it to a depth of 4 inches.
Spread one cup of 10-10-10 fertilizer along a 10-foot row, then fill the trench in with a layer of soil 2 inches deep.
Place your onion plants in the ground, spacing them 4 inches apart. Cover the roots with dirt, and water enough to moisten the soil.
Space rows of onion plants between 12 and 18 inches apart.