Onions offer a wide variety of options, starting from mild to pungent, tiny pearl onions all the way to the big Bermudas. Amateur gardeners can choose how they want to plant their onions too, sets, transplants, or seeds. A number of gardeners take advantage of all three techniques, according to the theory that you simply cannot have too many onions. Here is a simple post on how each technique works.
These are typically tiny onion bulbs, cultivated from seed and forced into dormancy during an immature stage. As soon as they are planted in your garden, they will resume growing. Sets are definitely the most straightforward of the 3 planting techniques as well as a superb way to produce a large number of big onions for storage.
You should plant sets two to four weeks prior to the average last frost date, you can find last frost dates for your area over at GardenAction. In moderate winter locations, plant onion sets in Autumn or Winter, place the sets within a shallow trench and cover with only ample garden soil to have their pointed tips right at the surface of the soil. The space in between your onions need to be approximately 4 to 6 inches, depending upon on the grown size for this variety, but you can place the sets closer together initially and harvest thinnings for use as green onions.
One particular downside of depending on sets often is the reasonably limited varieties available. The majority of garden centres or large DIY stores label their bins of sets by colour (white, yellow, or red) rather than cultivar. You will probably be enticed to choose the largest sized sets from the bin, but many of these can go to seed fairly quickly rather than developing a large bulb. Sets that are 1⁄2 inch in diameter (about the size of a 5p) are usually the best buy.
Packages of bare-root onion transplants are generally readily available from online stores practically all year round but you should ensure you pick ones that are right for the area, with a greenhouse (or even indoor) environment it is possible to also produce your own.
Sow seeds using a seed starting method 8 to 10 weeks before the last frost, that should be planted in the garden about 6 weeks later. The seedlings should be planted to be set out in Autumn or winter. Thin the emerging seedlings in the flat to stand at least 1⁄2 inch apart in rows about 2 inches apart so they will be easier to separate at transplant time.
Harden off the seedlings by setting the flat in a sheltered spot outdoors for a few hours a day, gradually acclimating them to a full day of direct sunlight. On the day they are to be planted in the garden, lift the seedlings carefully from the flat and shake the soil from their roots. If the tops have grown tall and wispy, trim them back to about 6 inches.
Dig a trench for the seedlings and place them slightly deeper than they were in the flat. As with sets, seedlings can be planted closer than their ultimate spacing of 4 to 6 inches, with the extras harvested as green onions.
To grow the biggest bulbs, onions benefit from the head start they get from sets or transplants. But bunching onions or scallions are quicker to mature, and they can be seeded directly into the garden. Sow seeds outdoors beginning about a month before the frost-free date and then again every few weeks through fall for continual harvests; in the south, the season is fall through spring. Start with fresh seeds or seeds that are no more than a year old, because onion seeds lose viability quickly in storage.