Timing is important
Mid-October is when the temperatures start to cool off enough to safely start onions. Short day varieties, the varieties that should be planted on the Gulf Coast, will have sufficient time to bulb as the days get longer. They will be ready to harvest between March and May, depending on variety.
Onions can be started indoors during late September, but there is no advantage to doing so. Onions that are too large during a hard freeze in December MAY have a tendency to bolt as soon as temperatures bounce back, depending on the variety. Mid-October is early enough to get started.
Onion seed is dependably viable for one year. It may hold viability for two or three years if it is stored perfectly in a dark, cool location, but vigor will be noticeably reduced. I am not saying to toss the seed out, just be prepared for less than expected germination.
Onion seed benefits from pre-soaking before planting. They can be soaked for as little as 15 minutes or as long as several hours. You can use plain water, compost tea, or Superthrive as you prefer. Drain and pat dry just before planting to make handling easier.
One of the most popular ways to grow onions is to use seedlings. Seedlings of several varieties are available at local nurseries and feed stores starting in January. But if you are interested in another variety, or just want to grow your own seedlings, it is easy to do. You can do this is a flat, or directly in the ground.
Broadcast onion seed fairly thickly on prepared soil – about 1/4″ apart each way. One of the easiest ways is to plant a band about 2″ wide and as long as your seed packet will allow. Square foot gardeners can broadcast a packet of seed over one square. Cover 1/4″ to 1/2″ deep. Tamp the soil over the seed and water well. Keep moist until germination is evident.
Allow to grow freely. If the seedlings grow tall too quickly, trim them back to 4″ tall. They can be trimmed several times if necessary. When the seedlings reach the 1/2 the size of a pencil to pencil size, gently dig and prepare for transplant. Trim the roots to 1/2″ long and trim the tops to 4″ – 6″ long. Keep the roots moist by settling them into a pot of compost or potting soil until you are ready to replant.
If garden space is at a premium, you can grow your seedlings in flats or pots. Prepare the seed and plant the same way. When you are ready to transplant in the garden, trim the tops as above, gently remove the root ball from the container, and tease the seedlings apart.
Some gardeners swear that better bulbs are achieved by planting onion seeds and letting them grow to maturity in place. For row gardening, make a 1/2″ deep furrow. Plant the seed 1″ apart along the row. Cover with 1/4″ – 1/2″ of soil, tamp, and water well. Thin the seedlings as they gain size, using the seedlings as scallions. Final spacing should be 4″ – 6″.
Square foot gardeners can plant 9/square for full size bulbs, or 16 per square for spring onions. Plant 3 seeds per hole, and thin to one each after germination.
For more information on onion culture, see the Growing Guide for Bulbing Onions.