Take the time before your onion starts come to prep your garden beds with some good compost, and your favorite high-phosphorous fertilizer. Something along the lines of 5-10-5 will work great. Forking your beds well to create good drainage and break up clots is a good idea too, as onions need the freedom to bulb-out.
When you receive your plants, plant asap into your garden beds at about 3-4” apart depending on variety. Water your seedlings in well, and adhere to the 1” of rain per week average for veggie gardens to keep your onions healthy. You’ll need to stop supplemental watering of your onions around mid-July so they don’t grow too fast and crack. As the onions start to “size-up” you’ll want to pull the soil away from the tops of the bulbs as they mature. You should see a fair amount of the onion, at least half, above the soil line. This will help the onions dry faster and prevent rotting.
You can harvest and eat your onions anytime, but refer to your variety specifics to know what size they are expected to be at maturity. If your onions are beginning to yellow heavily on the top of the greens, the bulbs aren’t likely to get larger and you should go ahead and harvest.
When harvesting, avoid pulling up on the greens, as this could damage the bulb and the curing process. Instead, use your digging fork to push up from under the onion and then gently shake the soil away.
Trim the greens down to about 2” for drying. If you want to braid your onions into a rope like garlic, you can leave the greens longer, but keep in mind that the greens will pull some of the sugars from your bulbs. Place your bulbs in a warm, dry, and dark area for curing. I spread mine in a single layer in open cardboard boxes and put them under my dining table until they are cured, but there are lots of options out there that might work better for you. There are also many options for long-term storage, as long as they stay dry and out of sunlight.
Onion, Allium cepa
Pollination, insect; Life Cycle, biennial; Isolation Distance, 1 mile
Onions are an insect pollinated biennial, and can be a challenge to keep pure. Keeping flowers covered and hand pollinating with a paintbrush is often the safe way to go. Allow seeds to ripen and dry on the plants, then harvest quickly to avoid losing seeds. Further drying in a paper bag will probably be necessary before separating seeds from the flowers. Flowers will likely need to be staked as they will become heavy with seeds.