They're also one of the more difficult garden vegetables to achieve good yields of. Onions are easy to grow, and difficult to grow well.
The simplest way to grow onions is from sets. In another post someday we'll go over onions from seed, but for now we're staying simple.
Typically, onion sets found in greenhouses or at chain stores are labeled either "Yellow", "White", or "Red". Not the most descriptive packaging and not really very useful information, but generally these are day neutral onions that will grow pretty much anywhere. Though they aren't the best for storage or the largest/sweetest/etc onions you could grow, onion sets are typically a safe choice that will produce a decent crop.
Onion sets are year old onions. They are started from seed, grown for a period of time and then harvested, dried, and stored. They are easy to plant and grow because they already have all the things needed to grow a respectable onion. They have a root system, and a store of energy to restart growing as soon as they're planted.
|Red onions are easily grown from sets.|
When and How
Plant onion sets as early as you can stick them in the ground, spaced about 3-4 inches apart. Rows waste space, plant them in a grid. You want just enough room to let them grow and so you can get your hands in there to pull weeds. Plant sets so the top is just barely poking out of the ground. The roots will be 1/2 to 3/4 inch deep that way. When I plant sets, I don't wait for the ground to dry out so it can be tilled. There might even be snow in the corners of the yard. I rake off whatever litter is on top of the soil and stick the sets right in the ground. If I have to pull weeds or last year's stalks first, I will, but I don't do any other prep to the soil. Keep in mind that I have good soil quality and I NEVER EVER EVER walk in my garden beds. I don't need to till to make the soil loose enough to grow onions, but if you have heavy or compacted soil, you may need to wait until it can be at least hand tilled before planting onion sets. The important thing is getting them in the ground as early as possible. The more cool wet days with sunshine that the onions get before it gets hot and the days start to shorten, the better your onion crop will be.
After planting, I immediately side dress with well aged compost. You can probably use a fertilizer, but I prefer compost. My compost is mostly made up of kitchen scraps, grass clippings, leaf litter, and straw from the chicken coop. Onions are very heavy feeders, and a good layer of nutrient rich compost will help make sure you have nice big onions. I usually put down about 1/2 inch of compost on the whole area, maybe more. Make sure the onions aren't buried under a thick layer. Brush some off the set tops if necessary. After adding compost, I cover the whole area with a light layer of grass clippings if I have them. If I don't, I wait until I do. Mulching onions is very important. Because they are heavy feeders, competition from weeds using the nutrients in the soil will hurt their growth significantly. Usually, the soil is plenty wet at this time of year so I don't need to water sets after planting. If it's dry though, water them well once, and then don't water again until the very top of the soil is dry. Fewer, deeper waterings will help your onions, and all your other plants, grow deeper roots and better withstand the upcoming heat of summer.
When the onions get to be about 6 inches tall, or after they've been in the ground for 30-45 days, I give them another sprinkling of compost fertilizer, and then another layer of grass clipping mulch over the top of that. The nutrients in these layers of compost will seep into the soil for the onions to take in.
Onions are biennial plants. This means they grow a plant the first year, go dormant for the winter, then re-grow again the second year and then flower. Onion sets, being 2nd year plants, are going to flower sometime during the middle of the growing season. For me, this happens in June. These flowers should be pinched off before they open and pollinate. This keeps the onion putting its energy into the bulb instead of seeds.
From here on out, keep the onions weeded, and well watered, and wait for the tops to yellow and tip over.
Harvest and Storage
Harvest onions when most, or more than half of the tops are yellowing and falling over. At this point, they are no longer growing larger, and are starting to go dormant. Pull onions, and lay them out in a warm but not hot, shady, well ventilated area to dry. I've got an old screen door sitting on sawhorses on the north side of my house where it's shady and the breeze blows through. If it looks like rain, bring the onions indoors until rain has passed. Leave the onions to dry outdoors for at least a week, 2-3 is better. Usually I leave them out for a few days and then move them indoors to finish drying. If they can't be left outdoors, spread them out in a dry area with good airflow to finish drying indoors. This process is called curing. When the onions are fully cured, the neck of the onion will be dry and brown and you can cut off the dry top, about 2 inches above the onion. Any onions that still have green necks are not yet dry, and those onions should be used up first as they are most likely to spoil fastest. Proper curing is critical to the storage life of the onion. I have kept properly cured onions in the root cellar for a full year before they started to spoil. You may not get onion grown from sets to store that long, but 6 months is certainly possible with proper curing.
So there you have it. Bigger onions through fertilization and reducing weed competition.