Onions are a wonderful staple crop in our household, and we like to store some of our onion bulbs for 6-8 months over the winter. We eat a lot of them, and we usually plant over 200 onions each year. If you follow a few simple steps, you can harvest a large amount of high-quality onions from a small garden.
Onions Need High Quality Soil
Bulb onions are a bit pickier about the quality of the soil than some other crops. They grow best in loose, friable, fertile soil with a healthy amount of organic matter. If your soil is still poor, you might want to wait a year or two to improve your soil before you start growing bulb onions.
Choose the Right Variety for Your Location
It’s important to select the right variety of onions for your climate and latitude. Short-day varieties, such as Vidalia, are best grown in the South below 35 degrees latitude, which runs roughly along the northern borders of Mississippi, Georgia, and Alabama. Many sweet onions are short-day types, and they aren’t usually good keepers. These plants start bulbing when the day length reaches 10-12 hours in early spring, so it’s best to plant these varieties in the fall.
Long-day varieties are best grown in the northern states. The best storage varieties are often long-day types. These plants will bulb up during the long 14-16 hour summer days, and produce best north of the 38 degree latitude – which is roughly from the northern half of Virginia west through southern Utah.
Some gardeners, including myself, are located between these two areas. I’m at latitude 37 degrees. It’s too cold here to overwinter onions outdoors, so I have to look for long-day varieties that will grow well in the upper south. I’ve had good luck with Copra and Pontiac – both excellent storage varieties. Most good storage varieties have strong, pungent bulbs.
The better seed catalogs will tell you where each variety will grow well. There are also a few day-neutral varieties available, such as Candy or Stockton Red, which can be grown anywhere.
You Can Plant Onions from Sets or Transplants
Sets are tiny 1-year-old onion bulbs. This is a very easy way to start your onions, especially if they are a minor crop for you and you don’t plan to store them long. Just plant these sets with the pointy side up so that they are just below the surface of the soil. Avoid planting the largest sets, as they are more likely to bolt (go to flower instead of producing a bulb).
Onion transplants sold commercially are bare-root plants sold in bundles of about 60. They may look half-dead, but they are tough plants and recover quickly after planting.
The Advantages of Planting Onion Seed
However, if onions are a major crop for you – like they are for my sister and myself – you might want to consider starting your onions from seed instead.
You have a wider choice of onion varieties when you use seed, and you can choose between yellow, white, or red varieties. Onion seed is much cheaper than a similar amount of onion sets or plants. In addition, storage onions grown from seed usually keep much longer than the same variety grown from sets.
When and How to Start Your Onion Seeds
Onions are the earliest vegetable seeds that I start for my garden. For long-day varieties, you need to plant them 8-10 weeks before your last spring frost. As my average spring frost date is around May 10th, I try to plant my onion seeds by March 1st.
They can be planted either indoors under lights, or outside in a cold frame or greenhouse. I’ve had great luck planting mine inside my unheated hoop house, though unusually cold spring weather may slow their growth. If you are in the north, I would recommend starting your seeds indoors under lights instead – though it would be worth experimenting planting some in cold frames, too.
Onions are one of the few vegetables where I will start multiple plants together in one container, instead of starting individual plants in separate pots. They readily tolerate being transplanted as bare-root plants, unlike most seedlings, and it’s easy to separate the onion seedling roots.
Given good growing conditions in the garden, the largest and most vigorous seedlings will produce the largest bulbs. So give your young plants deep soil and enough elbow room to grow. I try to allow each seedling at least 1-2 square inches of space in the seedling flat or containers.
You don’t need to be super exact in planting the seeds. If you’re using 3” square containers, just sprinkle 10-12 seeds fairly evenly over the surface of the soil, and then cover them with a thin layer of more soil. Your plants will grow OK in a 3.5” deep pot, but they will grow even better in a deeper 5” pot.
Since my sister and I grow such a large number of onions, we plant our seeds in our large 8” deep growing trays in our hoop house. When they’re ready to transplant, we just dig them out of the container, shake off the dirt, and transplant them bare root into our garden beds.
Transplanting the Seedlings into Your Garden
You can usually transplant the seedlings into your garden around 4 weeks before your last spring frost date. It's best to harden off the seedlings before planting them, to get them used to direct sunlight and wind. Another option is to plant them in the garden bed, and then protect them with row cover, gradually exposing them to more sunlight over a week or so.
If you want the highest yields per square foot and don’t mind harvesting smaller bulbs, you can space your plants about 3-4” apart. I prefer to use large bulbs in my kitchen, so I plant mine about 5” apart. Plant your seedlings at about the same depth they were growing in their containers. If you have more transplants than you need, plant your largest ones and dispose of the weaklings.
Onions don’t tolerate competition from other plants that are growing too close to them. I once planted some onions one foot away from my row of snap peas, and the onions became very stunted. The next row of onions, just 6” further away, grew to their full size. Competition can cut their yields in half, so keep your onions well-weeded and keep the soil evenly moist – especially when they are first planted, and when the bulbs are starting to size up. We also keep our onions lightly mulched, to reduce weeds and to keep the soil moister.
I also never plant my onions after anything in the cabbage family – such as broccoli, turnips, kale, etc. Your onion yield can be reduced up to 40% in this situation. I’ve noticed this effect in my own garden, and some university studies have found this to be true, too. This may not hold true for all soils and locations, though. Just be aware of this possible issue.
Onion bulbs grow mostly above the ground. Don't try to cover the bulbs with soil! It's not necessary, and it can hurt the plants.
How to Harvest Your Onions
Onions are ready to harvest when most of the leaves start falling over just above the bulb. In cooler climates, you can wait until most of the bulbs have fallen over, then pull them up and lay them on top of the soil and let them cure in the sun for a few days before bringing them under cover to dry further. It may take 2-3 weeks in a warm, dry and well-ventilated environment to fully cure the bulbs.
You’ll know they’re properly cured when the leaves and roots are fully dried and the bulb is covered with a couple of crisp, dried papery layers. You can gently brush off any dirt, and cut off the leaves and roots. Some people choose to braid the dried onion leaves together, and hang the bundled onions in storage, which can be very attractive. Others hang their onions in mesh bags.
In a hot climate, the sun can damage the onion bulbs if you try to cure them in the open. Instead, you should harvest the bulbs when only half the crop has fallen over, and bring all of them into a shady but well-ventilated location to dry out of the sunlight.
How to Store Your Onions
Onions store best in a cool, dry location. Around 40 degrees, with low humidity and good ventilation works best. However, I rarely have perfect storage conditions. My onions are kept in a dry area that averages 55-60 degrees. They would keep better in a colder location, but some of my harvest still lasts over 6 months despite the warmer temps.
Expect to lose some of your onions in storage. Rotting onions really stink, and they can damage any wood or cardboard surface they are sitting on. When I cured my onions on the floor of a spare bedroom, I always spread a sheet of plastic under the onions, to protect the floor.
We store our onions in ventilated plastic crates that are easy to clean out. Look over your stored bulbs at least monthly, and remove any that are becoming soft or moldy.
Bulb onions are little trickier to grow than some other vegetables, but by choosing the right varieties, getting the seedlings off to a vigorous start, and giving the plants excellent growing conditions in your garden, you can harvest a large amount of high quality onions from a small garden.