Onions are a cold-season crop, easy to grow because of their hardiness. By midsummer, you can begin harvesting individual onions! Learn more about planting, growing, and harvesting onions for an endless supply in your garden.
Onions can be planted in either the spring or fall. Onion plants grow well in raised beds or raised rows at least 4 inches high. We usually plant a mix of white, yellow, and red onions.
SHOULD I GROW ONIONS FROM SEED OR FROM SETS?
We prefer planting onion sets over starting them from seeds, simply because the sets establish quickly and are easier to plant. Onion sets are small onion bulbs that are sold specifically for gardening. Once planted, they develop into a full-size bulb after about 3½ months.
Also, onion sets can be planted without worry of frost damage and have a higher success rate than planting from onion seeds or transplants.
Of course, starting onions from seed is certainly doable, and may even be necessary in colder regions (Zone 5 and colder). If you’d prefer to try this method, check out our tips for growing onions from seed indoors.
PLANTING
WHEN TO PLANT ONIONS
- Onions can be planted in both the spring and fall seasons, depending on where you live. Generally speaking, plant onion sets outdoors when the weather is cool—not cold.
- A fall-planted crop of onions needs at least 4 to 6 weeks of warm temperatures to become established in the ground. They will remain dormant during the cool season, but be all primed and ready to grow when spring arrives. Fall onions often grow much larger and better-tasting than same-year planted onions.
- In regions with a frigid winter, it’s best to plant onion sets in the spring as soon as the ground can be worked—usually in March or April. Ideally, time your planting so that outdoor temperatures no longer dip below 28°F (-2°C) after the onions are in the ground.
- Fall-planted onions are a great way to enjoy an earlier and larger bulbed harvest from next year’s garden. Plant in the warm autumn soil so that they may establish a strong root system before winter sets in. As the cold chill of winter arrives, the crop goes dormant. Then, as the temperatures and soil warm again in early spring, the onions come back to life.
- If starting onions from seeds, start them indoors about 6 weeks before you plan to transplant them to the garden. Onion seeds need temperatures of at least 50°F (10°C) to germinate properly.
- Tip: Once the seeds have foliage that’s
PREPARING THE PLANTING SITE
- Growing great onions, whether in the spring or fall, all starts with soil that is well-drained, loose, and fertile. Compacted, rocky, or clay-heavy soil will affect bulb development and result in subpar onions.
- Tip: If you live in an area with heavy clay or hard soil, add in aged compost (humus) to improve soil texture. Contrary to popular belief, adding sand will only make it worse!
- Add aged manure or compost to the soil in the fall or early spring. Onion plants are heavy feeders and need constant nourishment to produce big bulbs.
- Select a location with full sun, where your onions won’t be shaded by other plants. The more energy they can get from the sunlight, the larger their bulbs can grow.
- At planting time, add about an inch of compost to the bottom of each row before planting. Or, dig a trench in the soil about 2 inches deep and 3 inches wide, and then fill the trench back in with about an inch of compost.
- Practice crop rotation with onions. In other words, don’t plant them in the same location year after year, as this can encourage the spread of diseases that affect the crop.
HOW TO PLANT ONION SETS
- Choose onion sets that are around ¾-inch in diameter and no bigger. Larger ones may produce stiff necks and go to seed too quickly.
- When planting onion sets, plant them between 2 and 6 inches apart.
- When planting larger transplants into the garden, space plants 4 to 5 inches apart in rows 12 to 18 inches apart.
- Set the bulbs with the point end up and don’t bury them more than 1 inch under the soil. It’s important that onions aren’t planted too deep, as this can affect bulb development.
CAN YOU PLANT A SPROUTED ONION?
Yes, you can plant a sprouted onion, though you won’t get more onions from it. You will get lots of tasty green sprouts, however! Here’s how to do it:
- Fill a pot with potting soil and make a hole in the middle that is about the depth and width of the onion.
- Place the onion in the hole and cover with soil.
- Water and put the pot in a sunny spot.
- Harvest the green sprouts as needed for cooking.
If you get a sprout with a flower, wait until the flower goes to seed. Save the seeds for planting in the spring.
ALSO READ RELATED POST ON: HARVEST/STORAGE – HOW TO HARVEST ONIONS
HOW TO CARE FOR ONIONS
Think of onions as a leaf crop, not a root crop (such as beets or carrots)!
- Fertilize every few weeks with nitrogen to get big bulbs. Cease fertilizing when the onions push the soil away and the bulbing process has started. Do not put the soil back around the onions; the bulb needs to emerge above the soil.
- Once bulbs have started to develop, mulch with a light ½-inch layer of straw (shredded leaves work well in the fall, too). Mulch will help retain moisture, suppress weeds, and allow air circulation.
- Generally, onion plants do not need supplemental watering if mulch is used. About one inch of water per square foot per week (including rain water) is sufficient. If you want sweeter onions, water more.
- Onions will look healthy even if they are bone dry, so be sure to water during drought conditions. This helps to prevent onions from bolting, which is when a stressed plant speeds up its development of flowers.
PESTS/DISEASES
- Thrips: To control thrips—tiny insects about as fat as a sewing needle—take a dark piece of paper into the garden and knock the onion tops against it; if thrips are present, you will spot their tan-colored bodies on the paper. A couple of treatments with insecticidal soap kills them. Follow the package directions. Spray the plants twice, three days apart, and the thrips should disappear.
- Onion Maggots: Cover your emerging onion crop with a fine mesh netting. Seal it by mounding soil around the edges. The onion maggot likes to lay its eggs at the base of plants, so the netting should prevent that. You should also keep mulch away because the insects like decaying organic matter, and make sure you completely harvest your onions as the season progresses. Onion maggots are usually a problem in very rainy periods, so these precautions may be unnecessary if you have a dry season.
