Growing Onions: The Complete Guide to Plant, Care, and Harvest Onions (2024)

I love to cook, so naturally, onions are always somewhere in my pantry or fridge. Red, yellow, white, and gourmet varieties have all found their way into my dishes at one point or another. I can’t recall the last time I cooked and didn’t chop up an onion.

I love onions raw, cooked, and caramelized. While I don’t love the tears that come with slicing up an onion, I’ve learned a few tricks over the years. I’m most fond of the time I wore goggles and a scarf to cut up an onion for risotto. People laughed, but it worked! Next time, consider donning a pair of ski goggles to prevent the stinging burn of onion-cutting.

I’ve only grown onions a handful of times, mostly because I often forget to start them early enough. They’re an incredibly rewarding crop, though. There’s something about harvesting a nice big onion. Tomatoes? Easy peasy. Everyone and their grandmother grow tomatoes, but for many, onions are a mysterious crop.

I think it’s because a lot of folks don’t know where most of their food comes from. I remember explaining to family members how Brussel sprouts grow – I got some incredulous looks! Even certain kale varieties seem a bit weird to the newly minted gardener. Onions, too, are a food we rarely think about, but couldn’t go without.

The guide below covers popular onion varieties, planting pointers, care tips, and more, so you can confidently grow onions in your garden space.

Onion Plant Info

• Hardiness Zones: 3, 4, 5, 6, 7, 8, 9
• Soil: Loam, sandy, clay, PH between 5.5 to 6.5, well-drained, fertile
• Sun Exposure: Full sun
• Planting:
  • Start Indoors: 4 to 6 weeks before the last frost date
  • Hardening Off: 7 to 10 days before transplanting
  • Transplant Outdoors: 6 weeks after planting indoors
• Spacing: 4 to 5 inches between plants and 12 to 18 inches between rows
• Depth: ½ to 1 inch seed depth
• Best Companions: Tomato, lettuce, chamomile, rose, strawberry, summer savory,
• Worst Companions: Beans, peas, sage, asparagus, leek, garlic
• Watering: Thoroughly after planting and once a week thereafter, 1 inch of water per week
• Fertilizing: Apply high nitrogen fertilizer every few weeks, stop fertilizer application once bulbing begin
• Common Problems: Black mold, botrytis leaf blight, downy mildew, fusarium basal plate rot, pink root, fusarium damping-off, rust, purple blotch, white rot, smut, leaf streak, onion yellow dwarf, pythium seed rot, leafminers, bulb mites, onion maggot, onion thrips,
• Harvest: 80 to 150 days after transplanting, when the leaves begin to yellow and fall off

Onion Varieties

There are many onion varieties, but here are a few of my favorites.

• Ailsa Craig: This classic Spanish onion tastes great raw. It produces large bulbs up to 6-inches in diameter and takes 105 days to maturity.
• Yellow Cipollini: Quite possibly my favorite onion variety, I’m always so excited to see Cipollini at the farmer’s market. They mature in about 80 days, and they have a pleasant flavor profile.
• Red Baron: Red Baron is a red onion that produces 3-inch bulbs. It takes 115 days to reach maturity.
• New York Early: An early-maturing variety that is ready to harvest in 98 days. It has lovely yellow flesh.
• Evergreen Bunching: This is one of the first onion varieties I ever planted. It’s a quick growing, with exceptional winter hardiness.
• Valencia: Valencia is another Spanish-type onion. It has beautiful brown skin and milder flavor than other varieties. It matures in 120 days.
• Gladstone: Gladstone is a white onion that’s somewhat sweet and suited for northern growing. It matures in 110 days.
• Walla Walla: Walla Walla is resistant to disease, features white flesh and produces uniform bulbs. It’s a long day onion.

Long or Short Day

Long-day onions are ideal for colder climates, while short-day onions are best for warmer southern climes. Bulb formation occurs in response to temperature and day length. Long day onions need longer days to spur bulb growth – about 14-16 hours of daylight.

Short day varieties will begin to bulb when sunlight hours are between 10 and 12. Most short-day types are poor candidates for long term storage. While it’s possible to grow short day onions in the north, you’ll likely experience setbacks and won’t get a useful harvest.

Planting Onions

Growing Zones

Onions are a cool season crop and typically have long growth periods. They grow best in zones 3-9.

Sun and Soil Preferences

Onions prefer full sun and well-drained, fertile soil with a pH between 5.5-6.5.

Starting Seeds

Onions are best planted in the early spring and harvested by the end of the season.

Start seeds in flats indoors 4-6 weeks before the last frost date. Alliums are the earliest seedlings to be started indoors. It’s possible to sow seeds directly in the garden (or plants onion sets), but starts will move things along much quicker. Germination of onion seeds takes 1-2 weeks, depending on conditions.

The key is starting onion seed indoors early enough so that the starts have a sufficient amount of time to mature.

Planting in the Garden

Transplant onions outdoors six weeks after starting indoors. Be sure to harden them off for 7-10 days before putting them in the ground. Some gardeners cut back the tops of seedlings when transplanting, but I think it’s better to leave them alone to allow for sufficient top growth, as it has a direct impact on bulb growth.

Plant onions in an area where the soil is fertile and loose enough to allow for the bulb to expand. Onions can also be grown in a garden bed or raised bed. Onions used for their leaves (green onion, chives) are well-suited to containers.

Spacing

Space plants about 5 inches apart with 18 inches between rows. If you’re a Square Foot Gardener, plant onions 9 per square foot. Put seeds 1-inch deep.

Onion Sets Versus Seeds

Sets are little dried onions that you can use to get a head start on the onion growing season without the need to start indoor seeds months prior. Onion sets are easier for beginners but don’t provide the same results as transplants started from seed. They typically produce smaller bulbs.

If you’ve purchased sets for planting but the soil isn’t warm enough yet, store in the fridge until planting time. Plant sets fairly shallow.

Caring for Onions

Here are a few tips for caring for your growing onions:

Watering

Water enough to prevent drought, but adequate mulching and rainfall, depending on where you live, could be enough. Watering evenly is vital to avoid misshapen bulbs. Give plants about an inch of water per week.

Weeding

Weed growing onions regularly to prevent competition for space, which can reduce bulb size.

Mulching

Keep onion beds well-mulched. Mulching helps to keep weeds down and conserves moisture.

Fertilizing

Onions are heavy feeders, so fertilize early in the season with nitrogen-rich fertilizer and then again at regular intervals during the season. Stop once the bulbs have formed. Too much nitrogen may negatively impact plants, so take care to test your soil first.

Crop Rotation

Do not plant alliums in the same place where you grew onions in previous years.

Troubleshooting Onions

Here are a few issues you may encounter while growing onions.

I planted an onion from my kitchen that was sprouting, but I dug it up at the end of the season and didn’t find any edible bulb. What happened?

You can’t plant a sprouted bulb and expect it to multiply into more onions. Instead, enjoy the delicious green tops that have sprouted.

My onions aren’t bulbing. Why not?

Did you choose the correct onion for your region? Onion bulbing occurs mainly in response to day length and temperature. Growing the wrong type of onion may result in tiny bulbs or none at all. No bulbs may also mean that your plants are getting excess nitrogen.

My onion bulbs are misshapen. Why?

To prevent uneven bulb formation, make sure to water evenly throughout the season.

Problems and Solutions to Growing Onions

Plant diseases are such a pain, but knowing what to look out for helps prevent the further spread of fungi and bacteria. Here are some illnesses that onions are susceptible to. While onions are an excellent companion for many plants due to their pest repelling properties, there are a few pests that prey on allium family plants.

Botrytis Leaf Blight and Downy mildew

Botrytis leaf blight disease is characterized by the white spots it causes on leaves. Downy mildew produces greyish leaf spots.

Both diseases slowly kill the plant off. Prevent them by watering from below and avoid planting onions in shaded areas. Crop rotation is also important. Get rid of infected plant material to prevent the spread of the disease.

Black Mold

Black mold is caused by the fungus Aspergillus niger. It can attack bulbs both as they’re growing or in storage. Ensure your garden has good drainage and air circulation. Your storage area should have good circulation as well. Avoid damaging the bulbs when harvesting, because that provides a place for the mold to enter.

Pink Root

As the name implies, this disease will give your onions pink roots, which eventually turn purple and then rot. It’s caused by a fungus found in the soil. Use good crop rotation practices, choose disease-resistant varieties, and solarize your soil if you have this disease to prevent it from returning next year.

White Rot

If you notice your onion leaves turning yellow or dying, with the decay starting at the base, you might have white rot. You may also see a fluffy white fungal growth around the base of the bulb.

The best way to tackle white rot is to avoid it. Clean your tools between use, buy certified clean seeds, and rotate your crops.

Yellow Onion Dwarf

You’ll know if you have yellow onion dwarf if your onion leaves have yellow streaks, which will eventually turn the entire leaf yellow. It also causes bulbs to be undersized. There is no chemical control for this virus, so don’t use sets if you struggle with it because they can be contaminated. Instead, choose certified disease-free seeds. Pull and destroy infected plants.

Leafminers

Leafminers create distinct linear marks towards the ends of onion leaves. The leaves may eventually twist and crinkle thanks to the damage. You may also find tunnels chewed through the onion bulb. These tunnels create the ideal condition for fungi and viruses to get in.

You can use a spinosad-based spray to control them, along with yellow sticky traps.

Bulb Mites

There are several kinds of mites that attack onion bulbs. Some are round and resemble tiny pearls, while others are elongated. Both attack growing onions and onions in storage. Both cause stunted growth and promote rot.

To control them, rotate your crops and don’t plant onions where you’ve had brassicas in the past few years. You can also soak seeds in 2% soap, 2% mineral oil and water for 24 hours before planting. Keep garden areas clean and well-weeded.

Thrips

These little tiny tan bugs can be controlled using natural pest spray like neem oil.

Onion Maggots

Onion maggots come from grayish-brown, hump-backed flies. Prevent these pests by covering your onions with insect netting or row covers and yellow sticky traps. If onion maggots are a problem, limit the amount of mulch you use around your onions. Don’t leave onions to rot in the soil; dispose of them to prevent attracting maggots.

Companions Planting for Onions

Onions are an excellent companion for many garden plants because the strong smell is a powerful deterrent to many pests.

Best Companions

• Brassicas
• Tomatoes
• Lettuce
• Beets
• Spinach
• Chamomile
• Rose
• Strawberry
• Summer Savory

Worst Companions

• Asparagus
• Beans
• Peas
• Sage
• Leek
• Garlic

Harvesting and Storing Onions

It’s time to harvest onions once a flower stalk has emerged or the tops have entirely died over (similar to potatoes). Onions are typically harvested at the very end of the growing season, usually 80-150 days after planting.

Pull up the onions if you plant to use them right away. For storage onions, remove surface dirt to expose most of the bulbs and wait for the skin to dry. Pull from the soil and leave to cure outside.

While the onions are curing, protect them from rainy weather to prevent rot. The drying process may take several weeks, depending on the weather. In rainy regions, it may be best to bring onions indoors to continue the curing process. Store in a cool, dry place once fully cured. Sweet varieties are more likely to spoil, so eat them first.

Onions are genuinely one of the pantry staples I couldn’t live without. I use them for virtually every single meal! In the summer, fresh sliced sweet red onions offer a zesty bite to garden salads, and in the winter, a hot cup of French onion soup is a cozy treat. Growing onions used to seem challenging, but now that I know the secrets, I don’t hesitate to have an onion patch in my garden.

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