Growing onions in your veg garden is a great way to protect nearby vegetables and herbs from pests, but it's also uniquely satisfying. Onions are particularly low-maintenance vegetables, so growing them to harvest-- especially from sets-- requires surprisingly little effort. Some onion varieties are also easy to store, making it easy to grow a few months' worth of onions in one raised bed.
This guide covers how to choose an onion variety, how to grow onions from seed or sets, basic onion plant care, and when to harvest onions for the best bulbs.
Skip Ahead: Long day vs short day • Growing onions from seed • Growing onions from sets • Onion spacing • Onion plant care • Harvesting onions • Onion varieties • FAQ
Long day vs short day onions
There are three main types of onions: short day, long day, and intermediate day. Each needs a different number of daylight hours to form a good bulb, so it's important to know which type is suited to your region before you buy seeds or sets.
Because the northern latitudes have longer daylight hours, northern gardeners need onion varieties that won't start bulbing up until they receive 14 to 16 hours of daylight. This prevents the onions from maturing too early in the season. Likewise in the US south, gardeners can get away with planting short day onions as early as the fall.
Here's how to choose between long day, short day, and intermediate (or neutral) day onions:
- Short day onions need 10-12 hours of sunlight daily to start forming a bulb. They are ideal for growing in the south-- zone 7 or warmer-- where they can be planted in the fall or early winter, giving them plenty of time to form a good bulb. Sweeter varieties tend to be short day.
- Intermediate day onions start forming bulbs at 12 to 14 hours of sunlight daily. They are ideal for zones 5 and 6 and can grow well in milder climates, but they don't produce well in the southernmost US states. Also known as neutral day onions, these varieties do best when planted in early spring in zones 5 or 6 or in late winter in more southerly zones.
- Long day onions need 14 to 16 hours of daylight to start bulbing. This makes them best suited for very northern zones-- zone 6 and above-- where they can be planted in late winter or early spring.
The energy generated by onion leaves will eventually grow a bigger bulb, so it's important to choose an onion variety that corresponds to your zone. This allows the onion enough time to grow healthy shoots before daylight hours increase and the plants start putting their energy into bulbing up.
Planting a short-day onion in zone 5, for example, would lead to plants growing fewer leaves and smaller bulbs. On the other hand, long day onion in zone 10 would likely not get enough daylight hours early enough in the season, resulting in an underdeveloped bulb.
How to grow onions from seed
To grow onions from seed, start indoors 10 to 12 weeks before your last frost date. This will give the onions enough time establish healthy roots and sturdy shoots before transplanting. (In southern zones, it is possible to sow short day onions directly in the early winter. But for gardeners in central and northern zones, growing onions for harvesting is only possible by sowing indoors and getting a head start on the season.)
Sow onion seeds into loose, fine, and moistened potting mix. It's best not to use a seed starting mix with onions. Because seed starting mix has no nutrients, seedlings sown into it need to be potted up into a fertile potting mix once true leaves emerge. Onion seedlings are too delicate and finicky to transplant at that stage; much easier to sow them directly into a fine potting mix with any larger pieces of wood sifted out.
Here's how to sow onion seeds, step by step:
- Sow onion seeds ¼" to ½" deep in a plug tray or soil blocks.
- Keep the soil moist (but not soggy) until germination.
- Once the seeds germinate, give them at least 12 hours of sunlight daily.
- Trim the seedlings to 4" or 5" tall as they grow. This encourages stronger, sturdier growth.
- Harden off seedlings before planting them out by moving them outside for increasing lengths of time for one to two weeks before planting out. This prevents transplant shock, which can kill or majorly stunt indoor-sown plants.
Once the hard frosts have passed, the soil has warmed, and your onion seedlings have grown on for a couple of months and are hardened off, they're ready to grow in the garden.
To plant out onion seedlings:
- Prepare the soil by amending it with plenty of compost.
- Prepare planting holes about 1" deep with a dibber (or your finger).
- Plant onion seedlings ½" to 1" deep.
- Trim the shoots down to 5" or 6", and remove any tattered or yellow outer leaves.
- Water the seedlings well to prevent transplant shock and ensure the roots are in good contact with the soil.
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Planting and growing onions from sets
Onion sets are immature onions that were harvested and stored when the bulbs were still very small. If the sets are small enough-- under an inch in diameter-- you can plant them as if they were seedlings. The sets send up green shoots before shifting their energy to sizing up their bulbs. The advantage to growing onions from sets is that it takes out the hassle of an indoor seed set-up and allows you to grow sizable onions even in shorter, northern growing seasons.
Here's how to plant and grow onion sets in the spring:
- Plant onion sets as soon as the soil is workable and the hard frosts have passed. In zones 5 through 7, this is usually sometime in late March or April-- but with spring weather being so unpredictable, it's best to go by how the soil feels rather than a calendar date.
- Plant onion sets 1" deep, so that the very top point of the set is just level with the soil. Give them plenty of compost in the soil.
- Water them in well, and consider interplanting them with an onion companion plant that will germinate quickly and be ready to harvest before the onions need that space; lettuce, radishes, and spinach are good options.
Onions are biennials, meaning they produce leaves in the first year and produce only flowers in the second year. If onion sets are too large, the plant will think that it's in its second year, produce flowers at the expense of the bulb, and die. So, when buying onion sets, look for smaller bulbs that haven't started to shrivel or send up shoots.
Onion spacing
When planting out onions, there are two approaches to spacing. Traditional onion spacing is to plant them 6" apart in rows spaced 12" apart. In theory, this gives the bulbs enough space to develop without competing for nutrients or sun.
An alternative approach is to plant onions in groups of three, spaced 6-8" apart in rows spaced 12" apart. In this method, the bulbs force each other outward as they grow. Some gardeners find that their onions actually reach comparable sizes when planted this way, but that overall they get a greater yield from their onion bed.
Both approaches can work well, and it's worth experimenting. If you want to maximize the space in your garden bed but don't want to try growing onions in groups, interplanting onions with leafy greens, carrots, parsley, and other space-efficient onion companion plants is another good option.
Onion plant care
As main season vegetables, onions need full sun, rich soil, and regular watering. They do best when given a very loose, fertile soil, a sunny position, and adequate spacing.
- Full sun: Onions require full sun, so they shouldn't be planted next to tall or bushy plants that will shade them out. Growing onions in a north-south orientation will also help ensure that the onions don't shade each other out as the sun moves from east to west.
- Rich soil: Onions like fertile, loose soil. They don't require fertilization if planted into healthy, rich soil.
- Watering: Onions have shallow roots that don't want to dry out, so the soil around onion seedlings should be kept moist. For established onion plants, water whenever the top inch of soil is dry. As the bulbs start to develop and the leaves start to droop, water them less; this encourages bulb growth and will avoid the bulb rotting in consistently wet soil.
Harvesting onions
For full sized bulbs, harvest onions when the tops have flopped over and started to yellow. The easiest way to harvest onion bulbs in by taking a firm grip on the base of the leaves and twisting as you pull up.
Most onions need to be eaten fresh; only certain varieties are suited for long-term storage. But if you grow a storage variety or want to harvest a few fresh eating onions for use in the near term, you'll need to cure the onions. To do this, leave them in a warm place out of direct sun with some airflow-- in the grass under a tree, on a patio table under an umbrella, or over a shaded fence all work. As long as they don't get wet, the onions will be ready to be braided or have their tops cut off and be stored after a day or two.
Here's how to harvest onions at other stages of development:
- Green onions: Green onions are easy to harvest in the spring and early summer by cutting the leaves down to the soil. They will regrow, providing a few harvests before the plants die back.
- Onion sets: If you've gotten a late start on growing storage onions from seed and don't expect to be able to grow sizable bulbs, you can harvest the onions to use as sets next year. To do this, pull the bulbs up when they are ½" to ¾" around. Cure them as usual and store them somewhere cool, dark, and dry.
Onion varieties
In addition to short day, long day, and intermediate day onions, there are multiple onion varieties to choose from. These include storage and fresh eating varieties, as well as sweet onions, yellow onions, and red onions.
Here are a few popular onion varieties:
- White onions: Snowball are long-day onions with a mild flavor. Crystal White Wax is a popular short day white onion.
- Sweet onions: Walla Walla and Utah Sweet are long day sweet onions, and Texas Sweet is a popular short day variety.
- Yellow storage onions: Patterson is a long day storage onion that can keep for up to a year; it might be the most popular yellow storage onion. Gabriela is a short day storage onion that can keep for 2-3 months. Sturon is another popular long day storage onion that's easy to grow from sets and somewhat resistant to bolting.
- Red storage onions: Redwing is a popular long day red storage onion. Red Creole is a short day onion suited to growing in the US south. It can store for upwards of 6 months.
- Red fresh eating onions: Red Candy is a sweet, intermediate day onion not suited to longterm storage. Cippolini Red is another intermediate day onion variety for fresh eating.
How long does it take for onions to grow?
Onions take 90 to 100 days to grow from seed, or about 60 to 90 days to grow from sets.
How do you know when onions are ready to harvest?
When onions are ready to harvest, the tops will fall over and start to yellow. This means that the bulb has reached its full size and can be pulled up. You can harvest onions at any time, though; if you want smaller onions, it's fine to harvest them earlier.
Do onions need full sun?
All onions need to be planted into full sun, with at least 10 hours of sunlight daily. It's also important to buy the right kind of onion for your zone. In northern zones, long day onions require 14 to 16 hours of sun. Intermediate day onions need 12 to 14 hours of sun, and short day onions need 10 to 12 hours.
What is the best soil for onions?
Onions grow best in loose, well draining soil that's rich in organic matter. Amending the soil with a couple of inches of garden compost will ensure that the plant can access the nutrients it needs to support leafy growth and a large bulb.
When should I plant onion sets?
Plant onion sets directly when the hard frosts have passed and the soil is workable and no longer cool to the touch. In central and northern zones, this is typically in mid to late March or April. For a head start, you can grow onion sets indoors for a week or two before planting them out.