The common onion (Allium cepa) is a biennial bulb that is closely related to garlic (A. sativum), shallots (A. ascalonicum), and chives (A. schoenoprasum). Onions have hollow, tubular, blue-green leaves that emerge from a bulb, which is actually a modified leaf structure with many layers. A shallow network of roots extend from the bottom of the bulb, and the bulb might push partially above ground as the plant matures.
Onions should be planted in the spring, and they have a moderate growth rate. They can be planted from seeds, transplants (seedlings that have just sprouted), or sets (small onion bulbs that are about to begin their second, final year of growth). Note that onions are toxic to pets due to their chemical compounds, so be mindful about where you plant them.
|Botanical Name||Allium cepa|
|Plant Type||Biennial, vegetable|
|Size||12–18 in. tall, 6–12 in. wide|
|Sun Exposure||Full sun|
|Soil Type||Loamy, well-drained|
|Hardiness Zones||5–10 (USDA)|
|Native Area||Uncertain; perhaps central Asia|
|Toxicity||Toxic to pets|
How to Plant Onions
When to Plant
Plant onion sets in the spring once the ground thaws and the temperature is reliably above 28 degrees Fahrenheit. Onion seeds are usually started indoors approximately six weeks prior to the outdoor soil temperature being around 50 degrees Fahrenheit. If you're starting with seedlings, plant them outside when the soil temperature is around 50 degrees Fahrenheit as well.
It’s also possible to plant in the fall in warmer climates. The onions will stay dormant over the winter and then continue to grow in the spring.
Selecting a Planting Site
Choose a sunny spot with loose soil. Compacted or rocky soil will hinder bulb growth. Also, avoid planting where other Allium species have been in the past few years. Pests and diseases that target the plants can linger in the soil. Container growth is also an option if you don't have proper garden conditions.
Spacing, Depth, and Support
To plant onion sets, press them into the soil so just the top is visible. Space the sets roughly 4 inches apart, and space rows 12 to 18 inches apart. Once the sets are in the ground, leave them alone; do not hill soil up around them. The sets are supposed to peek out of the ground at all times.
Plant seeds only about 1/4 inch deep. And thin seedlings to around 4 inches apart also in rows that are 12 to 18 inches apart. A support structure generally won't be necessary.
Onion Plant Care
Onions need full sun—at least six hours of direct sunlight per day—to grow properly. With onions, the more sunlight the better.
Proper soil is the key element to growing onions successfully. The soil needs to be extremely well-drained—even sandy—and it should have lots of organic matter. A loose loam will work well. And a soil pH that hovers around neutral to slightly acidic is best.
Onions need regular water to support the swelling of the bulbs. Give them 1 inch of water per week. But don't overwater or allow the bulbs to sit in soggy soil because this can lead to bulb rot. A light layer of mulch can help to retain soil moisture.
Temperature and Humidity
Onion seeds need temperatures of at least 40 degrees Fahrenheit to germinate. And the optimal growing conditions for onions are between roughly 68 to 77 degrees Fahrenheit. Humidity typically isn’t an issue as long as soil moisture needs are met.
Onions are fairly heavy feeders. Fertilize them every few weeks with a high-nitrogen fertilizer to support leaf growth, which will produce big bulbs. Once the soil begins to crack around where the bulb is forming, the foliage growing cycle is complete and no further application of fertilizer is required.
Most gardeners grow their onions as annuals and harvest before flower stalks appear. Bees and other insects, as well as the wind, serve to pollinate onion flowers.
Types of Onions
Onion sets or transplants purchased at a local garden center will usually be appropriate for your climate. But when buying mail-order seeds, make sure to choose the right variety based on your climate. There are three main categories of onions you can choose from:
- Short-day onions will begin forming bulbs when there are 10 to 12 hours of daylight each day. They work well in southern regions where summer daylight is comparatively short. Some common short-day onions include 'Southern Belle', 'White Bermuda', 'Granex', and 'Cipollini'.
- Long-day onions begin forming bulbs when there are 14 to 16 hours of daylight per day. They are good for northern climates where the summer days are relatively long. Some recommended long-day onions include 'Walla Walla', 'Ring Master', 'Red Zeppelin', 'Yellow Sweet Spanish', 'Italian Red Torpedo', and 'Redwing'.
- Day-neutral onions begin to form bulbs when they experience 12 to 14 hours of daylight each day. They are good for gardeners in the central U.S. Good varieties include 'Red Amposta', 'Early Yellow Globe', 'Cabernet', and 'Superstar'.
Onions vs. Garlic
Both onions and garlic are from the same plant family, and they both grow edible bulbs. Plus, they require similar growing conditions—especially loose, nutrition-rich soil. However, garlic bulbs generally mature at a much smaller size than onions. A garlic bulb also is a group of tightly packed cloves while an onion is many layers of leaves.
The time required for the bulbs to mature depends on the variety and whether they were started from seeds or sets. But you can harvest onions at any stage; even seedlings thinned from a row can be used as green onions.
Onion bulbs are fully mature when about half of the top leaves have collapsed and when the bulb skin has a papery feel. Bulbs allowed to remain in the ground until 50 percent or more of the green tops have collapsed will store longer. It’s best to harvest in dry weather.
Once you see that half the leaves have collapsed, gently coax the remaining leaves down without breaking them off the bulb. Then, allow the bulb to sit in the ground and cure for a couple of days. Next, dig up the bulb, rather than pulling it. You don’t have to dig deep—just enough to loosen the remaining roots. Brush off any loose soil, and trim the leaves to about 1 to 2 inches from the bulb. Also, trim off the roots.
You can use freshly harvest onions at any point, storing them in the refrigerator once they're cut. To store the rest of your harvest, set the onions outside in a warm, dry spot for a few days to cure. Then, hang them in a mesh bag in a cool, dry spot with good air circulation. The temperature should be roughly 40 to 60 degrees Fahrenheit. However, don’t store them in a refrigerator as that environment is too humid. Check regularly for signs of rot, and remove any culprit before it can impact the other onions.
How to Grow Onions in Pots
If your garden soil is too dense or you don’t have the right light conditions, container growth can be a good option for onions. Choose a container that’s roughly a foot deep. You can plant multiple onions per container as long as they have about 6 inches of space on each side. It’s also essential that the pot has drainage holes. Unglazed clay is a good container material to allow excess soil moisture to evaporate through its walls.
Onions generally don't require pruning. However, be sure to remove any damaged leaves promptly. If they're left to drag on the ground, they can introduce diseases or pests to the plant.
Besides growing from seeds and sets, you also can propagate an onion from scraps. This is a great way to stretch your harvest and get more out of what would’ve been waste. The best time to start this process is in the early spring. Here’s how:
- Cut roughly an inch off the bottom of a fresh onion, and remove the outer skin.
- Position the piece cut side up on a dry surface to dry out for a day.
- Place the bottom (root) side down in a container filled with moist soilless potting mix. Slightly cover the top with soil. Put the container in a warm spot with bright, indirect light.
- Keep the soil moist but not soggy. In about two weeks, you should start to see green leaves popping up through the soil. Roots will be developing at the same time. Once the leaves are several inches long and you feel resistance when you gently tug on them, you’ll know the roots have developed enough to be transplanted.
How to Grow Onions From Seed
If planting onions from seeds, plant them indoors in trays filled with seed-starter mix at least six weeks, and as much as 12 weeks, before outdoor planting time. Place the tray under artificial grow lights for 10 to 12 hours each day. Keep the potting mix damp but not soggy. When outdoor temperatures are routinely above 50 degrees Fahrenheit, transplant the seedlings into the garden.
Potting and Repotting Onions
Use a quality organic vegetable potting mix with sharp drainage for potting onions. You can mix in some compost to improve the drainage and nutrient content. Furthermore, as it's best to choose a container that can accommodate the onions' mature size, repotting shouldn't be necessary during the growing season.
Because onions are typically grown as annuals, overwintering won't be necessary. If you're planting a fall crop in a warm climate, consider raised garden beds. They will help to keep the temperature more consistent for the dormant onions throughout the winter.
Common Pests and Plant Diseases
Pests and diseases that afflict other Allium species also can impact onions. They include:
- Rot: During wet conditions, onions can develop stem or bulb rot. Avoid rot by making sure there is good soil drainage and air circulation.
- Splitting: Bulbs can split if the soil is allowed to remain dry while the bulbs are forming.
- Thrips: These small, yellowish-brown flying insects feed on leaves and can cause twisting and curling. Repeated attacks cause the foliage to stop growing, so the onion bulbs don’t mature. Plant resistant varieties, and don’t plant onions near grain crops. Neem oil and insecticidal soaps can provide temporary control.
- Onion root maggots: Root maggot larvae hatch from eggs laid by brown flies near the base of onion plants. The maggots burrow into the stems, feeding on the plants below the soil and eventually killing the onions. Rotate plants to a different location each year to avoid infestation. Using row covers for seedlings can prevent eggs from being laid. And diatomaceous earth can also be effective.
Are onions easy to grow?
Onions have a reputation for being difficult to grow, but with a little practice most gardeners can do it. Try growing from sets first, as they tend to have a good success rate.
How long does it take to grow onions?
In general, it takes roughly three to four months for an onion to mature, depending on the variety and method of planting.
Do onions come back every year?
Onions are biennial, meaning they complete their life cycle in two growing seasons. However, most gardeners grow them as annuals and harvest the bulb before the plant flowers and goes to seed.