Onions might be one of the most overlooked ingredients in the kitchen, but their journey from seed to harvest is anything but boring. Whether you’re a backyard gardener, a small-scale farmer, or simply curious about what goes into growing that yellow, white, or red bulb on your plate—understanding how onions are grown offers insight into both plant science and age-old farming techniques.
Let’s dig into it—literally.
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The Basics: Understanding Onion Types
Before you plant onions, it’s important to understand the different types available and how they affect your growing strategy.
Short-Day vs. Long-Day Onions
The first decision growers make revolves around day length. Onions are highly sensitive to how much sunlight they receive, especially during bulb formation.
- Short-day onions begin forming bulbs when daylight reaches about 10–12 hours per day. They are ideal for gardeners in the southern United States where winters are mild, and the daylight hours remain shorter for longer.
- Long-day onions require 14–16 hours of daylight to start forming a proper onion bulb. These varieties thrive in the northern states where summer days are long and cool.
There’s also an intermediate-day variety, suitable for gardeners in temperate zones. Choosing the wrong type for your region can lead to plants that produce only tops without ever developing bulbs.
Ways to Start Growing Onion Plants
You have three main options when it comes to starting your onion crop: onion seed, onion sets, or transplants. Each method comes with its own benefits and challenges, so it’s worth understanding which is best for your garden’s conditions and your own schedule.
1. Onion Seeds
Starting from onion seed is the most affordable method and offers the greatest diversity of varieties. However, it does take the longest—typically 100 to 120 days to maturity. Seeds need to be started indoors in many regions, especially in colder climates.
If you’re growing onion plants from seed, begin about 10–12 weeks before your area’s last expected frost date. Use seed trays filled with light potting mix, and provide consistent heat and light to ensure germination.
The benefit of growing onions from seed is better bulb uniformity and disease resistance compared to sets.
2. Onion Sets
Onion sets are small, partially matured onion bulbs grown the previous season and harvested before they reach full size. These are replanted in spring and mature more quickly than seeds—typically in 60 to 80 days.
They’re a popular choice for beginner gardeners who want fast, reliable results. However, onion sets may be prone to bolting (going to seed early), and they offer fewer variety options.
That said, if you want to plant onion crops in cool soil early in the season and don’t have time to start seeds, sets can be a great option.
3. Onion Transplants
Transplants are young onion seedlings that have already been started from seed and grown to several inches tall. They can be purchased from nurseries or grown indoors. Transplants strike a nice balance between the benefits of seeds and sets.
They’re less likely to bolt compared to sets, and they allow you to choose from a wider variety of cultivars than sets typically offer.
Preparing the Soil for Onion Success
Regardless of how you plant onions, they need soil that’s loose, fertile, and well-draining. Onions grow best in soil that allows their shallow root systems to expand easily and that doesn’t trap moisture around the developing bulbs.
Soil pH and Nutrients
The ideal soil pH for growing onion plants is between 6.0 and 6.8. If your soil is too acidic or alkaline, it can interfere with nutrient uptake.
Enrich the soil with compost, well-rotted manure, or other organic matter. Onions love nitrogen early in their growth phase to develop healthy green tops—but too much nitrogen once bulbs start forming can actually inhibit development.
Tilling and Raised Beds
Prepare raised beds or deeply loosened soil to encourage deep root growth and bulb expansion. Raised beds are especially helpful in areas with poor drainage.
Avoid planting onions in heavy clay soil unless it has been amended with sand and compost to improve texture.
When and How to Plant Onion Crops
Timing is everything when it comes to planting onions. Whether you’re starting from seed, using onion sets, or planting transplants, your local climate and the onion variety will dictate the right time to get started.
Planting Guidelines
- Seeds: Start indoors 10–12 weeks before the last frost date.
- Transplants: Set them outside 4–6 weeks before the last frost.
- Sets: Directly plant onion sets outdoors once the soil is workable, usually in early spring.
Spacing and Depth
Plant your onions in rows spaced 12–18 inches apart. Each plant should be spaced 4–6 inches apart within the row. For small onions like pearl or cipollini varieties, you can reduce spacing to about 3 inches.
Seeds should be planted about ¼ inch deep, while sets can be planted 1 inch deep, pointy side up.
Watering and Mulching
Onions require consistent moisture, especially in the early stages of growth and during bulb formation. Aim for about 1 inch of water per week, adjusting during hot or dry spells.
Mulching
Mulch helps retain soil moisture, suppress weeds, and keep soil temperatures stable. A layer of straw, grass clippings, or shredded leaves around your onions can make a big difference—especially for gardeners in warmer regions.
Weed Control and Maintenance
Weeds are a major concern when growing onion crops. Onions grow slowly and don’t develop a dense canopy, so they’re easily overshadowed by aggressive weeds.
Use shallow cultivation or hand-pulling to avoid disturbing onion roots. Mulching can significantly cut down on weed competition.
Understanding Bulb Formation
Perhaps the most exciting (and mysterious) stage in the onion lifecycle is onion bulb formation.
Initially, the plant focuses on developing roots and green tops. But once it reaches the correct day length, based on its variety, it will shift to forming a bulb.
This is why timing is so critical when planting. If you plant too early or too late, you may miss the day-length window necessary for successful bulb development.
Fertilizing Your Onions
Onions benefit from a feeding schedule that supports leaf growth early on and bulb development later.
Early Fertilizing
Use a nitrogen-rich fertilizer like blood meal or a 10-10-10 mix every 2–3 weeks during the first half of the growing season. The more green tops your plant develops, the larger your final bulb will be.
Tapering Off
As soon as you notice the plant starting to form bulbs, stop applying nitrogen. Continued feeding at this stage may result in overly soft bulbs or rot.
Common Onion Problems
While onions are relatively low-maintenance, they’re not immune to problems. Here are some to watch for:
Pests
- Onion thrips: Cause silvery streaks on leaves and stunted growth.
- Onion maggots: Feed on the base of the bulb and can cause rot.
- Cutworms: Cut through young stems at soil level.
Using floating row covers early in the season and rotating your crops annually can help reduce pest pressures.
Diseases
- Downy mildew: Fungal disease that affects foliage.
- White rot: Soil-borne fungus that affects bulbs.
- Neck rot: Affects onions during storage, usually due to poor curing.
Prevent disease by spacing plants well, using drip irrigation, and rotating crops annually. Avoid overhead watering whenever possible.
How and When to Harvest Onions
Knowing when to harvest is crucial to getting the best flavor and longest shelf life.
Signs of Maturity
- Tops start to yellow and fall over
- Bulbs feel firm and well-rounded
- At least half the tops have fallen over naturally
Stop watering at this point and allow the bulbs to sit in the soil for a few more days to firm up.
Harvesting Technique
Use a garden fork to gently loosen the soil around the bulb. Pull them out carefully and shake off excess dirt. Avoid bruising or nicking the bulbs, as this will reduce storage life.
Curing and Storing Onions
Freshly harvested onions are not ready to store right away—they need to be cured.
Curing Process
Spread onions in a single layer in a warm, dry, and shaded area with good airflow. Allow them to cure for 2–3 weeks. The outer skin should become papery and the neck completely dry.
Storing Options
- Store in a cool, dry, dark place (ideally 35–55°F)
- Use mesh bags, open crates, or braided bundles to allow air circulation
Spring onions or green onions, which are harvested early before full bulb formation, should be used fresh or stored in the fridge for a short time.
Sweet onions don’t store as long as pungent varieties and are best consumed within a few weeks.
Can You Regrow Onions?
Yes—though not always to full bulb form.
If you save the root end of an onion and replant it, it may sprout new green onions or tops. These can be used in cooking just like chives or scallions. With the right conditions, some may even form small onions, but they usually won’t reach full size.
Another method is to allow mature onions to flower and collect their seeds for next season. This completes the full growing cycle, especially useful for heirloom varieties.
Final Thoughts
So, how are onions grown? With a combination of timing, soil preparation, proper spacing, and careful maintenance.
Whether you’re starting from onion seed, using onion sets, or planting transplants, the key is to match your method to your climate and your level of experience. From the first signs of green shoots to the drying racks of curing bulbs, onions offer a rewarding gardening experience for both beginners and seasoned growers.
Whether you’re aiming for hefty storage onions or a fresh harvest of spring onions, the process of growing this humble crop brings both satisfaction and a deeper appreciation for the food on your plate.
Read more about growing onions from this article by University of Minnesota here.
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