Growing onions from sets is a straightforward shortcut to a harvest of crisp, homegrown bulbs. If you want to learn how to grow onion sets, you are in the right place. This method is faster than starting from seed and simpler for beginners, leading to reliable results.
Onion sets are small, dormant onion bulbs from the previous year. You plant them directly into your garden soil. With the right care, they quickly spring to life and mature into full-sized onions.
This guide will walk you through every step. We will cover choosing the best sets, preparing your soil, planting, care, and harvesting. You will have all the information needed for a succesful crop.
How To Grow Onion Sets
This section provides the complete roadmap for growing onion sets. Follow these steps in order for the best chance of a bountiful harvest.
Choosing The Right Onion Sets
Your success starts at the garden center or when ordering online. Selecting healthy, appropriate sets is crucial.
Understanding Onion Types: Long-Day vs. Short-Day
Onions are sensitive to daylight hours. Choosing the wrong type for your location is a common mistake.
- Long-Day Onions: Need 14-16 hours of daylight to form bulbs. Best for northern states (north of the 36th parallel, roughly from San Francisco to Washington, D.C.).
- Short-Day Onions: Need 10-12 hours of daylight. Ideal for southern states.
- Day-Neutral (Intermediate-Day) Onions: A flexible type that bulbs with 12-14 hours of light. They grow well in many regions.
Check the package or description to ensure you get the correct type. Planting the wrong one can result in no bulbs forming.
Selecting Quality Sets
Look for sets that are firm, dry, and about the size of a marble. Avoid any that are soft, sprouting heavily, or have a moldy smell. Smaller sets are less likely to bolt (flower prematurely).
When And Where To Plant Onion Sets
Timing and location are key factors for healthy growth.
Optimal Planting Time
Onion sets are cold-tolerant and can be planted in early spring, as soon as the soil can be worked. A good rule is to plant them 4-6 weeks before your last expected frost date. You can also plant in the fall in milder climates for an early summer harvest.
Choosing the Perfect Spot
Onions need full sun—at least 6-8 hours of direct sunlight daily. The site should have loose, well-draining soil. Avoid areas where water pools after rain, as this can rot the bulbs.
Preparing Your Soil For Success
Onions are heavy feeders and require nutrient-rich soil to produce large bulbs.
- Test Your Soil: Aim for a pH between 6.0 and 7.0. A simple test kit can confirm this.
- Amend the Soil: Work in 2-3 inches of compost or well-rotted manure. This improves drainage and fertility.
- Add Fertilizer: Mix in a balanced granular fertilizer (like 10-10-10) according to package instructions before planting.
Remove any weeds, rocks, or debris from the planting area. Loose soil allows the bulbs to expand easily.
The Planting Process Step-by-Step
Planting onion sets is a simple, satisfying task.
- Create Rows: Make shallow rows about 1-2 inches deep. Space rows 12-18 inches apart.
- Place the Sets: Set each onion bulb into the row with the pointy end up. The root end (flatter, sometimes with tiny dried roots) goes down.
- Spacing: Space sets 4-6 inches apart. This gives each bulb room to grow without competition.
- Cover Gently: Cover the sets with soil so that just the very tip of the pointy end is peeking out. Firm the soil lightly around them.
If planting in a raised bed or block, you can space them 4-6 inches apart in all directions.
Essential Care And Maintenance
Consistent care after planting ensures strong growth and prevents problems.
Watering Your Onion Sets
Onions have shallow roots and need consistent moisture, especially during bulb formation. Water deeply when the top inch of soil feels dry, providing about 1 inch of water per week. Reduce watering as the bulbs mature and the tops begin to yellow and fall over.
Weeding and Mulching
Weeds compete fiercely with onions for nutrients. Hand-pull weeds carefully to avoid disturbing the shallow roots. Applying a 1-2 inch layer of straw or shredded leaves mulch helps suppress weeds and retain soil moisture.
Fertilizing for Growth
Feed your onions every 2-3 weeks with a nitrogen-rich fertilizer (like blood meal or a 21-0-0 formula) until the bulbs begin to swell. After that, stop high-nitrogen feeding, as it can hinder bulb development.
Managing Pests and Diseases
Onions have few pest problems, but you should be aware of them.
- Onion Maggots: These are the most common pest. Use floating row covers to prevent the fly from laying eggs at the base of plants.
- Thrips: Tiny insects that cause silvery streaks on leaves. A strong spray of water or insecticidal soap can control them.
- Diseases: Good air circulation and crop rotation (don’t plant onions in the same spot more than once every 3 years) prevent most fungal issues like downy mildew.
Harvesting And Curing Your Onions
Knowing when and how to harvest is critical for storage.
Signs of Maturity
Onions are ready when their green tops turn yellow, brown, and flop over. This usually happens in mid to late summer. Once about half the tops have fallen, gently push over the remaining ones to signal the bulbs to stop growing.
How to Harvest
On a dry day, carefully loosen the soil with a fork and lift the bulbs. Brush off excess soil, but do not wash them. Handle them gently to avoid bruising.
The Crucial Curing Process
Curing toughens the outer skins for long-term storage. Lay bulbs in a single layer in a warm, dry, well-ventilated place out of direct sun—like a garage, shed, or covered porch. Let them cure for 2-3 weeks until the necks are tight and dry and the outer skins are papery.
Storing Your Homegrown Onions
After curing, trim the roots and cut the dried tops down to about 1 inch. Store onions in mesh bags, old pantyhose, or a ventilated crate in a cool, dark, dry place with good air circulation. Properly cured and stored onions can last for several months.
Troubleshooting Common Problems
Even with good care, issues can arise. Here’s how to address them.
Bolting (Flowering)
If an onion sends up a flower stalk, it has bolted. This is often caused by temperature fluctuations or planting sets that are too large. The bulb won’t grow larger and won’t store well. Harvest and use bolted onions first.
Small Bulbs
Small bulbs result from overcrowding, insufficient sunlight, poor soil nutrition, or inconsistent watering. Ensure you follow spacing, soil prep, and watering guidelines.
Soft or Rotting Bulbs
Rot is usually a sign of overly wet soil or a fungal disease. Improve drainage for future plantings and practice crop rotation. There’s no cure once rot sets in; remove affected bulbs.
Frequently Asked Questions
Can You Grow Onion Sets in Containers?
Yes, you can. Use a container at least 10 inches deep with drainage holes. Fill with quality potting mix and plant sets as you would in the ground. Ensure the container gets full sun and water more frequently, as pots dry out faster.
How Long Do Onion Sets Take to Grow?
From planting to harvest, onion sets typically take about 14-20 weeks, depending on the variety and your growing conditions. They mature faster than onions grown from seed.
Why Are My Onion Sets Not Growing?
If growth seems stalled, check for these issues: compacted soil, lack of nutrients, insufficient water, or planting the sets too deep. Correcting these conditions should get them back on track.
What Is the Difference Between Onion Sets and Onion Seeds?
Onion sets are small, partially grown bulbs from the previous year, offering a head start. Seeds take longer and require more precise conditions to start. Sets are generally easier and more reliable for home gardeners.
Can You Eat the Green Tops of Onion Sets?
Absolutely. The green tops are essentially green onions or scallions. You can snip a few from each plant throughout the season for cooking, but avoid taking to much from one plant if you want a large bulb to develop.