If you want a reliable and satisfying harvest, learning how to grow onion sets is a fantastic place to start. This simple step-by-step guide will walk you through the entire process, from choosing your sets to pulling plump bulbs from the ground.
Onion sets are small, immature onion bulbs from the previous season. They are the easiest and quickest way to grow onions, perfect for beginners. You skip the tricky early seedling stage and get a head start on the growing season. With just a little know-how, you can enjoy a bountiful crop of homegrown onions.
How To Grow Onion Sets – Simple Step-By-Step Guide
This main guide covers everything in order. Follow these steps for the best chance of success.
Step 1: Picking the Right Onion Sets
Your first task is choosing good sets. Visit a local garden center or reputable online supplier in late winter or early spring.
- Choose Firm Sets: Pick sets that are firm, dry, and have no signs of mold or soft spots. Avoid any that have started sprouting green shoots in the bag.
- Select by Day Length: This is crucial. Onions are classified by the ammount of daylight they need to form bulbs.
- Long-Day Onions: Need 14-16 hours of daylight. Best for northern regions (north of 36th parallel).
- Short-Day Onions: Need 10-12 hours of daylight. Best for southern regions.
- Day-Neutral (Intermediate): Form bulbs with 12-14 hours of light. Good for many areas.
- Consider Variety: Common set varieties include ‘Stuttgarter’ (yellow, long-day), ‘Red Baron’ (red, adaptable), and ‘White Lisbon’ (white, often for green onions).
Step 2: Timing Your Planting Perfectly
Getting the timing right makes a big difference. Onions are cool-season crops and can tolerate a light frost.
- When to Plant: Plant onion sets as soon as the soil can be worked in the spring. This is typically 4-6 weeks before your last expected frost date. The soil temperature should be at least 50°F (10°C).
- Soil Preparation: Onions need loose, well-draining soil. Heavy, compacted clay will stunt their growth. Work the soil to a depth of about 8 inches, removing rocks and weeds.
- Fertilize First: Mix in a balanced, slow-release fertilizer or well-rotted compost into the soil bed before planting. This gives them nutrients right from the start.
Step 3: Planting Your Onion Sets Correctly
Proper planting depth and spacing is key for bulb development. Here’s how to do it.
- Prepare the Bed: Rake the soil smooth, creating a level planting surface.
- Mark Your Rows: Use a string or hoe to create straight rows spaced about 12-18 inches apart. This allows for weeding and growth.
- Plant the Sets: Push each set into the soft soil so the tip is just barely covered. The nose (pointy end) should be facing up. Plant them just deep enough so they are secure, about 1-2 inches deep.
- Spacing is Key: Space the sets 4-6 inches apart within the row. For larger varieties, use the wider spacing. If you plan to harvest some as green onions, you can plant those closer together and thin them out early.
- Water Gently: After planting, give the bed a gentle but thorough watering to settle the soil around the sets.
Step 4: Essential Care While They Grow
Consistent care through the season leads to big, healthy bulbs. Don’t neglect these tasks.
Watering and Weeding
Onions have shallow root systems and need consistent moisture, especially during bulb formation.
- Watering: Provide about 1 inch of water per week. Water deeply at the soil level, avoiding wetting the foliage to prevent disease. Reduce watering as the bulbs mature and the tops begin to yellow and fall over.
- Weeding: Onions hate competition. Weed regularly but carefully by hand to avoid damaging their shallow roots. A light layer of straw mulch can supress weeds and retain soil moisture.
Feeding and Thinning
Proper nutrients ensure good growth.
- Fertilizing: About a month after planting, side-dress your onions with a nitrogen-rich fertilizer. Simply sprinkle it along the row and water it in. Avoid high-nitrogen feeds later in the season, as this can hinder bulb formation.
- Thinning: If you planted sets close together for a green onion harvest, thin them by pulling every other plant when they are pencil-thick. This gives the remaining bulbs room to expand.
Step 5: Knowing When and How to Harvest
Harvest time is exciting! Patience here ensures your onions store well.
- Watch for Signals: In mid to late summer, the onion tops will naturally start to turn yellow, bend over, and dry out. This is the plant’s sign that it’s finished growing.
- Stop Watering: Once most tops have fallen over, stop watering completely. This allows the outer layers of the bulb to dry and cure, which is vital for storage.
- Lift the Bulbs: On a dry, sunny day, gently lift the bulbs with a garden fork. Be careful not to bruise or pierce them, as this invites rot.
Step 6: Curing and Storing Your Harvest
Proper curing is the secret to long-lasting onions. Don’t skip this step.
- Curing Process: Lay the lifted onions in a single layer in a warm, dry, well-ventilated place out of direct sun. A garage, covered porch, or shed is perfect. Let them cure for about 2-3 weeks until the necks are completely tight and dry and the outer skins are papery.
- Preparing for Storage: After curing, trim the dried roots and cut the tops down to about 1 inch above the bulb. Brush off any loose dirt, but do not wash them.
- Ideal Storage: Store your cured onions in a cool, dark, dry place with good air circulation. Mesh bags, old pantyhose, or slatted crates are ideal. Check them periodically and remove any that show signs of softness.
Troubleshooting Common Problems
Even with good care, you might encounter a few issues. Here’s how to handle them.
Pests
- Onion Maggots: These are the most common pest. The small white larvae eat the roots and bulbs. Use floating row covers at planting to prevent the adult fly from laying eggs. Rotating your onion bed each year also helps alot.
- Thrips: Tiny insects that cause silvery streaks on leaves. A strong spray of water or insecticidal soap can control them.
Diseases
- Downy Mildew: Appears as fuzzy, purplish-gray patches on leaves. Improve air circulation, avoid overhead watering, and remove affected plants.
- White Rot: A serious fungal disease causing yellowing leaves and white, fluffy mold on the bulb base. There is no cure; remove and destroy affected plants and do not plant onions in that soil for 8+ years.
- Neck Rot: Often occurs in storage if onions were not cured properly. The neck becomes soft and mushy. Always ensure thorough curing and discard any onions with soft necks at storage time.
FAQ: Your Onion Set Questions Answered
Can you plant onion sets in the fall?
Yes, in milder climates (USDA zones 6 and warmer), you can plant certain varieties in the fall for a very early summer harvest. Choose short-day or intermediate varieties and plant them about 4-6 weeks before the first hard frost.
Why are my onions bolting (flowering)?
Bolting happens when the plant sends up a flower stalk, which ruins the bulb. It’s often caused by temperature fluctuations, like a cold snap after warm weather. Using sets that are too large can also trigger it. If a plant bolts, harvest and use that onion right away, as it won’t store well.
How do I grow large onions from sets?
For larger bulbs, ensure perfect spacing (at least 6 inches), consistent watering during bulb formation, and adequate fertilizer. Choosing the right day-length variety for your region is the most important factor for size.
Can I grow onions from sets in containers?
Absolutely. Use a container at least 10 inches deep with good drainage holes. Fill with a quality potting mix and space sets 3-4 inches apart. Container onions will need more frequent watering than those in the ground.
What’s the difference between onion sets and onion seeds?
Onion sets are tiny bulbs grown the previous year, offering a head start and easier planting. Seeds take longer and require more care to start but offer a much wider variety choice. Sets are generally recommended for beginners for their simplicity and reliability.
Growing your own onions from sets is a truly rewarding garden project. The process is straightforward and the results are incredibly useful in the kitchen. With this simple step-by-step guide, you have all the information you need to get started. Just remember to choose the right type for your area, plant at the correct time, and give them the basic care they need. Before you know it, you’ll be harvesting and storing your very own homegrown onions, ready to add flavor to your meals for months to come.