Growing your own onions is a rewarding project that connects you to your food from start to finish. Understanding the complete onion growing stages, from seed to harvest, is the key to a successful crop. This guide walks you through each phase with clear, practical advice.
Onion Growing Stages – From Seed to Harvest
Onions follow a distinct life cycle. They begin as tiny seeds, develop into seedlings, form bulbs, and finally mature for storage. Each stage needs specific care. Let’s break down what you need to do at every step to grow big, healthy onions.
Stage 1: Planning and Seed Starting
Success starts long before you put a seed in soil. Onions are sensitive to day length, meaning they bulb in response to how many hours of daylight they receive. You must choose the right type for your location.
- Long-Day Onions: Best for northern regions (above 35–40° latitude). They need 14–16 hours of daylight to bulb.
- Short-Day Onions: Best for southern regions (below 35° latitude). They bulb with 10–12 hours of daylight.
- Day-Neutral/Intermediate Onions: A flexible choice for many areas, bulbing with 12–14 hours of light.
You can start from seeds, sets (small dormant bulbs), or transplants. Seeds offer the most variety but take the longest. For a head start, begin seeds indoors 8–10 weeks before your last spring frost date.
How to Start Onion Seeds Indoors
- Fill a seed tray with a fine, sterile seed-starting mix.
- Sow seeds about 1/4 inch deep, spacing them roughly 1/2 inch apart.
- Keep the soil consistently moist and warm (65–75°F). A heat mat helps.
- Once sprouted, provide plenty of light. A grow light placed close by prevents leggy seedlings.
- Trim the tops back to about 3 inches tall every few weeks to encourage strong root and stem growth.
Stage 2: Transplanting and Early Growth
When seedlings are about the thickness of a pencil and outdoor soil is workable, it’s time to harden them off. This process acclimates tender plants to outdoor conditions over 7–10 days. Start with an hour in a shaded, sheltered spot and gradually increase their time outside.
Onions thrive in loose, well-draining soil with plenty of organic matter. A sunny spot is non-negotiable. Work some compost into the bed before planting. Space plants 4–6 inches apart in rows 12–18 inches apart. Plant them so the base is about 1 inch deep.
Water them in well after transplanting. The next few weeks are all about establishing a robust root system and green top growth. Keep the area weed-free, as onions hate competition for nutrients.
Stage 3: Vegetative Growth and Bulb Initiation
This is where the magic of bulbing begins. As daylight hours reach the required length for your onion type, the plant stops making new leaves and starts sending energy down to form the bulb. Each leaf a plant produces corresponds to one ring in the final bulb. More healthy top growth means a potentially larger bulb.
Your care during this phase is crucial. Onions have shallow roots, so they need consistent moisture. Aim for about 1 inch of water per week, from rainfall or watering. Drip irrigation or soaker hoses are ideal to keep foliage dry and prevent disease.
Feed your onions with a nitrogen-rich fertilizer early in this stage to support leaf development. However, once you see the very top of the bulb start to push up and the soil around it crack, stop applying nitrogen. A side dressing of a balanced or phosphorus-heavy fertilizer at this point can support bulbing.
Stage 4: Bulb Development and Maturation
The bulb swells noticeably during this stage. You’ll see the soil crack and the onion’s shoulders emerge. As the bulb matures, the focus shifts from growing to curing. The plant begins to prepare itself for dormancy.
Reduce watering as the bulbs near maturity. This helps the outer layers start to dry and prevents rot. When about half the tops have naturally fallen over and turned yellow-brown, it’s a sign to stop watering altogether. This usually happens in mid to late summer.
Do not bend the tops over yourself. Let it happen naturally. Forcing them over can damage the neck and invite rot during storage. The plant is slowly sealing off the neck, which is vital for long-term keeping.
Stage 5: Harvesting and Curing
Harvest time! Once most of the tops have fallen over, it’s time to pull your onions. Choose a dry, sunny day if possible. Gently lift the bulbs from the soil with a fork, taking care not to bruise or puncture them.
Curing is not optional for storage onions; it’s essential. The process dries the outer layers and neck completely.
- Lay the onions in a single layer in a warm, dry, well-ventilated place out of direct sun. A covered porch, garage, or shed is perfect.
- Let them cure for about two to three weeks. The necks should be tight and papery, and the outer skins rustle when touched.
- Once cured, trim the roots and cut the tops down to about 1 inch, unless you plan to braid them.
Stage 6: Storage and Saving for Next Year
Properly cured onions can last for months. Store them in a cool, dark, and dry place with good air circulation. Mesh bags, old pantyhose, or slatted crates work great. Check them periodically and use any that feel soft first.
If you want to save seeds, remember onions are biennial. They produce a bulb in the first year and a flower stalk in the second. You would need to overwinter a few best-quality bulbs and replant them in spring to get the flower head (umbel) that produces seeds. It’s a fun project for dedicated gardeners.
Common Problems and Solutions
- Bolting: When an onion sends up a flower stalk prematurely. It’s often caused by temperature fluctuations. Once it happens, the bulb stops growing. Use the onion soon, as it won’t store well. Choose bolt-resistant varieties and plant at the correct time for your zone.
- Thick Necks: Onions with thick, green necks at harvest often don’t cure well and rot in storage. These are usually caused by to much nitrogen late in the season or irregular watering. Use them first.
- Pests: Onion thrips and maggots are the main culprits. Use floating row covers to prevent maggot flies from laying eggs. For thrips, a strong spray of water or insecticidal soap can help control them.
- Diseases: Fungal issues like downy mildew or botrytis neck rot are often due to wet foliage or poor air circulation. Water at the base of plants, rotate your crops yearly, and ensure good spacing.
FAQ: Onion Growing Questions Answered
How long do onions take to grow?
From seed to harvest, onions typically take 90 to 120 days, depending on the variety and growing conditions. Transplants or sets will shave several weeks off that time.
Can you grow onions in pots?
Yes, you can! Choose a container at least 10 inches deep with good drainage. Use a quality potting mix and maintain consistent watering, as pots dry out faster. Select smaller or intermediate-day varieties for best results.
Why are my onion bulbs so small?
Small bulbs can result from insufficient daylight for the type planted, overcrowding, inadequate nitrogen during early growth, or competition from weeds. Ensure you’ve chosen the correct day-length type and given them space and food.
When should you stop watering onions?
Stop watering when about half of the tops have naturally fallen over. This allows the bulbs to begin the curing process in the ground and develop those protective dry layers.
How do you know when onions are ready to pick?
They are ready when the majority of the green tops have yellowed, weakened, and fallen over at the neck. The bulbs will often have a papery look at the top where they emerge from the soil.
Following these onion growing stages gives you a clear roadmap. With good planning, attentive care during bulbing, and proper curing, you’ll be rewarded with a hearty harvest of homegrown onions that will flavor your meals for months to come.