Growing onions is a rewarding project that takes you from a tiny seed to a flavorful kitchen staple. Understanding the onion growing stages is the key to a successful harvest. This guide will walk you through every phase, giving you the confidence to cultivate perfect onions in your own garden.
Whether you start from seeds, sets, or transplants, the journey follows a clear pattern. Each stage has its own needs for water, light, and nutrients. Getting these right means the difference between small, disappointing bulbs and large, firm ones you’ll be proud to store.
Onion Growing Stages
The complete cycle of an onion, from planting to harvest, can be broken down into several distinct phases. These stages are crucial to recognize because your care routine must change as the plant develops. Paying close attention will help you provide exactly what your onions need, exactly when they need it.
Stage 1: Germination
This is where life begins. Onion seeds are small and can be tricky to start. They require consistent moisture and warmth to wake up. If conditions aren’t right, germination can be slow or fail altogether.
For best results, plant seeds indoors 8-10 weeks before your last expected spring frost. Use a sterile seed-starting mix and plant the seeds about 1/4 inch deep. Keep the soil temperature consistently between 65°F and 75°F. A seedling heat mat can be very helpful for this.
You should see the first green loops (the cotyledons) emerge from the soil in 7 to 14 days. Keep the soil moist but never soggy during this tender time. A spray bottle is gentler than a heavy watering can for young seedlings.
Stage 2: The Seedling Stage
Once the first true leaves appear, your onions have entered the seedling stage. These leaves will look like tiny green blades. The plants are still very delicate and focused on building their root system and leaf structure.
Light is critical now. If growing indoors, provide 12-16 hours of bright light daily using grow lights placed just a few inches above the plants. Without enough light, seedlings become leggy, weak, and prone to falling over.
Water carefully at the base of the plants. Feed them with a diluted, balanced liquid fertilizer every two weeks to support their growth. This stage is all about establishing strong, healthy plants that are ready for the next step.
Thinning Your Seedlings
If you sowed seeds directly outdoors or crowded them in a tray, you’ll need to thin them. This gives each plant enough space to grow. Crowded onions will compete for resources and produce very small bulbs.
For direct-sown rows, thin seedlings to about 1 inch apart initially. Later, you’ll thin again to the final spacing. For indoor starts, gently separate seedlings if they are growing in clusters. Its a necessary step for good bulb development.
Stage 3: Vegetative Growth
This is a major growth period. The onion plant is now producing multiple green, tubular leaves. Each leaf is directly connected to a ring in the developing bulb underground—more leaves mean the potential for a larger bulb.
Your main tasks during vegetative growth are watering, feeding, and weeding. Onions have shallow roots, so they need consistent moisture. Aim for about 1 inch of water per week, including rainfall. Let the soil dry slightly between waterings, but don’t let the plants become drought-stressed.
Weeds are a big problem because they compete with those shallow roots. Mulch lightly with straw or shredded leaves to suppress weeds and retain soil moisture. Apply a nitrogen-rich fertilizer, like blood meal or a balanced vegetable feed, every few weeks to fuel all that green growth.
Stage 4: Bulb Initiation
This is the turning point. The plant stops just making leaves and starts forming a bulb. The trigger is daylight. Onions are classified by how much daylight they need to form bulbs:
– Short-Day Onions: Need 10-12 hours of daylight. Best for southern latitudes.
– Long-Day Onions: Need 14-16 hours of daylight. Best for northern latitudes.
– Day-Neutral Onions: Form bulbs with 12-14 hours of daylight and are adaptable.
When the daylight hours reach the required length for your variety, the plant gets the signal. It begins to divert energy from leaf production down into the base of the plant, which starts to swell. You’ll notice the tops stop producing new leaves and may begin to slightly weaken.
Care Changes at Bulb Initiation
Your care routine must shift now. Stop applying high-nitrogen fertilizer. Too much nitrogen at this stage will encourage more leaves instead of bigger bulbs. Switch to a fertilizer higher in phosphorus and potassium to support root and bulb development.
Continue watering consistently. The bulb is expanding rapidly and needs ample moisture to grow without cracking or becoming misshapen. This is not the time to let them dry out.
Stage 5: Bulb Development and Maturation
This is the final push. The bulb swells to its full size over several weeks. The outer layers of the bulb begin to develop their papery skin. As the bulb matures, the plant prepares for dormancy.
You’ll see the clearest sign of maturation in the tops. The green leaves will start to turn yellow, then brown, and finally flop over at the neck (where the leaves meet the bulb). This is called “lodging.” It’s the plant’s natural process of shutting down and preparing the bulb for storage.
Pre-Harvest Care: Curing On the Plant
Once about half to three-quarters of the tops have fallen over, stop watering. This allows the onion to begin curing while still in the ground. Let them sit in the dry soil for about a week to ten days. This crucial step helps seal the neck and develop the outer papery skins.
If rain is forcasted during this period, gently lift the bulbs and move them to a covered, dry area. Moisture now can lead to rot and ruin your storage onions.
Stage 6: Harvesting
Harvest time has arrived. Gently loosen the soil with a garden fork and lift the bulbs. Shake off any excess soil, but be careful not to bruise them. Damaged onions won’t store well and should be used first.
Leave the onions to dry in a warm, well-ventilated, and shaded place for 2-4 weeks. This is the final curing process. Spread them out on a screen or hang them in bunches. Good air circulation is essential. They are fully cured when the necks are completely tight and dry and the outer skins are papery and rustle to the touch.
Stage 7: Curing and Storage
After curing, trim the roots and cut the dried tops down to about 1 inch. Sort your onions. Set aside any with thick necks, soft spots, or bruises for immediate use. Only perfect, firm bulbs should go into long-term storage.
Store your cured onions in a cool, dry, dark place with good air flow. Mesh bags, old pantyhose, or slatted crates are perfect. Ideal storage conditions are around 35-40°F with 65-70% humidity. Check them periodically and remove any that show signs of spoiling.
Choosing Your Starting Point: Seeds, Sets, or Transplants?
You can enter the onion growing stages at different points. Each method has pros and cons.
– Seeds: Offer the widest variety choice and are most economical. They require the most time and care, starting indoors 8-10 weeks before transplanting.
– Sets: Are small, dormant bulbs from the previous year. They are the easiest and fastest to plant, but can be more prone to bolting (flowering prematurely). Varieties are limited.
– Transplants: Are young seedlings started by a grower. They offer more variety than sets and give you a head start over seeds. You’ll find them at garden centers in spring.
For beginners, sets are often the easiest route. For serious growers wanting specific storage or flavor types, seeds are best.
Common Problems and Solutions
Even with perfect care, issues can arise. Here’s how to handle common problems.
– Bolting: When an onion sends up a flower stalk. The bulb stops growing and won’t store well. Caused by temperature fluctuations. Choose bolt-resistant varieties and plant sets at the right time for your climate. If a plant bolts, harvest and use it right away.
– Onion Maggots: Small white larvae that feed on roots and bulbs. Use floating row covers to prevent the adult fly from laying eggs at the base of plants. Practice crop rotation each year.
– Downy Mildew: Shows as pale spots on leaves that turn into a fuzzy gray mold. Improve air circulation, avoid overhead watering, and remove infected plants.
– Thrips: Tiny insects that cause silvery streaks on leaves. They can reduce yeild significantly. Use insecticidal soap or neem oil, and encourage beneficial insects like ladybugs.
Step-by-Step Planting Calendar
Follow this general calendar for a successful season. Adjust dates based on your specific growing zone.
1. 8-10 Weeks Before Last Frost: Start seeds indoors under lights.
2. 2-4 Weeks Before Last Frost: Harden off seedlings outdoors for a week. Prepare garden bed with compost.
3. On Last Frost Date: Plant sets and transplants outdoors. Sow seeds directly if desired.
4. Spring to Early Summer: Water, feed with nitrogen, weed consistently.
5. Early to Mid-Summer (Daylength Trigger): Bulb initiation begins. Switch to low-nitrogen fertilizer.
6. Late Summer: Tops begin to yellow and fall over. Stop watering.
7. 1-2 Weeks After Tops Fall: Harvest bulbs.
8. 2-4 Weeks After Harvest: Complete curing process, then trim and store.
FAQ Section
How long do onions take to grow?
From planting to harvest, onions typically take 90 to 120 days, depending on the variety and whether you started from sets, seeds, or transplants. Full maturation for storage onions takes the longest.
What are the growth stages of an onion plant?
The main stages are germination, seedling growth, vegetative leaf growth, bulb initiation, bulb development and maturation, and finally, curing and storage. Each stage requires slightly different care.
Can you leave onions in the ground over winter?
Most standard onions are not winter-hardy. However, special varieties called “overwintering” onions can be planted in fall for a very early summer harvest. They have a different growth cycle than spring-planted onions.
Why are my onions so small?
Small bulbs can result from several factors: planting the wrong day-length type for your region, overcrowding, insufficient nutrients during vegetative growth, inconsistent watering, or harvesting too early. Its important to match the variety to your location.
How do you know when onions are ready to pick?
Onions are ready when the bulbs have pushed up slightly out of the soil and the green tops have turned yellow or brown and have fallen over. Once half to three-quarters of the tops are down, its time to stop watering and prepare for harvest.
Growing onions successfully is a matter of understanding these stages and providing simple, consistent care. With good planning and attention to timing, you can enjoy a bountiful harvest of homegrown onions that will flavor your meals for months to come. The process is straightforward once you know what to expect at each phase of the plants life.