Growing onions from seed can be difficult, but it doesn’t have to be! Onion seeds take a long time to grow and must be started indoors during mid to late winter. It would be nice if you could just sprinkle them over your flat and be done with it, but they will turn into a tangled mess before long. So let’s learn the tips & tricks to grow onions from seed all the way to harvesting big, beautiful bulbs, plus how to cure and store them for use over the winter.
Growing onions from seed offers many benefits like more variety options, cheaper costs, and earlier harvests. However, it also comes with unique challenges not found when planting onion sets or transplants. Onion seeds can be tricky to start and grow successfully.
Below are some of the most common problems when growing onions from seed, along with tips to help troubleshoot these issues.
Problems with Onion Seed Germination
Getting onion seeds to reliably sprout can be one of the biggest obstacles. Here are a few reasons you may struggle with germination:
Starting Too Early
It’s tempting to get an early jump on the growing season by starting onions inside weeks ahead of your last expected frost. But onion seeds germinate best at soil temperatures between 55-85°F. They simply won’t sprout if it’s too cold. Wait to start seeds until about 4-6 weeks before your average last spring frost date.
Planting Too Deep
Onion seeds need light to germinate. Plant them too deep and they’ll struggle to emerge. Sow seeds just 1⁄4-1⁄2 inch deep in seed trays or pots. Cover lightly with seed starting mix and mist to water in.
Overly Wet Soil
While onion seedlings need consistent moisture, soaked soils can inhibit germination. Allow the starting mix to dry out slightly between waterings. Onion seeds may rot if planted in a constantly soggy medium.
Improper Storage
Onion seeds lose viability when stored in hot humid conditions. For best germination rates store seeds in a cool, dark place in an airtight container. Refrigeration can extend viability. Only buy seeds from reputable sources and check expiration dates.
Young Onion Seedling Problems
Once sprouted onion seedlings face hazards like damping off, nutritional deficiencies, and more.
Sowing Too Densely
When starting onions indoors it’s tempting to plant seeds close together to maximize space. But overcrowded containers cause seedlings to stretch and become leggy as they compete for light and nutrients. Sow seeds 1⁄2-1 inch apart and thin later.
Damping Off Disease
This fungal disease causes onion seedling stems to rot at soil level, killing the plant. It thrives in cool wet conditions. Allow containers to dry between waterings, provide plenty of airflow and avoid overcrowding to prevent damping off.
Nutrient Deficiencies
Onion seedlings need consistent nitrogen feeds to grow healthy greens and develop bulbs later on. Use a balanced liquid fertilizer every 2-3 weeks at half strength after seedlings emerge. Signs of deficiencies are yellowing, stunted plants.
Light Issues
Without sufficient sunlight onion seedlings get tall and weak. Make sure to grow seedlings in a bright south-facing window or under grow lights. Supplement natural light with 14-16 hours per day of artificial light.
Troubleshooting Young Onion Transplants
Transitioning onion seedlings from indoors to the garden comes with potential pitfalls:
Transplanting Too Early
Eager to get seedlings in the ground? Don’t rush it! Onions can’t tolerate any frost. Wait until soils have warmed and nighttime temps stay above 40°F before hardening off and transplanting.
Poor Hardening Off
If onion seedlings aren’t properly hardened off, transitioning from indoors to outdoors, they’ll suffer transplant shock. Harden off gradually over 7-10 days, bringing plants outside for increasing periods of time.
Overcrowding
Give those onion roots room to expand. Overcrowded bulbs will be smaller. Follow seed packet spacing guidelines, usually 4-6 inches apart in rows 6-8 inches apart. Wide spacing also allows for air circulation.
Burying the Stems
Don’t plant too deep. Onion bulbs form right at soil level. If stems and leaves are buried, they can rot. Plant onions with just the root end in the ground and stems exposed.
Forgetting to Water
Seedlings dry out quickly outdoors. Until established, water transplants daily or every other day if rain is lacking. Prioritize consistent moisture the first 2-3 weeks after transplanting.
Preventing Common Onion Growing Problems
Here are some tips to avoid issues through the growing season:
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Test soil and amend as needed – onions like fertile, well-draining soil with plenty of nutrients. Incorporate compost or aged manure before planting.
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Use drip irrigation – wet foliage is susceptible to fungal diseases. Drip irrigation targets water to the soil.
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Weed religiously – onions do not compete well with weeds for water and nutrients. Keep area around bulbs weed-free.
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Side dress with fertilizer – scatter high-nitrogen fertilizer around plants every 3-4 weeks to fuel bulb growth.
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Control pests – onion flies, maggots and thrips can damage developing bulbs. Use floating row covers.
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Allow drying before harvest – bulbs left wet in ground are prone to rotting. Allow 2-3 weeks drying time before gathering bulbs.
While growing onions from seed has challenges, paying close attention to seed starting, plant care, and growing conditions can help overcome common issues. With experience and troubleshooting, you can successfully grow onions from seed for an inexpensive, steady harvest.
How to Grow Onions
Onions are fairly shallow-rooted so they make a great choice if you garden in raised beds or soil that has been amended on the top few inches but sits on clay. Again, they don’t like soggy feet and can be prone to rot, so be sure the soil does drain adequately.
Keep the soil from drying out (straw or hay mulch works wonderfully for water retention) and use a foliar fertilizer, such as fish emulsion, every 2-3 weeks until they start to form bulbs.
Onions do not do well with competition from weeds so you will need to keep the weed pressure down. Another great reason to consider using an organic mulch.
When to Harvest Onions
The day you transplant seedlings in the garden is the day you start counting your days to harvest. So for example, I grew Bridger onions from seed last year. They are a 90-day onion and were started from seed on February 19th and transplanted on April 25th.
Onions are ready to harvest when the tips of their leaves begin to turn brown and fall over. The neck, right above the bulb, will be starting to shrivel up, but not fully dry. The Bridger onions were ready and I harvested them on July 27th.
Why Not Trim Onions Before Planting + Growing Onions from Seed Tips and Tricks
FAQ
Are onions hard to grow from seed?
While many folks choose to buy onion sets (dried tiny onions) or starts (started green plants), they are actually very easy to grow from seed. Growing onions from seed is much cheaper than buying started plants, so they are more economical if you are growing a lot of them.
Why are onions so hard to grow?
Onions roots are shallow and not very efficient at taking up moisture, so they need a steady supply of water to grow without interruption. Although they actually recover well from drought and start growing again when watered, it is best to keep the soil consistently moist until the bulbs enlarge.
How do you know if you are overwatering onions?
Yellow, swollen, or translucent leaves are screaming “overwatered!” at the top of their lungs. If they fall off with a gentle poke, you’ve got your answer.
How long does it take to grow an onion from a seed?
Onions require 90 to 100 days to mature from seed, which is around four months. We start seeding our vegetables shortly after the New Year and then in spring we transplant them into the garden.
Are You planting the wrong type of onions?
You may be surprised to learn that one of the most common mistakes when it comes to growing onions is planting the wrong type. This is especially true if you’re buying your seeds, sets, and transplants from an online source. How to grow a large onion? To grow large onions, keep the weeds out of their turf.
Can you grow onions from seed?
While growing onions from seed has challenges, paying close attention to seed starting, plant care, and growing conditions can help overcome common issues. With experience and troubleshooting, you can successfully grow onions from seed for an inexpensive, steady harvest.
How many seeds do you need to grow onions?
Drop 3-4 seeds in a module for growing bulbing onions and up to 8 seeds per module for growing bunching onions. Onion bulbs will push each other apart, and while you won’t be growing giant onions with this method, you’ll triple your harvest in the same amount of space.
Why do onions have a lot of problems when grown from seed?
Onions are grown from seed often have the most problems from seed incubation, planting in the ground, soil drainage, watering, light, temperature, etc. Onions grown from the set usually have fewer problems because of their vitality and ability to adapt to the outside environment.
How do onion seedlings grow?
If you’re growing onion seedlings for the first time, they’re like nothing you’ve ever grown before. They spring from the soil like blades of grass – when emerging, both ends of the seedling are stuck underneath soil level, and that’s perfectly normal. As the onion seedlings emerge, they straighten out into a single narrow leaf pointing up.
Why are my onion seedlings stuck in the soil?
After germination, my onion seedlings often struggle to pull away from the seed. The stuck seedling appears as a little green loop, with the smaller end still in the soil, weighted down by the black remains of the seed husk.