Growing Onions from Seed: A Complete Beginner’s Guide
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Growing onions from seed takes a little more planning than planting sets, but it offers big advantages. Starting with seed gives you hundreds of varieties, larger bulbs, and less disease issues.
Onions (Allium cepa) have a long growing season, so starting onions early is the key to success. With the right timing, a good seed starting mix, and adequate light, you can raise strong onion seedlings ready to move into the garden.
For years I settled for whatever onion sets were at the hardware store, but I’ve become an onion seed believer. Now I grow several hundred of these beauties every year, many of which go into homemade salsa and spaghetti sauce.
Table of Contents
Long Day, Short Day and Day Neutral Onions
Before planting onion seeds, it’s important to choose a variety suited to your location. Onions form bulbs based on day length.
There are three main types:
Long Day Onions
- Best for northern regions like Wisconsin
- Begin forming bulbs when days reach about 14–16 hours of daylight
- My current favorites: Ailsa Craig Exhibition, Yellow Sweet Spanish, Rosa di Milano
Day Neutral or Intermediate Onions
- Good for mid-latitude regions
- Begin bulbing at 12–14 hours of daylight
- Often planted in spring in the north and fall in the south.
Short Day Onions
- Best for southern climates
- Start bulbing at 10–12 hours of daylight
When to Start Onion Seeds
Because onions need a long growing season, they are usually started indoors.
For most northern gardeners:
- Start planting onion seeds 8–10 weeks before the last frost
- Transplant seedlings outdoors 4–6 weeks before the last frost
Onions tolerate cool temperatures well, so they can go outside earlier than many garden crops. Plants need around 100+ days to mature, but you can harvest bulbs before they reach full growth.
How to Start Onions from Seed Indoors
Starting onions from seed indoors is simple if you provide good light and consistent moisture.
Materials
- Seed trays or shallow containers
- Quality seed starting mix
- Onion seeds
- Bright window or supplemental light
Step-by-Step Instructions
- Fill containers with seed starting mix
Use a light, well-draining mix. Onions have shallow roots, so heavy soil can slow early growth. - Plant seeds shallowly
Plant seeds about ¼ inch deep (roughly a seeds inch spacing if sowing in rows). Mark trays with variety and planting date. - Water gently
Keep soil evenly moist but not soggy. Cover seedling tray with plastic cover or plastic bag to conserve moisture. Place in a warm location (70-75°F (21-24°C)) to stimulate germination. I like to use a seed starting heat mat under my trays. - Provide strong light
Onions may take a while to sprout, so be patient. Once they sprout, remove the cover. place seedlings under grow lights or another supplemental light source for 12–16 hours daily. - Fertilize
To give your onions extra TLC, water with a natural liquid fertilizer every two weeks.
Trimming Onion Seedlings
One trick that helps grow onions successfully is trimming the tops of seedlings. When the seedlings reach about 4–5 inches tall, trim them back to about 3 inches.
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Benefits of trimming:
- Prevents floppy seedlings
- Encourages thicker stems
- Makes seedlings easier to transplant
Use trimmed greens like mild chives in the kitchen, or compost.
Transplanting Onion Seedlings Outdoors
When the soil can be worked and temperatures stay above freezing, it’s time to begin planting onions outdoors.
Hardening Off
Before transplanting:
- Move seedlings outside for a few hours each day
- Gradually increase exposure for about a week
This process toughens the plants and reduces transplant shock.
How to Plant Onion Seedlings
- Prepare loose, fertile soil. I like to add some aged manure, compost, or worm castings.
- Separate onion seedlings carefully.
- Plant them about 1 inch deep. Snug the seedlings into the soil in your desired planting pattern. Water gently to remove air pockets around the roots.
- Space plants 4–6 inches apart for full-size bulbs.
Because onions have shallow roots, consistent moisture is important during early growth. I prefer to plant my onions in triple rows in a grid pattern, with roughly a handspan between plants. Sometimes I plant in rows with companion plants in between.
5 Tips for a Great Onion Harvest
- Make sure your onions get plenty of water. Onions tend to be shallow rooted, so they need regular rain or hand watering. Stop watering about 2 weeks before harvest to let them start drying up for storage.
- Keep weeds in check for bigger bulbs. Because of their shallow roots, onions are not great weed competition. Use mulch (which also conserves moisture) or shallow cultivation to reduce weed pressure.
- Use companion plants such as deep rooted carrots, turnips or beets to help bring up water and nutrients. Or pair onions with leafy greens to spread between the onion rows to keep down weeds. Onions also make good companion plants to the nightshade family (tomatoes and peppers) and the cabbage family.
- Don’t use old seed! Onion seeds do not tend to maintain germination rates well in storage. I try to use fresh seed each year, or one year old seed at the oldest. By the second year, germination rates drop through the floor.
- Don’t overdo the nitrogen! Too much nitrogen will grow big green tops and small bulbs. Side dress with some compost in spring, or water with balanced liquid fertilizer once a month, but don’t go crazy.
When you’re ready, see:
When to Harvest Onions (Plus Harvesting Tips)
How to Store Onions So They Last
Why Do Onions Make You Cry? (and How to Help)
Why I Choose Growing Onions from Seed
For us, we opt for growing onions from seed mainly due to the varieties available. Starting with seed is less expensive, too. Our seed grown onions are more vigorous and disease resistant.
Our current favorites produce large bulbs, and 2 out of 3 keep great in storage. (Ailsa Craig Exhibition is best fresh, but they last for a while.) We are able to root cellar onions from one harvest to the next.
Growing onions from seed takes a little more time, but for us, it’s definitely worth it.
This article is written by Laurie Neverman. Laurie and her family have 35 acres in northeast Wisconsin. They grow dozens of varieties of fruiting trees, shrubs, brambles, and vines, along with an extensive annual garden. Along with her passion for growing nutrient dense food, she also enjoys ancient history, adorable ducks, and lifelong learning.
Originally published in 2016, last updated in 2026.
Hi Laurie, I want to try planting onions from seed this year! Just wondering what company you buy your seeds from.
Hi Shelley.
It varies. This year I purchased my Ailsa Craig and Sweet Spanish from Pinetree Garden Seeds, and the Rosa di Milano from Siskayou Seeds. I’ve purchased the first two from a few different seed sources over the years, but so far only found the Rosa di Milano at Siskayou. There are more yellow and white varieties that store well, but Rosa di Milano has thus far outlasted any other red I’ve tried, plus I like the shape and size. I don’t care for the concave tops on many red onion varieties.
What depth should the seedlings be planted? Thank you for your wonderful tutorial. I may have missed this detail but all I saw was to snug the seedlings in. How deep ? I so want this endotoxins next successful.The spacing and watering etc. was so helpful. But how deep to plant the seedlings. They are past the chive like stage and have a bit of a bulb.
Thanks so much ,
Nancy
Hi Nancy.
Put them in the ground so the bulb is covered. Leave the green part poking out.
If I want to plant onions in the tropics, should I plant them directly?
Hi Alfred! My name is Amber and I live in South Carolina on the coast. Our zone is 8b and very humid. Although I have never planted onion from seed (I always buy bulbs) I can offer my experience as our climates may be similar. We plant onions in fall and in late winter-early Spring. They like full sun and nitrogen rich soil. Ours do best if planted in cooler weather and are ready to harvest as the weather warms up. We rarely get below 20° F and our Springs get hot fast. If nothing else, you can at least harvest the tops! Best of luck to you.
How/When do you collect the seeds from the flower bulb?
Collect the seeds when the flower heads are dry. As they approach dryness, you may want to snip them off into a paper bag or bin and let them dry completely. The flower heads will become quite brittle, and the tiny seeds like to go everywhere. Letting them finish drying in a paper bag (or open top bin with closed bottom and good air circulation) ensures that the seed will drop where you want to collect it.
It’s my first time of making onion seedlings and it’s going better than I was expecting. The seedlings reached about 2 inches now and I can’t wait for transplanting them outside in the garden. Thank you for sharing this great and helpful information. Happy gardening!
Yeah! Glad you tiny plants are doing well.