I spent three seasons growing sad little onion bulbs that never bulked up the way the seed packets promised. Honestly? I almost gave up. Then I figured out the real rules for how to grow onions and everything changed. Now I pull fat, sweet storage onions every summer and still have plenty left for green onions along the way.
This is the guide I wish someone had handed me on day one. No fluff. Just what actually works in a regular backyard plot.
Why Most People Fail at How to Grow Onions
See, the biggest mistake is treating every onion the same. They are not. Day length controls everything.
I live in a northern zone. The first year I planted short-day sets because they looked pretty at the garden center. The tops grew fine. The bulbs stayed the size of golf balls. I felt defeated.
Long-day onions need 14 to 16 hours of daylight to start bulbing. Short-day ones trigger at 10 to 12 hours. Day-neutral sit in the middle. Plant the wrong type for your latitude and no amount of water or fertilizer will save you.
Match the variety to your location first. Everything else comes second.
I now stick to long-day types like Yellow Sweet Spanish and Walla Walla. Southern gardeners I know swear by Texas Grano and Red Creole. Day-neutral Candy works almost anywhere if you want an easier middle ground.
Choosing Your Starting Method
You have three solid options. I have tried them all.
Onion Sets
These are the tiny dried bulbs you find in mesh bags every spring. Easiest method for beginners.
I plant sets when I want quick results and fewer steps. Push them into the soil pointed end up, about one inch deep. Space them four to six inches apart in rows twelve to eighteen inches apart. Done.
The downside is limited variety and a higher chance of bolting if the sets are too large. I pick the smallest ones in the bag now. Bigger sets often flower instead of forming good bulbs.
Seeds
Cheapest route and the widest choice of varieties. Also the slowest.
I start seeds indoors ten to twelve weeks before my last frost. Sow them a quarter inch deep in seed-starting mix. Keep the soil moist and under grow lights. When the green shoots get long and floppy I give them a haircut down to three inches. This keeps the plants stocky.
Transplant outdoors after hardening them off for a week. Space the same as sets. Expect 100 to 120 days to mature bulbs.
Transplants
My current favorite. These are young seedlings sold in bundles or grown yourself.
They give the best yields in my experience and bolt less than sets. Plant them two to three weeks before the last frost. Same spacing rules apply. Trim the tops to four inches and the roots to half an inch if they look leggy.
I order transplants from reliable catalogs when I want specific storage varieties. The extra cost pays for itself in bigger harvests.
Soil Prep That Actually Matters
Onions hate compacted ground. Their roots stay shallow and the bulbs need room to swell.
I dig the bed six to eight inches deep and work in two to three inches of finished compost. The soil should feel loose and drain well. Target pH sits between 6.0 and 7.0. I test every couple of years and adjust with lime or sulfur as needed.
Heavy feeders need nitrogen early. I mix a balanced organic fertilizer into the top layer before planting. Later I side-dress with more nitrogen once the plants establish. Too much late nitrogen creates thick necks that refuse to cure properly. I learned that the hard way.
Full sun is non-negotiable. At least six hours, preferably eight or more. Shade equals small bulbs.
Planting Day Step by Step
I wait until the soil can be worked in early spring. Light frost is fine. Hard freezes are not.
- Clear weeds completely.
- Rake the bed smooth.
- For sets: press each one so the tip sits just at soil level or slightly below.
- For transplants or seedlings: plant so the roots and base sit one to one and a half inches deep.
- Firm the soil gently around each plant.
- Water thoroughly right away.
I mulch lightly with straw once the soil warms. This keeps moisture steady and blocks weeds. Never hill soil over the developing bulbs. They need to sit partly above ground as they swell.
Water and Feed Through the Season
Consistent moisture is the difference between marble-sized onions and proper ones. I aim for one inch of water per week. Deep watering once or twice a week beats daily sprinkles.
When the weather turns dry during bulb formation I watch the plants closely. Dry spells cause splitting or small size. Soggy soil invites rot. Balance is everything.
Fertilizer routine for me:
- At planting: balanced fertilizer worked into the bed.
- Three to four weeks later: nitrogen side-dress six inches away from the row.
- Stop nitrogen once the bulbs start to push the soil and the tops begin to fall.
Weeds compete hard for the same shallow root space. I hand-pull them early and often. A light mulch helps but never covers the bulbs.
Watching for Trouble
Bolting (flower stalks) ruins bulb size. Cool weather followed by sudden heat can trigger it. I remove any flower stalks the moment I see them. Energy that goes into flowers never returns to the bulb.
Onion maggots and thrips show up some years. Row cover early in the season keeps the worst pests off. I rotate the onion bed every three years to cut disease pressure.
Yellowing leaves late in the season are normal. Early yellowing usually means water stress or nutrient issues.
Harvest Timing and Curing
The plants tell you when they are ready. Tops yellow and fall over on their own. When about two-thirds of the foliage has flopped I stop watering and wait a few more days.
I pull the onions on a dry morning and leave them right in the garden for a couple of days if rain is not in the forecast. Then I move them to a shaded, airy spot with good airflow. A covered porch or garage works. Leave the tops on during curing.
After one to two weeks the necks dry and the outer skins turn papery. I trim the roots and cut the tops to one inch. Store the cured bulbs in a cool, dry place with good air circulation. Mesh bags or crates work better than plastic.
Green onions can come out anytime once the tops reach six inches. I thin aggressively early and eat the extras.
Common Mistakes I Still Catch Myself Making
Planting too deep. Burying the bulb more than an inch slows growth.
Ignoring day length. Still the number one reason for failure.
Overcrowding. Four inches minimum for decent bulbs. Closer if you only want greens.
Stopping water too early. Bulbs need moisture right up until the tops fall.
Skipping the cure. Soft necks mean short storage life and mold.
Frequently Asked Questions
Can I grow onions in containers?
Yes. Use a pot at least twelve inches deep and sixteen inches wide. Fill with loose potting mix and keep the water consistent. I grow a few red onions this way on the patio when garden space runs short.
How long does it take to grow onions from sets?
Usually 90 to 100 days to mature bulbs. You can start pulling green onions much earlier.
Why are my onions so small?
Wrong day-length variety, not enough sun, inconsistent water, or overcrowding. Fix those four and size improves fast. IMO the day-length issue trips up more gardeners than anything else.
Do I need to soak onion sets before planting?
No. Plant them dry. Soaking can encourage rot in cool soil.
Can I save seeds from my onions?
Onions are biennial. Leave a few healthy plants in the ground over winter (or dig and store them) and they will flower the second year. Collect the black seeds once the heads dry.
I still get excited every spring when the first sets go in the ground. The process feels simple once you respect the day-length rule and keep the soil happy. Start with a short row of sets this season if you are new. Taste the difference between a store onion and one you grew yourself. That first bite usually turns people into onion gardeners for good.