Gardeners can choose to plant onion seeds, onion bulbs/sets, or onion seedlings from a nursery, seeds started indoors, or a sprouted onion.
Starting onions from seed allows you the largest selection of onion varieties to choose from. When choosing which onion varieties to grow the best place to start is determining whether you should be growing short day onion or long day onions.
This depends on where you live in the country. Short day onions are commonly grown in the south, south of the 35th parallel, and long day onions are commonly grown in the north.
How To Plant Onion Seeds
Plant onion seeds aggressively. This will help to keep weeds out, onion plants are notorious for being outcompeted by weeds, and onion seeds do not have great germination rates. As onions grow you can thin them down when the top growth reaches 4-6″ high and eat the culled plants as green onions.
Check out “How Far Apart To Plant Onions?” for more on correct spacing.
In the north where I live onion seeds need to be started indoors. There’s not enough growing season for onion seeds to produce root growth and sufficient foliage growth above ground before day length signals the onion plant to focus on bulb growth.
In the south onion seeds can be planted in the late summer/fall and they will develop a healthy root system and a small bulb that will go dormant over the winter and start to grow in the spring as soon as the weather warms up.
Sunrisesunset.com will tell you the average amount of daylight hours your city will get in a specific month.
The Old Farmer’s Almanac will tell you expected last frost in the spring and first frost in the fall for a given area.
How To Plant Onion Bulbs & Sets
Onion sets are seeds that were grown into miniature bulbs the previous growing season. In the north plant sets outside in the early spring about 4 weeks before your last expected frost date. In the south plant sets in the fall to harvest the following spring/summer.
In the north this still won’t give enough time for the onion plant to get as big as it could if grown indoors.
Also growing from sets make onion plants more prone to bolting and disease, lessens the amount of onion varieties you can choose from, and you don’t know how the bulb was grown the previous year.
How To Plant Onion Starter Plants
To plant onion starter plants make a row where you want the onion plants to be about 3″ deep with a hoe or other gardening tool. Plant the onion seedling so that the root system and about 3/4 of the white section above the root system is covered with dirt.
You want to leave some of the white area below the green onion exposed. As the bulbs from it’s common for 1/3 or so of the growth to be above the ground.
How Do Onion Farmers Plant Onions?
Onion farmers typically start onion plants indoors and then transplant them outside in the field by hand. In areas where climate permits onion seeds can be direct sown into a field and that’s done by a planter pulled along by a tractor. The planter disperses onion seeds at a high rate along a cultivated row.
Check out this onion production guide by the University of Georgia Extension for more on how commercial onion growing is done.
Should You Soak Onion Sets or Onion Seeds Before Planting Them?
It is not necessary to soak onion sets or seeds before planting them but it may help them germinate faster. The main factor that will determine how fast onion sets or seeds sprout is soil temperature.
If you do decide to soak onion sets or seeds soak them overnight and plant in the morning. Do not soak them for more than 24 hours.
Onion seeds will germinate with temperatures 40 degrees or above but the optimum temperature that onion seeds will germinate at is 75 degrees. Use a soil thermometer to test your soils temperature.
Starting Onions Indoors
By starting onion seeds indoors 8-12 weeks before transplanting them outside you can ensure that the onion plant has a good root system and top growth which will enable it to produce a good sized bulb/onion when you plant it outside.
The problem with direct sowing onion seeds into the garden in most areas is that the plant doesn’t have enough time to grow before daylight length triggers it to focus on bulb growth.
If the plant isn’t big enough when this happens it will be unable to grow a good sized onion.
To plant onion seeds indoors use a starter tray and grow light. If you want to just start one tray indoors you can use a small grow light and if you have many trays you want to start indoors I recommend a T-5 fluorescent fixture.
Plant seeds heavily in the starter tray and then thin them down when they reach 4-6″ high. You can eat the culled onion plants as green onions.
It can be a good idea to trim all of the onion plants when they reach this height so that they regrow into stockier and sturdier plants.
After planting the seeds water the tray in gently from above or preferably water from the bottom. To do this fill a flat tray with about a 1/4″ of water and set the starter tray inside of the flat tray.
After about 20-30 minutes water should have wicked its way up and the top of the soil should feel moist. Get a good idea for heavy the tray feels while its dry and watered to help you know when you should water again.
To plant onion sets inside use a similar method but give the bulbs more space, about every 2″ apart.
Please comment below about your own thoughts and experiences surrounding how to plant onions.