When you grow onions indoors, make sure your plants have enough space and light. Onion plants need six to ten inches of depth for proper root growth and bulb formation. Leave three inches between each onion plant. Scallions or smaller varieties can be planted more closely together.
Onion plants need between 12 to 16 hours of light, varying by whether they are long-day or short-day onions. Supplement sunlight with artificial light if needed. While you can start with onion seeds or sets, you can easily turn leftover onion bulbs or sprouted onions from your kitchen into an indoor garden.
People read also: How to Grow Garlic Indoors
How to Grow Onions From Seed
Onions take a while to develop from seed. Sowing inside in January or February under growing lights then transplanting to the garden in early spring is the only way that I can grow onions from seed and have them mature in my zone 5 garden. If you live in more southern areas, you can plant onion seeds in late summer to early fall, overwinter, and they will begin growing when the weather warms.
The way to grow onions in container gardens is much like growing onions in the ground. You need good soil, adequate drainage, good fertilizer, and plenty of light.
Really, the only difference between what you do when you grow onions in the ground and when you grow onions in pots is choosing the container you’ll be growing them in.
Since you need several onions planted to get a decent crop, attempting to grow onions in pots that are only 5 or 6 inches (13-15 cm.) wide would be cumbersome. If you choose to grow onions in pots, choose a large mouthed pot. It needs to be at least 10 inches (25 cm.) deep, but should be several feet (1 m.) wide so that you’ll be able to plant enough onions to make it worth your while.
Many people have success growing onions in a tub. Plastic tubs are much cheaper than a comparable sized pot, therefore, growing onions in a tub is economical and efficient. Just make sure that you put holes in the bottom of the tub to provide drainage.
You can also grow onions in 5 gallon (19 L.) buckets, but realize that you may only be able to grow three or four onions per bucket as onions need at least 3 inches (8 cm.) open soil around them to grow properly.
How to Regrow Onion Bulbs Indoors Using Water
Another method for growing onions indoors is to use water. With this method, you don’t need any potting soil. All you need is a jar that has a wide enough mouth for your onion bulb to rest on the top. Just add enough water to the jar, so that the bottom of the bulb rests in the water. Place your jar in a sunny location, and add more water when necessary to keep the onion roots submerged.
Onions can be harvested young as green onions or left to mature fully. Onions are finished growing when the tops flop over. Stop watering at this point and wait for a dry day to harvest. Harvest, lay out to cure, and store for winter.