Learn how to grow various types of onions, including their sunlight and temperature requirements.

By Deb Wiley
December 14, 2015

Add delicious, savory flavors to cooked or raw dishes by growing your own onions.

Long-day onions take a long time—about five months—to grow from seed. Northern gardeners who want to grow onions from seed should consider growing short-day varieties or scallions (green onions).

Plant seeds in well-drained soil in full sun 1/2 inch deep, sowing one to five seeds per inch and thinning them as they grow. If you are growing onions to a large size, thin them so they are about 3 inches apart. For green onions, thin to 1/2 to 1 inch apart.

Keep the soil consistently moist when the green tops are actively growing.

How to Plant an Onion Bulb

If you want to grow large onions, it's easiest to start with small bulbs, sold as transplants or sets.

If you're growing sets into scallions or green onions, plant the bulbs 1-1/2 to 2 inches deep and 1 inch apart. If you're growing large onions, plant the bulbs 1/2 inch deep and 4 inches apart. When to plant them depends on how soon you can work the ground in spring. Onions can survive light frost.

Keep onions moist until they reach the size you want and the green tops begin to tip over. When the soil dries out, dig up the bulbs and allow them to dry and cure in the sun (or a warm, dry, sheltered space if rain is forecast) for a week. This curing step helps the onions keep longer.

How Do Onions Grow?

Here's something you might not know: Some onions need more daylight hours to grow than others.

Long-day onions stop growing their green tops and start forming bulbs when they receive 14 to 16 hours of light per day, making them a great choice for northern states (roughly the upper two-thirds of the United States, above the 36th parallel). In the Northern Hemisphere, the farther north you go, the longer the summer day length.

Short-day onions form bulbs when days contain 10 to 12 hours of sunlight, so they're a good choice for planting in the spring and fall in the lower third of the United States.

Here's another interesting tidbit: The ultimate size of an onion depends not only on the type it is, but the number and size of green leaves it forms. Each leaf indicates one ring of onion forming in the bulb below ground. The larger the leaf, the larger the ring is.

Onions grow their roots and leaves when temperatures are still on the cool side, 55 to 75 degrees F. When bulbs start to form, however, onions need warm temperatures and prefer low humidity. If there are a lot of cool, overcast days during a bulb's growing period, onion growth stalls.

How to Grow Green Onions

Growing green onions is easier than growing storage onions because the bulb doesn't have to grow as much.

To grow green onions, you can plant any type of onion and simply harvest it when young. Or choose sets or seeds of bunching or scallion onions, which don't form bulbs.

Plant seeds for green onions in full sun, placing them 1/4 inch deep and 1 to 2 inches apart in well-drained soil, thinning them as they grow. Harvest scallion onion plants by the time they reach about 10 inches tall. If you wait, the flavor becomes bitter as they grow larger.

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