How to Grow Onions

Types of Onions In the grocery store produce section onions are typically broken out into categories based on their color, size and sweetness, but for gardeners we’re also interested in their “day-length.” An onion’s day length refers to the number of hours of daylight that will trigger it to start growing a bulb. Essentially the closer you are to the equator, the shorter your days will be during the peak of summer. So if you’re in the deep South you’ll want to grow short-day onions, and if you are far in the North you’ll want to grow long-day onions. If you’re in that gray area between the far North and deep South there are varieties categorized as intermediate-day that will probably work well for you.

Starting Onions Onions grow from seeds or small bulbs called sets. If you’re looking to produce large round onions, starting them from seed is the best way to ensure success because the sets, which are produced the previous season, sometimes get confused by the refrigeration process and think it’s time to start bulbing very early in their growth. You will also have a much bigger selection of onion varieties to choose from if you grow from seed.

Start early if growing from seed, at least eight to 10 weeks before you transplant them outside. Sow them in a 2- to 3-inch deep flat filled with potting soil so that there are 4 to 6 seeds per square inch. Cover lightly with soil or vermiculite, then put your trays in a warm place (75 to 80°F is ideal) and keep the soil moist. Don’t give up if you don’t see growth for a while, they may take up to two weeks to germinate. Once they sprout, put them under a grow light or in a sunny south-facing window. At this point they are pretty hands-off; just remember to water every time the top of the soil dries out.

You can plant your onions outside two or three weeks before the average last frost date in your area, once the danger of a really hard frost has passed. Harden off your seedlings at least a week before you plan to transplant them by moving them outside to a semi-protected area each day and bringing them back inside at night. Now you’re ready to plant.

How to Grow Onions Onions don’t typically compete well with other plant species so they should be grown in a bed completely free of weeds. To avoid weeding your onion bed as they grow, you can spread a good amount of straw or leaf mulch around the plants to smother weed seedlings. While they don’t compete well with other species, they actually grow fairly well when clumped together in bunches of three or four. You can also grow onions singly in rows, but the bunching method might help maximize your yield if you have limited growing space.

Botanically speaking, onions are essentially a big clump of fleshy leaves that grow in a bulb. The green tops of onions that are typically referred to as "leaves" are actually just the tips of the leaves. The full leaves extend all the way down to the root and includes the part of the onion that we eat. This is important to note because leafy growth requires lots of nitrogen and that’s exactly what onions love. You want to make sure you are growing them in a rich, well-fed soil full of compost and nutrients. If you are working with poor soil, you can mix in some compost and then apply a fertilizer solution like fish-meal or kelp-meal throughout the onion’s growth.

When to Harvest Onions Depending on the variety that you choose, onions take around 90 to 100 days to reach maturity. The onion tells you it’s ready to be picked when the green tips of the leaves will start to wither and flop over on the bulb. This means that the onion is finished growing and has no more need to photosynthesize.

You can also harvest your onions as green onions or spring onions during any part of their growth, and you can even pick them as microgreens to top a savory dish. Keep in mind that if you pick them in any of the green stages, they will not store well outside of the refrigerator.

How to Store Onions If you allow your onions to reach full maturity, they can store for a relatively long time in the right conditions, but again this depends on the variety you grow. Most seed companies will specify if the onion variety stores well in the description. If something didn’t go quite right during the growing process or your onions didn’t have enough time to reach full maturity, you can also pickle them or cook them down into an onion jam to preserve them through the winter.

I’ve heard of people who don’t like the flavor of onions, but I find it very hard to believe. They have grown and evolved alongside us for thousands of years and the smell of chopped onions sautéeing in oil is intoxicating on an instinctual level. If you are looking to round out your homegrown meals this next growing season, I highly recommend finding some space for a few onions in your vegetable garden.