11 Tips for Growing Great Onions

There's a certain photo of my garden that, when I post in on social media, always seems to inspire a lot of onion-growing questions. While I've given some onion-growing tips on this blog's Facebook page, today I want to expand on those and give you my very best tips for successfully growing your own onions.

1. Select the right type of onion for your area. This is the most important thing to know about onions and can literally make or break your success. All onions are broken down into three main types, called long-day, day-neutral, and short-day.

Long-day onions are best for Northern parts of the U.S. (37 - 47 degrees latitude) and should be planted in early spring (or in the fall, if allowed to overwinter in the garden and form bulbs in spring); long-day onions begin growing bulbs when days have reached 14 - 16 hours of sunlight. (The onions in the photo above and below are Walla Wallas - a day-neutral type.)

Day-neutral onions (sometimes called "intermediate" onions) are best for the central states (32 - 47 degrees latitude). They begin forming bulbs when there's about 12 - 14 hours of light per day and should be planted in early spring.

The photo that has everyone asking for onion growing tips.

Short-day onions grow best in Southern parts of the U.S. (25 - 35 degrees latitude). They start growing bulbs when there's 10 - 12 hours of daylight and are usually planted in fall and allowed to overwinter in the garden, to begin growing bulbs in spring.

2. Choose from seeds, sets, or plants. If you're new to growing onions, I recommend growing them from plants (bundles of bulbs with green or brown shoots) or sets (dormant bulbs without shoots). That said, some people find onions grown from sets tend to go to flower before the bulb fully matures.

3. Make sure your soil is well draining. Onions will not grow well - and may even rot - in soil that retains a lot of water.

4. Follow the planting advice that comes with your onion seeds, sets, or plants. If needed, do an internet search to get information specific to the variety you're growing. You need to know how deep you should plant and how far apart your onions should be, in addition to what time of year to plant.

5. Keep your onions well irrigated. Onions that don't have enough water can't grow big and juicy. If you're unsure whether or not you need to water, stick your finger in the soil. If it's dry one inch down, irrigate. Mulching around your onions will reduce the amount of watering needed.

6. Keep your soil fertile. Because I use no-till methods, which includes laying organic matter on top of the soil to allow it to decompose and add nutrients to the garden, I do not need to fertilize my plants. If you garden differently, you'll want to use a balanced fertilizer (though Phosphorus and potassium are most important for bulb growth) every 2 - 4 weeks; onions are heavy feeders.

7. Do not push soil up around your onions. Many people tell me they've been taught that if their onion bulbs appear over the soil, they need to cover them up. This isn't true. As onions grow, their shoulders will naturally rise above the soil line; let them be!

As onions grow, they will push themselves up above the soil line.

8. If your onion bolts, eat it. When an onion develops a thicker middle stem and a flower bulb appears on top of it, it's bolting (i.e. going to seed). Harvest the onion and eat it right away; it won't store well and it won't grow bigger if left in the garden. In fact, if left in place, it may rot. (Bolting is a reaction to stress, such as temperature fluctuations or lack of water, but these stressors may not affect all your onion plants the same way.)

9. Understand when to harvest. Some onions taste great at any size (like my Walla Wallas), so it's fine to harvest one as needed. But don't harvest onions for storage until the leaves fall down and turn yellow or brown. Stop watering your onions when this happens and leave them in the ground for another week or two. Then dig out the onions, brush the soil off them, cut the leaves to about 4 inches long, and cure the bulbs in a single layer in a warm, dry environment (ideally, 7-80 degrees F.) for a couple of weeks. When the necks are fully dry, trim the leaves to 1 - 2 inches and trim the roots close to the bulbs. Store in a dry, cool, dark location, ideally at 40 to 60 degrees F. Stored onions need to breathe, so it's best to place them in a basket (just be careful not to let the basket poke holes in the onions, which will lead to rot), mesh bags, or in a single layer on top of newspaper or cardboard. (If you chose the latter, the onions will last longer if you rotate them periodically.)

Part of last year's onion harvest,

10. Know that not all onions store well. If you want onions that will keep in a cool location all winter long and well into spring and possibly summer, look for a storage variety, like Patterson, Cortland, or Yellow Globe. About once a month, check your stored onions for soft spots or decay. Remove and eat any onions that are no longer in good condition or they may cause nearby onions to also begin decaying. For onions that aren't storage types, dehydrate, freeze-dry, or freeze to preserve them.

11. Consider special types of onions. Last year, after many years of wanting to, I added walking onions to my garden. Walking onion types (sometimes called "topset onions") grow not only a bulb in the soil, but also bulbs on top of their leaves. As the bulbs get heavy, the leaves bend down to the soil and the topmost bulbs take root into the ground. Think of them as a perpetual onion; you'll never have to start onions from seed (or buy sets or plants) again.

Egyptian walking onions. Courtesy of Dave Whitinger.

To grow scallions (i.e. "green onions") you should select bunching types, which don't grow large bulbs; they are really grown only for their mild green tops. As long as the bulbs remain in the soil, they will reproduce for many years. (Do note that you can simply trim regular bulb onion leaves and use them as scallions, but they may be more strongly-flavored and you should stop harvesting leaves by late spring.)