How to Grow Onions in Containers – Get a Bumper Crop with Hardly Any Soil
Onions have such shallow roots they look like they're just sitting on top of the soil. Harness this oddity of the garden world to get a huge container harvest.
People tend to think of onions as large-scale farm crops, probably because they’re such grocery store staples that they’re rarely seen in gardens. They’re actually quite easy to grow, and since they have shallow root systems and grow mostly aboveground, you can produce a massive harvest in a very small footprint and with very little soil.
In other words, they’re perfectly suited to container life! You can grow onions in containers on patios, balconies, and even indoors. The secret lies in finding the right onion variety for your climate, and in getting the soil just right. Below we’ll cover everything you need to know about growing onions in containers
How to Grow Onions in Containers
Growing onions in containers is very similar to growing them in the ground. You need good soil, adequate drainage, good fertilizer, and plenty of light. Really, the only thing you need to do differently is find the right container.
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The Best Containers for Potted Onions
Container grown onions need very little soil depth, and several of them can be planted in the same container. You already have a good idea of how wide onions can grow, so you have a sense of the size of container you’ll need. An ideal onion container would be 10 inches (25 cm) deep, and several feet (at least 60 cm) across. A long trough-style planter like this one from Amazon is a good bet.
Potted onions also need extremely good drainage. Leave an onion sitting in soggy soil, and it’ll rot within days. A good option to ensure drainage is a trough-style fabric grow bag, like this one from Amazon.
There’s nothing that says you can’t grow onions in smaller containers. Just know that you’ll either have a small crop, or you’ll be planting in lots of pots.
Choosing the Best Location
As we covered above, onions need a whole lot of light to develop bulbs. Once it’s warm enough to move your containers outdoors, the sun ought to take care of that. If you’re starting onions indoors, however, or just growing them as an indoor crop, you’ll need supplemental lighting. A grow light like this one from Amazon should have you covered.
Which Type of Onion Should You Grow?
It’s important to choose a variety of onion that matches your zone. Otherwise you can wind up with lots of green leaves but tiny, disappointing bulbs.
Onions are photoperiodic, which means they’re affected by the number of hours of sunlight they get in a day. Until they reach a minimum number of hours of light in a day, they won’t start forming bulbs.
Different varieties of onions are classified according to the number of hours of light in a day (daylength) needed for onion bulb formation. There are three main groups:
- Short-day onions: These need at least 10-12 hours of daylength for bulbs to form. This usually falls around May or June. It’s best to plant them in the fall and let them overwinter. In the spring, bulbs will start to develop. Good for southern states (zone 7 and up).
- Intermediate/Day-Neutral Onions: These need 12-14 hours of daylength to start bulbing. Start them indoors in late winter, and they can be harvested in June to early July. They’ll grow pretty much anywhere.
- Long-Day Onions: These need 14-16 hours of daylength. They grow best in northern states (zone 6 and lower).
Ways to Start Onions
There are two different ways you can start growing onions. They both work perfectly fine in containers. Which you pick just depends on your skill level (or bravery) and how much time you have.
- Seeds: These are cheapest, but take a long time. You should start onion seeds indoors 8-10 weeks before the last frost date.
- Sets: These are seedlings, shipped in bundles. They tend to produce large, reliable bulbs, and are easier than seeds. Plant onion sets 1 inch (2.5 cm) deep 2-4 weeks before the last frost date.
Burpee offers several dozen varieties of onions as both seeds and sets.
Watering Potted Onions
While drainage is essential, so is moisture. It’s a tough needle to thread, I know. Onion soil should be kept consistently moist, like a wrung-out sponge. If the soil dries out completely, you run the risk of your onions bolting, which will ruin the bulb.
Container grown onions dry out faster than those grown in the ground. Check them daily, and if the top of the soil is dry to the touch, give them some water. Onions grown in containers will need at least 2 to 3 inches (5-8 cm) of water per week. If the weather’s dry, they’ll probably need more.
