Onions have a reputation for being fussy, which is a little unfair. Most of the time when someone ends up with a bag of thick green leaves and no bulb underneath, the problem was decided before planting even started.
So, can I grow onions in grow bags? Yes, and onions suit bags better than most crops people put in them. The reason is simple once you know how an onion root system works. The catch is that one particular decision matters more than everything else combined, and it has nothing to do with the bag.
Why Onions Do Well in Bags
Their Roots Stay Shallow
An onion keeps most of its roots in the top 15 to 20 centimetres of soil. It is not reaching down the way a tomato or a carrot does.
That means you do not need a deep container, which is exactly where bags tend to struggle with other crops. A moderate bag gives an onion everything it wants.
They Hate Sitting in Wet Soil
This is the big one. Onions rot easily when the soil around the bulb stays soggy, and heavy garden soil holds water precisely where you least want it.
A fabric bag drains freely from every surface, so excess water leaves instead of pooling around the bulb. Plenty of gardeners who struggle with onions in clay beds have much better luck in bags for this single reason.
You Can Move Them
Onions want as much sun as they can get, ideally six hours or more. A bag can be shifted to follow the light, or pulled under cover during a wet spell. A garden bed cannot.
The One Decision That Decides Everything
Here is the part that catches people out, and it is worth understanding before you buy a single seed. Onions do not form bulbs because they reach a certain age. They form bulbs when the day length reaches a certain number of hours. Until that trigger happens, the plant just keeps making leaves.
Varieties are grouped by the trigger they need.
Short-Day Onions
These start bulbing when days reach roughly 10 to 12 hours. They suit lower latitudes, including the tropics and warmer southern regions.
Long-Day Onions
These need around 14 to 16 hours before they bulb. They are bred for northern regions where summer days stretch out long.
Why This Matters So Much
If you live near the equator, your days are close to 12 hours all year round. A long-day onion planted there will never receive its trigger. It will grow healthy leaves for months and never form a bulb, and nothing about your bag, soil or watering will change that.
The reverse is also a problem. A short-day variety planted in a northern summer bulbs too early, while the plant is still small, and gives you tiny onions. So before anything else, check what day-length group a variety belongs to and match it to where you actually live. Seed suppliers list this, and it is the single most useful thing on the packet.
Choosing the Right Bag
Depth
Aim for at least 20 to 25 centimetres of usable depth. More is fine, but it is not where the benefit comes from, since the roots will not use it. What matters more is that the depth is real. A bag that only just holds that much medium once settled will dry out quickly.
How Many Onions Per Bag
Spacing is the actual limit, not volume. Full-size bulbs need roughly 10 to 15 centimetres between plants. Crowd them closer and you get a lot of small onions instead of a few good ones.
As a rough guide, a bag around 35 to 40 centimetres across comfortably holds four to six onions grown for full bulbs. A bag in the 20 to 40 litre range is a sensible choice. If you want spring onions instead, you can plant far more closely, which is covered further down.
Fabric or Plastic
Fabric bags drain and breathe from every surface, which suits a crop that dislikes wet feet. Plastic planter bags hold moisture longer, which can help in very dry climates but needs good drainage holes. Either works. Just make sure water can genuinely get out.
Get the Growing Medium Loose
Onions want a medium that stays open and drains. Heavy garden soil in a bag compacts into something closer to a brick, and that is where bulb rot starts.
A mix of good compost with coco peat or coir, plus perlite or coarse sand for structure, gives you what the crop wants. Aim for something that feels crumbly rather than sticky when you squeeze it. Slightly acidic to neutral suits onions best. Very acidic medium limits how well they take up nutrients.
Sets, Seeds or Seedlings?
Sets are small part-grown bulbs, and they are the easiest starting point. You push them in and they go. The trade-off is a higher chance of bolting, meaning the plant throws up a flower stalk instead of building a bulb.
Seeds are the cheapest route and give you the widest choice of varieties, which matters a lot given the day-length issue. They just take longer, usually needing a couple of months before they are ready to go into their final bag. Seedlings or transplants sit in between, and for most people growing a handful of onions they are the easiest balance of speed and reliability.
One planting detail regardless of which you choose: do not bury the bulb deep. The shoulders of an onion should sit close to the surface. Buried too deep, bulbs stay small and are more likely to rot.
Watering Is Where Most People Slip
While the plant is building leaves, keep the medium consistently moist. Not wet, just never fully dried out. This phase is when the plant builds the capacity it will later turn into a bulb.
Then the shift happens, and this is the part that gets missed. As bulbs swell and the tops begin to soften and lean over, start easing off the water. Onions approaching maturity do not want moisture around them.
Stop watering entirely about one to two weeks before you plan to harvest. This lets the outer skins firm up, which is what makes an onion store well rather than going soft in a few weeks. Watering right up to harvest day is one of the most common reasons homegrown onions do not keep.
Feeding
Onions want nitrogen early, while they are putting on leaf growth. A balanced feed through that stage works well.
Once bulbing starts, ease back on nitrogen. Too much late in the season produces thick necks and soft bulbs that will not store. Shifting towards something with more potassium suits the bulbing phase better.
Harvesting and Curing
Harvest when roughly half to two thirds of the tops have yellowed and fallen over. Do not wait for every leaf to collapse.
Lift the bulbs gently rather than pulling hard, and leave the tops attached for now. Then cure them somewhere warm, dry and airy but out of direct sun, for around two to three weeks. The necks will dry down and the skins will tighten. Only after curing should you trim the tops and store them. Skipping this step is why some harvests rot in the cupboard within a month.
If You Just Want Spring Onions
There is a much easier version of this whole project.
Spring onions, also called green onions or scallions, are harvested for their stems and leaves rather than a bulb. That means day length does not matter, spacing barely matters, and you can plant them far more densely in a smaller bag.
They are also fast, usually ready in a couple of months rather than a full season. For anyone short on space or patience, or in a climate where bulbing varieties are difficult to source, this is the sensible place to start.
Common Mistakes
- Planting the wrong day-length variety for your location is the biggest, and it is the one nobody notices until the season is over.
- Crowding too many into one bag comes next. It feels efficient and it quietly halves your bulb size.
- Using heavy garden soil instead of a loose mix traps water around the bulb and invites rot.
- Burying sets or seedlings too deep keeps bulbs small.
- Feeding nitrogen all the way to harvest gives you lush tops and soft bulbs that will not keep.
- Watering until the day you harvest, and then skipping the curing stage, undoes a whole season of work in the storage cupboard.
Easier in a Bag Than in the Ground
So, can I grow onions in grow bags? Not only can you, but a bag solves the thing onions complain about most, which is wet soil around the bulb.
Get three things right and the rest is fairly relaxed. Pick a variety whose day-length requirement matches where you live. Give each onion enough room, around 10 to 15 centimetres, in a bag with at least 20 to 25 centimetres of loose, free-draining medium. Then taper the water as the tops fall over and cure the bulbs properly after lifting.
Do that, and the crop is genuinely straightforward. Get the variety wrong, and no amount of care afterwards will produce a bulb, which is why it is worth checking the packet before anything else.
Frequently Asked Questions
Can I grow onions in grow bags?
Yes, and they suit bags well. Onions root shallowly and dislike waterlogged soil, so a free-draining bag often produces better results than a heavy garden bed.
How deep should a grow bag be for onions?
At least 20 to 25 centimetres of usable depth. Onions keep most of their roots near the surface, so extra depth beyond that adds little.
How many onions can I plant in one grow bag?
Allow 10 to 15 centimetres between plants for full-size bulbs, which works out to roughly four to six onions in a bag 35 to 40 centimetres across. Spring onions can be planted much more densely.
Why are my onions growing leaves but no bulbs?
Most often the variety does not match your day length. Long-day onions will not bulb near the equator, because the trigger they need never arrives. Check whether your variety is short-day or long-day.
For more information about Planter Bag please contact: Whatsapp/Mobile Phone: +62 822 9933 3938 (Ms. Panni) or Email : info@urbanplastic.id