A Guide To Growing Onions In A Grow Bag

Grow bags add a lot of flexibility to your garden. They are more environmentally friendly than plastic pots and can grow an abundance of vegetables. Onions are well suited to any container gardening, and this article will show you how to grow onions in a grow bag.

Choose a grow bag that allows around 25cm (10inches) of soil depth. Fill your grow bag with a mix of compost, well-rotted manure, and soil. You can grow them from sets or from seeds. Keep your grow bag thoroughly weeded, and keep your onions well watered until mid-August. Once half the onion tops have fallen over on their own, bend the rest over. Wait one more week, and then harvest your crop of onions.

Grow bags are a great way to save space or create a temporary and semi-portable garden. Onions are well suited for grow bags, and here is how to grow healthy, delicious onions in your grow bag.

Which Grow Bag To Use?

There are many different grow bags on the market. Some last for years while others are only meant to be used for a season or two. There are grow bags designed for particular crops, such as small access flaps to harvest potatoes. Some are small for a few herbs, and others are large enough to lie down in. And of course, you can even choose your preferred colour.

For growing onions, you can really choose any grow bag you want. The size you get will depend on how many you want to grow. You will be spacing your mature onions about 8cm (4inch) apart on average, so this will give you an idea of how many you can fit in a bag. Try and choose a grow bag that will accommodate a soil depth of 25cm (10inches). Onions have a shallow root system and they can be grown in less soil, but you don’t want to strangle the roots so provide extra space if possible.

Upcycled grow bags

Old, ripped or holey cloth shopping bags make excellent recycled, and free, grow bags.

Preparing The Grow Bag

Onions are heavy feeders, meaning they consume a lot of nutrients as they grow. Thus it is important to start your onions off right by properly preparing their growing conditions. In other words, give them good soil and compost. Mix a good amount of compost or well-rotted manure into your soil and fill the grow bag. We like to fill about 1/3 of the grow bag with compost and manure and the remainder with soil. Compost adds beneficial humus to the soil, and chicken or horse manure is particularly beneficial as it contains a high amount of nitrogen for the onions.

Good soil by Carmen Edenhofer
Image by Carmen Edenhofer

You can use soil out of your garden or you can use bags of soil from the garden centre. Potting soil is ideally suited for growing in bags. Topsoil will often compact too much in containers and constrict root growth. If you are using topsoil, add extra compost to improve the texture or add some vermiculite or perlite for aeration and drainage.

Since onions are such heavy feeders, they also benefit from glacial rock dust, which contains a lot of valuable minerals.

Onions like a soil pH between 5.5 and 6.5. Again, adding heaps of compost will benefit your onions as this will slowly and naturally lower the pH of your soil.


Onions are generally divided into two categories: long day and short day onions. Long day onions should be grown in Canada, northern US, and other countries where summer days are noticeably longer than winter days. Short day onions, on the other hand, are for places where the length of summer days is not as pronounced over the winter days. Be careful of this if you buy your onions online as long day onions will not grow in the south and vice versa. Your local garden centre should supply onions that are appropriate for your area.

Another division in onion planting is whether you should grow for onion sets or from onion seeds.

Growing From Sets

Onion sets are onions that are pulled when they are immature. When you plant them in your garden, they continue growing to mature into a full-sized onion to harvest. Onion sets have a head start on the growing season and are generally planted outside about 4 weeks before the last frost date. The advantages of growing onions from sets are that they are easy to plant, and they are often ready for harvest quite quickly (about 3 1/2 months).

Plant the onion sets about 2cm to 3cm (1inch) deep in the grow bag, and keep the soil moist but not overly wet. Space the onions about 5cm to 8cm apart, but space them further for larger varieties. Sets will establish quickly, and you should soon see green growth from the bulb.

An onion top by Carmen Edenhofer
Image by Carmen Edenhofer

Growing From Seeds

Onions can also be grown in your grow bag from seeds. Onion seeds should be started about 6 to 8 weeks before the last frost date in your area. If your grow bag is large and cannot be moved, it is best to start the onion seeds in trays and then transplant them into your bag. Alternatively, you can set your grow bag inside for germination and then transition your seedlings to the outside around the average last frost.

It is important to remember that when onion seed packets say they will mature in about 100 days that this is from the transplant date. This means that your onion seeds will be ready in around 5 months (50 or so days indoors plus 100 days to maturity).

The optimal temperature to germinate onion seeds is 16°C to 25°C (50-75°F). At this temperature, the seeds will have a 75% germination rate and will emerge in 6 to 12 days. They will germinate in lower temperatures of course, but it will take longer and their success will be decreased.

Sow the seeds 1cm (1/2inch) deep. You can sow them fairly close to make sure you get a good emergence, but then thin the seedlings to 5cm to 8cm (3-4inches) for standard size bulbs and further apart for large varieties.

Water and Weed

Whether you grow your onions from seeds or sets, make sure they have ample water while growing but they don’t do well in wet or soggy soil. Because grow bags are made of breathable material, the soil in them tends to dry out faster than in the garden or in conventional containers. So make sure you keep and eye on the moisture level and to keep them from drying out.

If you start your onion grow bags indoors, remember that this breathable material also lets water seep out the bottom. Make sure you put the bags on a tray so you don’t ruin your floor.

Onion plants are easily choked out by weeds, especially when they are young. So it is important to pull any weeds that sprout in your grow bags as soon as you see them. In this case, it is advantageous to use purchased bags of potting soil as they are usually sterilized to be weed-free.

Harvest Time

While the green tops of onions are edible and delicious, it is generally best not to harvest them if you are aiming for hearty bulbs. The greens feed the plant through photosynthesis, so the bigger and healthier the tops are the bigger and healthier the bulb will be.

Around mid-August, stop watering your onions so the bulbs can start to dry while still in the garden. If your growing season is long enough (or if you started them early enough), the green tops of the onions will start to dry and fall over. Once this happens, push over the rest of the tops. This will discourage further growth and start getting them ready for storage. Wait about a week after all the tops have been bent, and then start to pull your harvest.

The onions can then be cured and stored, and they should last quite well until the late spring. If your onions do sprout in storage, you can still eat them, use them to propagate more onions, or grow an abundance of onion greens for fresh eating.