Written by Dale Richardson - Updated: June 23, 2023
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Did you ever have an onion sprout greens in your pantry and wonder what to do with it? Do you want to take an onion from a store and turn it into a sustainable harvest? Here is how to grow onions from the grocery store.
To grow an onion from the grocery store start by sprouting the onion by either using the onion bottom or the onion buds. Then, pot out the new onions into a pot or your garden, and cultivate them until harvest. These propagated onions will produce green tops, seeds, or more onion bulbs.
Most onions are grown in the garden from sets, which are immature onions whose growth was interrupted. when you plant them in the garden, they continue to mature into a full-sized onion. An onion that you buy from the grocery store is already mature, but with a little preparation, you can grow them for a whole new harvest. Here's how.
The outer layer of an onion is the brown, dry peels that protect the bulb. Underneath this skin is the layers of flesh that encircle the core. The core is made of one or more buds, which are the parts that sprout green shoots if you leave them in your cupboard too long. Each of these buds will grow into a new onion, and these are the parts that you will use to propagate your new onion patch.
You could stick the whole onion in the soil or in water and watch it grow, but this would be a waste of a mature and perfectly edible onion. Thankfully, there are two ways you can eat most of the onion, and use the leftovers to grow with: using the bottom of the onion or using the onion buds.
Image by Carmen Edenhofer
With this method, you are going to use the bottom 1/3 or so of the onion.
Using a sharp knife, cut off the bottom 3cm to 4cm (1-1.5inch) of the onion. This is the root base of the onion which you will use to propagate more onions. The top of the onion can be used for culinary purposes. You can tell which side is the base because it usually has a flat tip with hair-like roots that are dry and brown, while the top is often has a dried stem that looks like a neck.
Place the onion bottom in water so that the roots are submerged. A 250ml mason jar is usually the right size that the onion bottom can rest on the rim with the root end hanging down into the jar. Fill the jar with water until the roots are covered. Alternatively, you can see in this other article how to suspend the onion over a glass with toothpicks, or put the onion bottom in an egg carton.
Refresh the water every day or so and keep the roots submerged. In a few days to a week, the onion should start to sprout roots and it is ready to move into a pot. Fill a pot with your choice of soil, such as soil from your garden or bagged potting soil, and make a shallow bowl on the surface of the soil that is the size of your onion. Nestle the onion bottom into the bowl, making sure the roots are settled well in the soil. In a few weeks, you should have healthy, vigorous green growth.
Each bud in the core of the onion will send up one shoot, and each onion bulb usually has between 1 and 5 shoots. Each of these shoots is an individual onion plant. Once you have vigorous greens, dig up your onion bottom from its pot. Separate each plant by carefully cutting down between the buds, or carefully break them apart with your fingers. Be sure to leave part of the roots on each bud.
Here is a link to an article that goes into great detail about growing the onion bottom accompanied by pictures.
In this method, you are going to separate the individual buds out of the core of the onion. To start, let the onion sprout green shoots out the top. You can do this by leaving it to sit in the cupboard, but a better method is putting it in water just like the onion bottom, only leave the onion whole. You can also "plant" the onion in the soil by burying the bottom third of the bulb in the soil.
Once the bulb has sprouted, carefully peel away all the layers of the onion until you come to the sprouted buds at the core of the onion. Carefully separate these buds, making sure to leave roots on the bottom of each one.
This video is well worth watching to see this method in action. It also gives a lot of great pointers, and don't forget to read people's experiences in the comments.
Image by Carmen Edenhofer
Now you can plant your individual onion plants in soil. It is generally a good practise to bury most of the white part of the onion under the ground since this part has never been exposed to the sun and could easily wither and burn. If your bud has a lot of green growth, it is a good idea to cut off most of the greens, leaving about a third of them still on the bud. Cutting back the greens allows the plant to establish itself in its new environment without the tops drawing away energy and nutrients.
Keep your onion watered as it grows. It shouldn't get too wet, but you don't want it to dry out either.
You can also plant your onions in your garden. However, the onions that you bought from a store might not be suitable for your area. In my area, most of the onions in grocery stores come from nearly 3,000km away. This is a drastically different temperature zone and our short, cold season could not grow this variety. In this case, it is better to grow your onion in a pot as an ornamental and edible houseplant. If you do want to grow store-bought onions in your garden, shop around for local onions or try sourcing them from a local farmer's market
What will you get when you plant an onion from the grocery store? You will get three useful products: greens, seeds, and more onions.
The most likely outcome is you will get a good growth of onion tops (often called spring onions) that are edible and very versatile. The onion greens can be cut off the bulb and they will grow back several times.
Along with the greens, onions from the grocery store are likely to produce a flower. Since the bulb is already mature, the next stage of its growth would be to produce a flower and reproduce. If you let these flowers mature, you can harvest the seeds to plant in your garden next year.
The last option (and the one most gardeners are hoping for), is for the store-bought onions to grow another onion (or maybe a few more). While this is possible, it is, unfortunately, the least likely. However, with a lot of patience and dedicated care (and also a bit of luck), many gardeners use an onion from the grocery store to produce more onions that they propagate year after year. It will take about 90 to 120 days for the onion to form into a decent-sized bulb. Bear in mind that the bulb that you grow will probably not measure up to the parent plant in size and uniform shape.
Image by Carmen Edenhofer
If you want to grow the onion for a bulb, don't harvest the greens. It is the top of the plant that feeds the root, so by continually cutting the greens, you are starving the plant of everything it draws from above the ground. In general, the bigger the greens, the bigger the bulb
It is important to remember that onions from the grocery store are not as viable as onion sets bought from a garden centre. While most of the onions from the supermarket will sprout and grow, some of them might die off. Regardless, you will still get a good harvest from their growth if you follow the guidelines below.
It is easy to think that an onion that has sprouted in your fridge is "bad" but this is not the case. In reality, this onion is simply starting to grow after dormancy to continue its life cycle. Not only is it still edible, but it is also perfect for growing since it has already started for you. You can peel off the outer layers to expose the sprouting buds within and you are ready to plant.
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