How to grow onions — for beginners

Onions are a great crop. Along with carrots and potatoes, they are one of the main crops that beginners should try. Why? Because they are relatively easy and can actually replace a significant amount of your annual supermarket onion purchase. In other words they provide significant bang for your buck. In my household, our garden has provided well over half of the onions we consume each year at a cost of less than £20 per annum all in.

Onions can be harvested from May to October and eaten straight away. That covers six months of the year. For the remainder of the year you will need to store them which is trickier. Storing onions is where most of the losses tend to occur, but with care you can keep eating stored onions right round until March.

Onion Sets

Typical onion sets

Onions can be grown from seed, but this method is less reliable, so for beginners I would recommend growing from onion sets. Onion sets are tiny bulbs grown from seed which have been dried perfectly so they are fully dormant.

Once planted they grow away quickly and are generally very reliable, with each tiny 1cm set growing into one full-size onion. [Note: shallots are different to onions as each shallot will grow into a bunch of 5 to 10 shallots.]

Onion sets are available to purchase in autumn and spring from most garden centres; usually in packs of 50 or 100

100 sets will fill a 4ft x 4ft bed. There are many varieties — white, brown and red skinned. Some claim to be better for storing, but the differences are marginal. Try a mix of varieties so you can learn which ones do well for your soil. Personally, we like to use red onions raw in salads, whereas we store mainly brown skinned ones.

Autumn onion sets are planted in October or November and grow slowly over winter, producing an early crop in late May or June. Onion sets purchased in spring may be planted from March until April for harvest from July to October.

Preparing the soil

Onions need somewhere in full sun, with a good loose soil of moderate to high fertility with adequate moisture. Avoid waterlogged or shady areas. Raised beds are excellent as long as you water them in dry weather.

Dig over your bed and remove all weeds. Most normal garden soils are fine for growing onions, but if your soil is particularly sandy dig in plenty of compost to help retain moisture. If, on the other hand, you have a heavy or sticky clay, dig in sand to help with drainage and compost to open up the soil structure so that the roots can get down easily.

If your soil needs a feed or has had other crops on it for a few years, then dig in a general purpose fertiliser such as fish, blood and bone. Avoid high potash fertilisers (such as tomato feed) as these encourage flowers which in the case of onions we do not want. Bone meal is good for nourishing root crops.

Planting the sets

The sets should be spaced about 4″ to 6″ apart (10 – 15cm). At 4″ spacing you will get more but smaller onions; There is little point in spacing them further than 6″ unless you are trying to grow giant show-onions! Stagger the rows to achieve a diamond layout (as shown below) as this maximises the amount you can grow in a given space.

space onion sets at 4 to 6 inches in staggered rows

When planting check the each set is firm with no evidence of mould. Reject soft, ’empty’ or mouldy sets as they will inevitably rot in the ground.

The sets need planting with the root end down and the papery tip pointing up. Many people push them just into the surface of the soil, half-in half-out, but I prefer to partially bury them. This helps prevent them pushing themselves out of the ground as their roots grow. I also plant them in small depressions made with a dibber, as this helps with watering when they are young.

TIP: If you find you have some spare sets you can plant one area more densely for early picking as spring onions.

Looking after them in the next few weeks

The onions will sprout in the next couple of weeks, pushing up little shoots. Keep them watered in dry weather, and watch out for pigeons who love to pull them out of the ground for no obvious reason! Replant any that get yanked out.

After four to six weeks (for spring planted sets) or by March (for autumn planted sets) they should look something like this:

I often mulch between the onions with clean grass-clippings, compost or bark. This helps conserve moisture and can be used to prop up shoots that have fallen over.

If your site is prone to strong wind you may want to protect the onions with shelter from the sides, as the stems are rather brittle. I have a rectangular frame that is 6″ high I can place around a bed for this purpose.

These onions are growing well. They have another month or two to reach maturity, but some can be pulled early and used as spring onions or if large enough as small onions.

It is a good idea to pick some early as it extends the season forwards, and can be more successful than trying to store onions for longer. With this in mind, it can be a good idea to plant a small patch of onions closer together — say at 3″ (7cm) spacing — specifically for an early harvest.

Once bulbs are a couple of inches across you can start picking them for early use. The skin may not yet have fully formed, and the onions will have a green look in the kitchen, but they are delicious. These early green onions are great — really fresh and juicy.

Bolting

Tall flower spikes rise above the foliage from onions that have bolted.

Some of your onions will produce flower spikes. Onions that do this are said to have bolted. If you leave them, the onions will develop a hollow centre and will be useless for eating or storing. The best strategy is to immediately harvest and use them. The earlier you catch a bolting onion the more of it will be usable. Even so, when you cut up the onion in the kitchen you will have to discard the centre of the onion (where the flower stem has started growing) which will have become fibrous, hollow and ‘woody’. The surrounding flesh is still fine to eat.

Alternatively, you can leave bolted onions and let them flower. The large white allium flower heads are ornamental and attract pollinators.

Can you stop onions bolting?

It is said that onions bolt more if they are stressed by irregular watering (especially early summer dry patches) or sudden changes in temperature. In my experience bolting is inevitable and is just something you have to live with. When flower spikes start rising all over your onion bed it looks like they are all going over, but it is usually only 10% to 15%.

Another suggestion is that larger sets are more likely to bolt. Some say sets should be no thicker than a pencil. Most commercial onion sets contain many that are larger than this and might explain why so many bolt however careful I am with watering.

Solution? Plant enough sets that you can live with the inevitable loses.

Harvest

You can tell when your onions are getting ready to harvest as the foliage spontaneously bends over.

Onion stalks bend over at the neck when they have finished growing

At this point you will see that the onions have a clear skin, but not as complete as typical shop-bought onions. They are still ‘green’, and if you want to store them they will need ripening off. You will see a range of sizes, mostly 2 to 3″ across, along with some 1″ runts, and the occasional giant 4″ specimen.

In my experience, the largest onions are the hardest to dry-off and the first to rot in store, so harvesting those now may be prudent. I have found that the 1½” to 2½” (4cm to 7cm) are the best size for storing

As already explained, green onions are excellent in the kitchen so you can take some right away. Harvest as many as you want at this stage for use in the next few weeks. They will keep well in the fridge for up to a month. Top and tail them with a clean knife, scissors or secateurs, leaving a section of stalk.

Large green onion, just harvested. Will keep in the fridge for up to a month.

With the remaining crop you have three choices: freezing, drying, or leaving in the ground. Each of these has advantages and you can even do some of each to try and maximise your onion season.

Storage Methods

1. Freezing

  • All of the pain of stinging eyes is out of the way in one go
  • Provides onions all year round
  • None of your harvest will go to waste (rot)
  • Can only be used in cooking, not suitable for salads

Chopped onions freeze well. There is no need to blanch them, just cut them into ½” pieces, and pop them in sealed freezer bags to prevent their odour getting into other foods.

Some people in my family never buy fresh onions and only use frozen chopped onions in all of their cooking. Their reason is mainly that they cannot tolerate the effect on their eyes of cutting up fresh onions, but they also find frozen onions speed up cooking and they can take just the amount they need.

2. Drying

  • The traditional method for storing onions
  • Provides onions over winter in the traditional format
  • You should allow for losses (rot) which in some years will be extensive especially towards the end of the storage season (February)
  • Takes up space and requires regular checking to avoid rot.

To dry onions: a week or two after the tops have turned over, use a garden fork to lightly life the onions so they are sitting on top of the soil. This will trigger the roots and stem to dry up, and become papery. Do not water them in this time. If it is hot and dry the process will be quicker. If rain is due, move them under cover.

Avoid bruising onions as they will tend to rot.

Place the onions on a rack or garden bench — preferably under a porch or in a greenhouse to keep them dry — allowing air to circulate freely. Over a couple of weeks, the stems will shrivel and a papery skin will form over the bulbs. The roots will shrivel too.

After drying, these red onions were topped and tailed. Some were brought in to use in the following two weeks. The remainder were stored in a dry, cool space.

Once dry you can cut off the excess roots and reduce the stems to 3″ long, ready for storing. Discard any that are showing signs of going soft or mouldy: they will not keep and will quickly infect the others. Any that are bruised or not fully dry can be used immediately.

To dry onions: a week or two after the tops have turned over, use a garden fork to lightly life the onions so they are sitting on top of the soil. This will trigger the roots and stem to dry up, and become papery. Do not water them in this time. If it is hot and dry the process will be quicker. If rain is due, move them under cover.

Here is my main crop dried then stored in a special rack in an outbuilding. A shelf in a garage, or any dry, frost-free space out of direct sunlight will do.

You will need to check the onions every week in case any start to go soft. Remove any that do otherwise the infection will spread and you will quickly end up with a stinking mushy mess!

3. Leaving onions in the ground

  • In mild areas, onions will remain in perfect condition if left in the ground, even after the top growth shrivels.
  • You can keep picking such onions right up until Christmas: perfectly fresh.
  • This method ties up space in your garden.
  • Heavy frosts or snow can ruin the crop.

I discovered this by accident. There was a small patch of onions that I never got round to harvesting that just sat in the ground over winter. Several times I went and pulled some of them, and they were always in perfect condition. They even survived some mild frosts and a short period of snow cover.

In the spring they started sprouting, but were still good. I have not tried this with a large quantity, but it is certainly worth experimenting with.

I have also tried planting some sets late (in July) to see if I could get a winter crop. Earlier in the year I held back half a pack of spring onion sets, placing them in the fridge to stop them spouting so they were still viable in July. It was a partial success, but they didn’t get very large. I think it would have been more successful if I’d planted them in early to mid June.

Overview of the onion year

As you can see from the calendar below, autumn planted sets finish just as spring-planted sets begin. Autumn planted sets do not store well, so best use them fresh or freeze them. Spring planted sets can be dried and stored from the end of August, left in the ground or frozen.

One way or another, with care, you can eat your own onions all year round!


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