How to Grow Onions

How to Grow Onions

Onions are one of the most popular vegetables for growing in home gardens and are one of the first crops of spring. They can be stored over winter thus making a versatile crop. They are used in a huge range of culinary dishes, both raw and cooked and are quite easy to grow.


Site Preparation
If adding manure or composted organic matter then add a few weeks before sowing / planting out.
You can tread the soil gently to firm it up a bit prior to sowing.

Sowing Onion Seed & Planting Sets

Onions can be planted from seed or from sets (small partly grown onion bulbs). Sets are more expensive but they tend to be more reliable in their results and also require less work - no thinning and reduced onion fly risk.

If sowing from seed then sow in drills about 2cm deep with about 1 inch between seeds. If sowing in rows then space the rows about 30cm apart.

The soil should be moist before sowing so check the soil the day before sowing and water if the soil is dry.

If planting onion sets then they can be planted around Mid to Late March (earlier if a cloche / polytunnel is used). Again space rows about 30cm apart. Sow sets around 10cm apart as they shouldn't require any thinning. Dig a small hole for each set and place the set in neck upwards. When covered back up with soil the tip of the neck should just show through the soil surface.

Spring onions (scallion) can be sown from April and planting should be staggered every few weeks to ensure a continuous crop throughout the growing season.


Onions will grow in most climates and are frost resistant.

Soil type

Onions will grow in almost any soil from sandy loams to heavy clay. The soil should be firm. If your soil is heavy then you can introduce some organic compost or manure into the soil to help its moisture retaining properties.

Onions prefer a slightly acidic soil - PH 5.5-6.5 is a good PH for growing onions.

Planting Onion Sets

Onions will need a realtively fertile soil with a good tilth and good dranage. If you have a heavy wet soil we recommend using riased beds to avoid potential disease caused by damp conditions. Avoid using fresh manure. Onions will enjoy a warm, sunny site. Growing onions from sets is much easier than growing from seed and perfect for the beginner. Onion sets are just small, immature onions from the previous year. Plant the onions from early March till the end of April. Red onions are more likely to bolt (go to seed) and a later April planting can reduce the chance of this happening.

Once you have your bag of little golden onions you will need to pick the best ones to sow. If you do this correctly you will end up discarding almost half of the sets so bear that in mind when deciding what quantity to order.

Here are some tips to help you pick the best:

  1. 1 You don’t want any shoots, you might think you’re getting a head start but you’ll just get a very poor quality bulb.
  2. 2 Avoid any skinny looking ones.
  3. 3 Discard anything with mould or brown patches on the skin.
  4. 4 Any very big sets are more likely to run to seed.
  5. 5 Nice, tidy and round and oval bulbs will produce an excellent onion.


Push the sets about 1 cm into loose soil approx 10cm apart with 25cm between rows, leave about half of the onion showing above the soil. Birds seem to love pulling them out of the ground and scattering them round the garden. You can solve this by placing enviromesh or cloches over them for the first month which I strongly recommend. Firm the bed before planting either by standing on a timber plank of leaving the soil to settle a couple of weeks before planting. If the soil is very loose the roots don't get a firm hold, the plant thinks it's starving and can run to seed (Bolt). If you want to be clever and save weeding later on you can plant through a sheet of black plastic. Spread the plastic over your prepared bed, make a hole just large enough for each set and just pop them in.

Onion Crop Care

Your onions should be well rooted now so it will be safe to remove the cloche. It is important to hoe regularly around the plants as onions don't have foliage to suppress weeds. Take care when hoeing as onions have very shallow root systems and you may damage them with deep, vigorous strokes. Hoe regularily to control weeds and to stimulate the growth of the plant. One of the best gardening tools you'll ever buy is the oscillating stirrup hoe. It's a very effective old fashioned tool that really works properly. Hoeing not only removes the weeds but it also breaks up the surface of the soil and creates a fine texture or 'tilth'. A good tilth lets air and moisture in to the roots of your plants thus increasing their vigour.

Watering and Mulching Onions 

If the summer is proving hot and dry keep your onions watered. Onions have a very shallow root system and so will dry out a lot quicker than other plants. This can cause the onions to mature early leaving you with small bulbs. When watering you are better to give the area a thorough soaking to a depth of 6 inches rather than watering all the time. Also it's better to water early in the morning as midday irrigation can evaporate quickly. Evening watering will leave the plants cold and damp over night like going to bed in wet pyjamas! Mulching onions when the tops are 10 - 12 inches tall can also help as the mulch will retain moisture.

Onion Flower Stems 

Break off any flower stems which may appear, this is the onion trying to bolt. A hard stem will be produced inside the onion making it unsuitable for storing. Breaking the stem off will not prevent this from happening but will stop the onion using the energy in the bulb to produce a flower. The bulb around the hard stem will still be fine but should be used when pulled.

Preparing Onions For Harvest 

Spring planted onions are perfect for storing through the winter. You can tell when your onions are ready when about 75% of the stems have tuned yellow and fallen over. At this point some of the stems should have begun to yellow and should be ready for harvest in about 2 weeks time. If you have had a wet year the stalks can stay green for too long and prolong the ripening process. You can speed up the harvesting time by pulling up the bulbs a little bit to break some of the roots, about 10 days later lift your onions completely.

Pests and Diseases

It's very possible you won't suffer from any of the pests or diseases listed below but I've included them so you know what to look out for.

Onion Fly 

The tell tale signs are yellow, drooping leaves. This will be most common in the dry soils of midsummer. Young plants will be killed and older ones won't develop properly. If you lift the plants and look at the bulbs you will find white maggots inside.Lift and burn badly affected plants. If you have a problem in your garden growing from sets is better than growing from seed.


Eelworm are tiny worms which live in the soil and will effect the bulb and stem by distorting them. The onions will produce soft bulbs which cannot be stored. A strict crop rotation will help keep the population down. Chickweed, bindweed and mayweed are also hosts for eelworm so your work with the hoe will pay off! Avoid planting onions, peas, beans or strawberries for 4 years if badly affected.

White Rot 

White rot is the most serious onion disease as the spores will remain in the soil for up to seven years. Symptoms include yellow and wilted plants, pull them up and you will discover white mould on the base. Stick to a strict crop rotation to avoid this disease. 

Downy Mildew

Downy grey mould covers leaves which will die back and shrivel from the tips. Onions will be soft and unsuitable for storage. The disease thrives in wet conditions so if you have a damp garden raised beds will do a lot to avoid this problem. Hoeing around the plants to increase air circulation will also be a big help.

NB: Don't overfeed onions as this causes soft tissue which is more prone to the disease.


Onions are ready to harvest a week after their tops have started to fall over and are yellowed. Use a fork to lift the onions out of the ground. Take care not to damage the skins as this invites decay organisms in to attack the onion flesh.

Onions should be harvested on a sunny day, cleaned of any soil still attached to them and then placed on top of the soil where they will dry out with the help of the sun and wind.. Leave the onions out for a few days (until the tops dry out).

Remove the tops with a sharp knife about 2cm above the onion top so that decay organisms do not have direct access to the onion bulb. Never cut the stalks off your onions before drying is complete or they will rot. If you wish to store them loose wait till the stalks are dry and papery which will occur about 2 months after pulling.

Discard any onions that show signs of decay or damage as these can affect healthy onions if they are stored .

If you want to store the onions over winter then you can cure them by hanging them in a well aired place like a dry shed. Mesh bags or strings can be used to group and hang the onions and they should hang for about 3-4 weeks.

Spring onions can be harvested when they are about 30-40cm in height and will store for up to a week in the fridge. To prepare spring onions simply remove the outer set of leaves and wash.