Welcome to the top of the crops! Today you will learn how to grow spring onions in a polytunnel.
Spring onions are a crop that you can sow and grow in your polytunnel practically all year round. They will not take up much space at all, and can not only provide you with a useful edible yield but also make excellent companion plants for a number of other crops.
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Spring onions, also known as salad onions, green onions or scallions, are plants that belong to several species in the Allium genus, to which other onions, garlic and leeks also belong.
They are most commonly Allium cepa var. cepa, and so are of the same species as the common bulbing onion but are grown for their leafy growth rather than their bulb. But there are also other onions, such as bunching onions, which belong to different allium species.
They are usually harvested either before a bulb forms, or when only slight bulbing has taken place, or snipped off at the base, leaving the roots in place to continue to grow.
Spring onions can be successfully grown in a range of different settings. They can be grown in the ground, or in raised beds. They also, of course, make good container plants due to the limited amount of space that they require.
But to make sure that spring onions can thrive, you need to make sure that they have a site in full sun, ideally with light and reasonably free-draining soil or growing medium.
To grow spring onions you will need:
Spring onion seeds
Seed trays/ small pots/ growing medium etc. when sowing indoors.
A suitable growing location/ container.
Facility to water your plants.
Organic matter for mulch.
Spring onion seeds are widely available and these can be sown any time between around March and October.
Seeds are sown indoors both in the early spring and towards the tail end of the season in order to extend the length of the growing season. With a polytunnel you may even be able to sow a little earlier and a little later than usual.
When sowing spring onions, it is a good idea to successional sow – sowing small batches of seeds every couple of weeks rather than sowing a great many all at once. This will extend the duration of the harvesting period and make sure that you pretty much always have spring onions when you need some.
Begin sowing seeds inside in early March and then sow directly all through the warmest months. Seeds sown in August or September will overwinter and provide you with a useful harvest in the spring. For seeds sown for overwintering, be sure to choose a seed variety that is suitable for late planting.
If you are sowing directly into the ground or a raised bed, rake the soil to a fine tilth, removing any lumps or stones. Sow the seeds thinly into shallow drills around 1cm deep. If you want multiple rows then you should space these around 10cm apart.
The main elements of care are discussed below. But when tending spring onions, one other thing that you may need to do if you sowed the seeds directly where they are to grow is to thin the seedlings.
The seedlings should be thinned if necessary to an eventual spacing of around 2.5cm since closer spacing can make problems like downy mildew more likely to occur. Remember however that you can eat the small spring onions that you thin.
Lift your spring onions for use as needed, while they are still young and the bulb (if any) is still small. You can use the spring onions in salads or finely chopped as a garnish on a range of dishes. But there are also plenty of other spring onion recipes that you might consider.
To increase your yield and truly make the most of your spring onion crop, you can also choose to use all but the root and base of a spring onion. The rooted base section, left in the soil or growing medium, will regrow a new upper green portion, so you get further harvests from the same plant.
You can also place the part with roots and a short section of the base of any spring onion, whether one you have grown yourself or one you have purchased, with the roots in some water, and the upper portion should regrow.
Spring onions are one of a number of different plants that you can use to grow more food from vegetable scraps.
Spring onions will keep for longer when they have water. If the root section is intact, you can store spring onions in a glass or small vase of water on a sunny windowsill. This way the spring onions will stay fresh and the plants will also still continue to grow.
You can also store spring onions wrapped in a damp cloth in your fridge for a week or two, or chop and freeze spring onions for up to around 6 months.
If you are growing spring onions in pots then you only need to move them to a larger container when the roots emerge through the holes at the base of the pot. If growing continuously in the same container, you should replace the growing medium every year to ensure fertility and to avoid any potential compaction issues.
As mentioned above, tending for spring onions involves a few main jobs. But this is not a crop that will take up a great deal of your time.
Water spring onions when the soil or the growing medium is dry. But take care not to overwater and ensure that excess water can always drain away freely. Don't water the foliage of the plants – aim for the soil at their base. Remember that you will need to water more frequently if you are growing spring onions in pots or other containers.
One of the biggest dangers with spring onions is accidentally weeding them out before they get a chance to grow. It is easy to mistake a spring onion for a blade of grass, which is why it is best to plant them in rows rather than scattering randomly.
It is important to weed carefully and regularly around your spring onions as, like other alliums, they do not like too much competition. Weed little and often and it will not seem like too much of a chore.
A good quality organic mulch will work well to retain soil moisture and keep down weeds. Upon planting or sowing, it is a good idea to lay an organic mulch of compost or well-rotted manure around your plants.
With some protection, or in a polytunnel, spring onions can often remain in a garden throughout the coldest part of the year. They are fairly hardy but do need protection in colder regions and when the weather is more severe. Cloches or some fleece are usually sufficient if you are not growing undercover.
Recommended varieties of spring onions or salad onions to grow in the UK include:
‘Apache’
‘Feast’
‘Guardsman’
‘Ishikura’
‘Katana’
‘Matrix’
‘Parade’
‘Photon’
‘Savel’
‘Summer Isle’
‘White Lisbon’
‘Winter White Bunching’
Spring onions are usually relatively trouble-free plants to grow, and, indeed, can help as companion plants with organic pest control in your garden. However, there are a few pests that can plague these onions, along with other alliums, and a handful of fungal diseases that are quite common when growing onions in your garden.
One problem that can arise is onion white rot. This is a fungal problem more likely to occur in damp growing conditions. It causes the foliage to yellow and wilt and the roots and bulb (if there is one) to decay. This problem can unfortunately be serious, and will remain in the soil for many years.
Also caused by a fungus-like organism, onion downy mildew can also occur on spring onion plants. It causes affected leaves to turn yellow and die off from the tip downwards, and sometimes a whitish and then purplish mould can form on affected plant parts. Avoid overcrowding and damp conditions, and ensure good ventilation and airflow to reduce the chances of this problem taking hold.
One of the most common pests that can trouble spring onions are onion flies, which can be an issue for all members of this family. Companion planting with carrots is one well-known method to help reduce problems with onion fly. But where these are present in larger numbers, physical barriers over your crop are really the only solution.
Spring onions can be grown in a polytunnel all year round. There are types and varieties suitable for cultivation throughout the whole of the year.
Spring onions can be very useful as companion plants, and work well around the edges of a bed where they can protect against pests by confusing or repelling them but won't take up too much space. They can also, of course, be intercropped with carrots or other crops and can be popped into any gaps that open up in a garden for a relatively quick crop.
However, keep spring onions and other onions away from legumes (peas and beans) as they may stunt their growth.
Aim to rotate alliums with other important families in crop rotation and try not to grow them in the same growing area year after year or this can potentially lead to disease and pest problems.
Make sure you provide enough space for your crop, and ventilate well to reduce the chances of a fungal infection taking hold.
Gould, M., (2023) Spring Onion Recipes That Unlock The Alliums Vibrant Flavours. Delish. [online] Available at: https://www.delish.com/uk/cooking/recipes/g33008581/spring-onion-recipes/
