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How to Grow Onions
Home grown onions are full of flavour and can be stored for months after harvesting. Read our step by step guide to growing onions below.
Most common Onion plant questions
When can I plant Onions?
Onions are best planted from mid-March to mid-April.
What kind of soil do onion plants need?
Onions will grow well in fertile, well-draining soil. If you are planting your onions in the ground it is recommended that you rake in a layer of garden compost into the surface layer as onions have very shallow roots. If your soil is very clay-heavy or sandy, then mix in quality compost at a rate of three parts compost to one part topsoil to use as your soil. If possible, cover the soil you would like to use with well-rotted manure the previous Autumn to give it some slow-releasing nutrients and to also improve its texture.
How much sun do onion plants need?
Onion plants need to be planted in full sun. Position them where they will get 6-8 hours of sunshine every day.
How to plant onions
If you are planting onions in the ground, plant them roughly 10cm apart with about 25cm in between rows. If you are growing onions in containers then select a container which is at least 20L, as you will need to plant them 7-10cm apart. Dig a small hole slightly larger than the roots of the plant, place the roots in the hole and then fill in with the soil and then gently firm in with your fingers. If you are planting onion sets (immature onions) then place them into the ground with the roots (flat end) facing down and the tip facing up and cover with soil so only the very end of the tip is poking out the ground.
How much water do onion plants need?
When you first plant your onion plants give them a good water, then keep the soil consistently moist. Usually until June the British rain will take care of that, but, if there are any dry spells, you should go out and feel the soil. If the soil is dry to the touch, then give the plant some water. As Onion plants have shallow roots it is important to always be vigilant and to ensure that the soil does not dry out. It is also important to see if the soil is draining away well. If the soil becomes soggy this will rot the onion bulbs.
How to care for onion plants
It is a good idea to cover young onion plants in a horticultural fleece to prevent attack from pests and birds. Make sure you keep the area around onions free from any weeds by hand picking them out of the ground.
How to harvest onions
Onions are ready to harvest from late summer to early Autumn, usually August-September. They will be ready when the leaves start to turn yellow and wilt. Wait until a day where the soil is dry and gently lift the onions out of the ground with a garden folk, being careful not to accidentally pierce any.
How to store Onions
Once you have harvested the onions you should leave them to dry for about two weeks. This can be done somewhere such as a shed or a greenhouse. Simply space the onions apart individually somewhere it is dry with a good airflow. Then you can hang onions up to store, in mesh bags, bundles or onion strings somewhere it is dry and cool. Onions stored properly should keep well for months so you can enjoy your garden crop up until the new year. Never keep onions in the fridge or wrapped in plastic as this will cause them to rot and go off. Like most root vegetables, they are best stored simply in a cupboard or even in the shed if you can ensure nothing will get to them!
Common Onion plant problems
There are four main pests and diseases that affect onions in the UK. They are Onion Fly, the Allium Leaf Miner, White Onion Rot and Allium Rust. A horticultural fleece and sound gardening hygiene are your best protection against these onion scourges - see below for information about each.
Onion Fly
The Onion Fly hatches eggs at the base of the plant and the resulting maggots eat the bulb and roots.
They will cause the leaves to wilt and become yellow.A horticultural fleece is your best protection.You can also plant Mint near your Onions to deter and confuse Onion Fly. You may want to plant your mint in a pot as it is a relentless grower which will spread and be very difficult to contain.
Allium Leaf Miner
The Allium Leaf Miner is a small fly about 3mm in length. Its maggots eat the plant from the inside out causing white blotches to form on the stems. A horticultural fleece is your best protection.
White Onion Rot
White Onion Rot is a fungal infection that causes the stems and leaves to go yellow and wilt whilst the bulbs underground rot, becoming covered in a white fungus. There is no remedy for White Onion Rot, you can only do your best to avoid it by maintaining good garden hygiene practices such as ensuring people from other gardens wash their shoes and tools to stop the spread of contamination. If you get White Onion Rot, burn all the onion plants in your garden, clean all of your tools and shoes thoroughly and do not plant onions in that ground again for another 15 years.
Allium Rust
Allium Rust is a fungal infection which causes tiny orange, rusty coloured bumps to appear on the leaves. Allium Rust is not as serious as White Onion Rot, but if the plant suffers from a serious infection it will decrease the overall yield of that plant. Rust thrives in crowded, humid conditions. To prevent Rust it is best to properly space apart your plants and to water plants at the base in the morning, giving the day enough time to evaporate the water on the leaves. Rust is also unable to live on dead plant matter, so if you suffer one year from a Rust infection, simply clear away the ground thoroughly of all live vegetation over winter and this should rid the garden of the fungus – this may require getting rid of any Leeks you are growing over winter because Onions and Leeks are all a part of the same Allium family.
