How Often to Water Onions
Compared with most vegetables, onions need surprisingly little water. Their roots are shallow, but in a typical British spring and summer, rain does most of the work, and too much water can do more harm than good. The skill lies in spotting the few moments when a drink genuinely helps, and knowing the point after which watering does more harm than good.
How often should you water onions?
Help young onions through any dry spells while their roots are getting going. Once they're growing well, leave them to the rain unless it stays dry for a couple of weeks or more, and then give the bed one long soak every seven days or so. When the bulbs are fat, usually by the middle of summer, stop watering completely. Onions in containers dry out faster and need checking every day or two in warm weather.
Watering newly planted onions
Onion sets and young plants have very small root systems when they first go in. If spring is dry, keep the soil lightly damp until you can see strong new leaves. Seed-grown onions are even more delicate as seedlings and shouldn't be allowed to dry out.
For planting times, see when to plant onions in the UK
Watering onions in garden beds
From May into June, as the foliage lengthens and bulbs start to fatten, onions do use more water. In most years, rainfall provides enough. Only water if there's been a long dry spell, and when you do, give a deep soak rather than a light sprinkle, so the water reaches down to the roots.
Pour at ground level rather than over the tops. Onion foliage that stays wet is more prone to fungal diseases.
When to stop watering onions
This is the step many gardeners miss. As soon as the bulbs have reached a good size, usually in July, stop watering, even in a dry spell. Extra water at this stage keeps the plants growing when they should be ripening, and spring-planted onions watered late in the season tend not to store as well.
Yellowing, toppling foliage is the sign that lifting time is close. Drier soil at this point helps the skins firm up, which is what lets onions keep for months.
For lifting and drying, see when to harvest onions
Overwintering onions
Onions planted from overwintering sets in autumn go through the winter without any watering. The weather supplies more than enough, and soggy ground is far more of a threat. Start watching them in spring, and water only if it turns dry while the bulbs are swelling in May and early June.
Watering onions in containers
A pot holds a limited reserve of moisture, and on a bright, breezy day it can run dry quickly. Check the compost every day or two in warm weather, and water whenever it's dry below the surface, until water drains from the base. The same cut-off applies in pots: no more water once the bulbs are fully formed.
For container advice, see growing onions in containers
Avoiding overwatering
Onions don't cope well with soggy soil. Waterlogged ground can make the roots struggle and the bulbs rot, and it encourages fungal problems. Plant in free-draining ground, skip watering if the soil is already moist, and don't stand pots in saucers of water.
You can read more in how often to water a vegetable garden
Signs of watering problems
Small bulbs: dry weather while the plants were establishing, or weeds competing for water.
Bulbs rotting in the ground: waterlogged soil.
Onions that rot in store: often watered after the bulbs had finished swelling, or lifted and stored before they were properly dry.
Plants flowering early: usually cold weather in spring rather than watering, though stress from drought can play a part.
For other causes, see why onions don't form bulbs
Healthy onion growth
Get young onions established, help them through long dry spells, keep the foliage dry and the rows weed-free, and switch off the water once the bulbs are full-sized, so they ripen and keep.
For growing advice, see how to grow onions in the UK
Growing spring onions? They're watered differently, so see how often to water spring onions
Common questions
How often should I water onions?
Mainly while they're young, and after that only when it's been dry for a couple of weeks, when one long soak every seven days or so is plenty. Stop once the bulbs are full-sized, usually by July.
When should I stop watering onions?
When the bulbs reach full size, typically in July. Carrying on after that holds up ripening and shortens how long they'll keep.
Do onions need a lot of water?
Not much. Once established, onions usually get enough water from rain, and too much water can cause rot. Water only in prolonged dry spells.
Should I water onion sets over winter?
No. Overwintering onions get all the water they need from winter rain, and waterlogged soil is the bigger risk. Water only if it turns dry in spring while the bulbs are swelling.
Watering advice based on RHS guidance.