Growing Onions in Scotland: The Complete Guide

Allium cepa

Everything you need to grow onions in Scotland, from autumn-planted sets that sit quietly through the winter to a July crop of bulbs drying in the sun. We have done the research so you don't have to.

Onions are the crop most gardeners start with, and they are straightforward: give them a sunny bed, keep the weeds down, and wait. The one decision that changes the outcome is when you plant them, and in Scotland that decision matters more than almost anywhere else in Britain. A cold spring can trigger bolting, which ruins the bulb. The answer is autumn planting, and the reasoning is worth understanding because it changes what you do every year.

Sets or seed, and when to plant

Onions are grown from either sets or seed, and the choice settles the planting date and the harvest in one go.

• Sets are small, part-grown bulbs that go straight into the ground. They are the easier route and they need no indoor start, no pricking out and no thinning. Plant them in early to mid-spring for a crop from July to September. Push each set gently into the soil so the tip is just showing, about 2cm deep. Heat-treated sets are worth seeking out. They greatly reduce the bolting risk, which is exactly the thing a Scottish grower needs.
• Autumn-planted sets go in during autumn and sit through the winter. They are less sensitive to cold than spring-planted sets, which means they are far less likely to bolt when a late cold snap hits. They crop in June and July, two months ahead of a spring planting.
• Seed gives you more variety choice and it is cheaper, but it needs an early indoor sowing in mid-winter to give the plants enough time. Sow outdoors from early to mid-spring. Expect a harvest from July to October. Onion seed does not stay viable for long. Germination declines after a year or two, so fresh seed each year is worth the small cost.

For most Scottish gardens the best approach is an autumn planting of sets for the early summer crop, followed by a spring planting of heat-treated sets to carry you through to autumn. Seed is the enthusiast's route and it works, but sets get you there with less work and less waiting.

Varieties

The overwintering autumn-planted types are the ones worth knowing about here, because planting date changes the outcome more than variety name. These carry the RHS Award of Garden Merit, a UK-wide award, not a Scottish one.

Also worth considering

The real variety decision is between spring-planted and autumn-planted types. Autumn-planted sets are bred for cold tolerance. The cold sensitivity that causes spring-planted sets to bolt is what autumn types were selected to handle. If you only grow one onion crop, make it an overwintering one. If you want onions that will last through winter, Centurion's long-term storage and Hercules's bolting resistance are the two traits worth choosing on.

Soil and site

Onions want an open, sunny site and free-draining soil. They have a limited root system, so improving the soil with plenty of organic matter before planting makes a real difference. Dig in a bucketful of garden compost or well-rotted manure per square metre. Do not add fresh manure.

The allium family prefers soil around pH 6.5. Below 5.8 growth is increasingly restricted, which matters in Scotland because our soils are naturally acidic and plenty of gardens sit below that mark. A cheap soil test before planting tells you in minutes whether you need to lime.

They cannot stand competition from weeds, and their thin, upright leaves cast almost no shade, so every weed that germinates will get the light it needs. Prepare the bed thoroughly and rake it to a fine, level surface. A clean start makes the rest of the season simple.

Planting

Spring sets: push each set gently into the soil so the tip is just showing, about 2cm deep, spaced 10cm apart with rows about 25cm apart. Wider spacing gives you larger bulbs, so go to 15cm if you want big ones. Birds sometimes pull newly planted sets out of the ground thinking the tips are worms. Protect with fleece or netting until the sets have rooted in. If your soil is cold and wet in early spring, start the sets in modules indoors and transplant them out when conditions improve.

Autumn sets: plant in autumn, timing it so they have a few weeks to root before the cold arrives but are not putting on much top growth. The same spacing applies. They will sit through the winter and start growing as soon as the soil warms in spring, giving you the earliest onions of the year. Avoid autumn planting on heavy ground that sits wet all winter. The sets will rot. On heavy soil, start them in modules instead and transplant in spring, which still gives them a head start.

Seed: sow about 1.3cm deep in rows 25 to 30cm apart, and thin to 5 to 10cm between plants once they are up. Indoor-sown plants need hardening off before they go out. Multi-sown modules of three or four seeds can be planted as a clump. The bulbs will be smaller, but you get more of them in the same space.

Containers: onions grow well in a large pot. Use a container at least 45cm wide and deep, filled with soil-based compost, placed in a sunny spot. The spacing rules above still apply.

Watering and feeding

Onions in the ground rarely need watering unless there is a prolonged dry spell, which in most Scottish summers is unusual. The early stages, when roots are forming, are when watering makes the most difference. Stop watering once the bulbs have swollen in mid-summer. Watering spring-planted crops after that point can mean they store less well, and wet foliage sitting overnight encourages fungal diseases. Water the soil, not the leaves.

If the soil was prepared with compost or well-rotted manure beforehand, feeding is rarely needed. On poor ground, autumn-planted onions benefit from a nitrogen-rich organic fertiliser in late winter. It boosts growth and can suppress premature flowering.

Weeding is the job that matters. Onions cannot shade out competition, and every weed that establishes itself next to a bulb is taking light and water that the bulb needs. Hoe between the rows regularly, and hand-weed close to the plants. A clean onion bed in July is the sign of a crop that will store well.

White rot, the one that stays

White rot is the serious disease of the allium family, and once it is in your soil it is there for years. The first sign is yellowing, wilting leaves. Lift the plant and you will find a white fluffy fungus at the base with tiny black dots embedded in it. There is no treatment.

The defence is rotation, and it needs to be a long one: keep onions, garlic, leeks and shallots off the same ground for four to five years. That is longer than the usual four-year convention, and the reason is that white rot spores survive in soil for a very long time. Leeks are less susceptible than onions, but the rotation rule holds across the whole family.

If you have never seen white rot in your garden, the best thing you can do is keep it that way. Do not accept onion plants or soil from other gardens, and do not move soil or tools from an affected bed to a clean one.

Harvesting and storing

Autumn-planted sets are ready in June or July. Spring-planted sets and seed-raised onions are ready from July through October. The signal is the leaves: once they turn yellow and flop over naturally, the bulb has finished growing. An old tip says to bend the foliage over to hasten ripening. This is no longer recommended. Lift the bulbs carefully with a fork, taking care not to bruise them, because damage at harvest is damage that will rot in storage.

Dry the bulbs for about two weeks before storing. Lay them out in full sun on wire racks so air circulates beneath them. In a wet summer this is easier said than done, so here is the backup: spread them on racks in a greenhouse or well-ventilated shed. The skins need to be papery and rustling before the onions go into storage. A bulb with a thick, soft neck will rot and take its neighbours with it, so use those ones first.

Store somewhere light, cool and dry. Storing onions in the dark encourages them to sprout, so a shed with a window or a cool pantry with natural light is better than a dark cupboard. Hang them in strings or lay them in single layers in trays. Check through them every few weeks and remove any that are softening. Autumn-planted onions will store until early winter. Spring-planted and seed-raised onions can last well into the following spring.

Common problems

• Bolting. The plant runs to flower and the bulb stops swelling. Triggered by a cold spell after spring planting. Autumn-planted sets and heat-treated spring sets largely avoid it. Snap the flower stem off as soon as you see it and use that onion first. It will not store.
• Onion white rot. White fluffy fungus at the base, no treatment, rotation is the only defence.
• Rust. Bright orange raised spots on the leaves. The fungus spreads most readily at 10 to 18°C, with August to October the worst months. Frost knocks it back, and it is not soil-borne. It survives only on living green tissue. More common on leeks, but onions in the same family can pick it up. Good spacing and airflow reduce the risk.
• Allium leaf miner. The fly larvae bore into the stems and bulbs. Widespread in England and parts of Wales. The extent of its establishment in Scotland is not well documented.
• Birds pulling out sets. Newly planted sets with their tips showing look like worms. Protect with fleece or netting until the sets have rooted in, and check the row daily for a few days after planting.

Common questions

Sources