Onion Ailsa Craig

Allium cepa 'Ailsa Craig' Heritage Scottish exhibition onion, large globe-shaped bulbs

The Scottish exhibition onion that has been winning prizes at British vegetable shows for over a century, and quietly providing the country's largest, finest culinary onions in the meantime. Ailsa Craig was developed by gardener David Murray in the 1880s at Culzean Castle in Ayrshire, named for the dramatic granite island off the Scottish coast, and registered formally in 1887. Almost 140 years later, it remains the standard against which large culinary onions are measured — producing huge straw-yellow globe-shaped bulbs typically weighing 500g and occasionally exceeding 1kg in the hands of an expert grower.

The flavour is genuinely fine for so large an onion — sweet, mild, never sharply pungent, with the kind of clean culinary character that makes Ailsa Craig outstanding raw in salads, in sandwiches, sliced thin on top of cheese, and in any preparation where the onion is meant to be tasted rather than just used as a background note. The texture is firm and crisp when fresh, softening to silken sweetness when slowly cooked. The skin cures to a beautiful pale straw-gold colour, and well-cured bulbs store well into winter — typically through to February or March from an October harvest.

Two practical decisions are worth making at sowing time. First, Ailsa Craig is exceptionally large, so plan for the size: at full development the bulbs are tennis-ball to small-grapefruit dimension, and crowding them dramatically reduces final size. Second, growing the biggest possible bulbs requires sowing in January or February under cover for a long growing season — later sowings produce smaller but still excellent onions.

Ailsa Craig is open-pollinated heritage. Seed saved from second-year flowering plants will grow true the following year.

A note on growing

For exhibition-size bulbs, sow indoors from January in a heated propagator at 15–18°C in seed compost at 0.5cm depth. Germination takes 10–14 days. Grow on in cooler conditions to prevent leggy seedlings. Alternatively, for culinary-size bulbs, sow from February to April in modules under cover, or direct outdoors from March to April once soil temperatures reach 7°C.

Plant out from April to early May, spacing 15–20cm apart in rows 30cm apart for very large bulbs (closer for smaller bulbs). Choose a sunny, open position in fertile, well-drained soil that has had well-rotted manure or compost dug in the previous autumn. Avoid freshly-manured ground for direct sown crops, which can cause splitting.

Water consistently through the growing season, particularly during bulb formation in June and July. Stop watering and feeding in late July to encourage the bulbs to ripen properly. Bend the foliage over gently in early August once tops start yellowing — this signals the plant to direct its remaining energy into bulb sizing rather than continued leaf growth.

Harvest from August to September once the tops have yellowed and fallen over naturally. Lift bulbs gently on a dry day and leave them on the soil surface (or on slatted shelves in a greenhouse) for 7–10 days to cure — this allows the skin to harden, the neck to seal, and the bulb to develop full storage quality. Store in net bags, trays, or strings in a cool, dry, well-ventilated place.

Where it shines

In the kitchen, Ailsa Craig is the onion you actually want to taste rather than hide. Slice raw onto open sandwiches with mature cheese and chutney. Use rings in salads. Char on the griddle as a side. Caramelise slowly with butter for onion marmalade, French onion soup, or as a base for slow-cooked stews. Bake whole, slicing the top, with butter and herbs, as a substantial side. The single-bulb-as-portion size makes Ailsa Craig genuinely useful for entertaining — one bulb stuffed and baked feeds a whole family.

In the garden, three or four rows of Ailsa Craig provides 50–80 substantial bulbs from a relatively modest bed area, with the largest specimens going on display at autumn village shows. Pair with Red Baron (red culinary onion) and White Lisbon (spring onion) for the complete domestic onion range.

Plant alongside

Onions are themselves valuable companion plants — their sulphur scent deters aphids, carrot fly, and many soil-dwelling pests. Plant alongside carrots, beetroot, lettuce, and brassicas. Calendula 'Neon' attracts beneficial predators. Avoid planting near beans and peas, which share competing nitrogen requirements.