How to Grow Onion
'Ailsa Craig' from Seed
The Scottish heritage exhibition onion introduced in 1887 at Culzean Castle -- globe-shaped bulbs with golden straw skin and mild sweet flavour, 500g-1kg for the kitchen and up to 2.3kg for the show bench; a long-day variety perfectly adapted to UK latitudes; sow January-February under glass for exhibition size or March-April for an excellent kitchen crop; plant pencil-thick seedlings into rich, deeply-prepared ground; water consistently during bulb development; harvest when tops fall; cure and use within 1-2 months (not a storage onion)
Onion 'Ailsa Craig' has a story behind it that most vegetable seeds do not: it was introduced in 1887 by David Murray, head gardener at Culzean Castle in Maybole, South Ayrshire, Scotland, for his employer the Marquis of Ailsa. The onion takes its name from Ailsa Craig -- the distinctive 338-metre volcanic plug that rises from the Firth of Clyde, some 16km off the Ayrshire coast, which is itself named after a Celtic saint. An onion named after a dramatic offshore Scottish rock, introduced by a Victorian head gardener for a Scottish marquis, that went on to become the most famous exhibition onion in the UK and one of the most widely grown show vegetables of any kind in British horticultural history: the provenance is as distinctive as the bulbs themselves.
The practical qualities that have maintained Ailsa Craig's status for over 130 years are equally distinctive. The globe-shaped bulbs are large -- substantially larger than most kitchen onions from sets or other seed varieties, with average kitchen weights of 500g-1kg and show-bench specimens regularly exceeding 2kg -- with a characteristic golden straw skin and a mild, sweet flavour that is markedly different from the more pungent onions suited to long storage. Ailsa Craig is not a storage onion (maximum 1-2 months, after which quality declines) but an eating-fresh onion of exceptional quality: one that rewards being sliced raw into salads, grilled halved alongside summer barbecue food, or used whole as the centrepiece of a French onion soup where its mild sweetness and large size are exactly what the dish requires.
Quick Facts at a Glance
Plant Type
Hardy Annual vegetable -- Scottish heritage 1887; globe-shaped exhibition onion
Size
500g-1kg kitchen bulbs; show specimens to 2.3kg (5lbs!); globe-shaped; golden straw skin
Flavour
Mild and sweet -- NOT a storage onion; best eaten fresh within 1-2 months of harvest
Exhibition
January-February sowing for show bench; March-April for general kitchen crop
Long-day
Needs 13+ hours daylight to form bulbs -- perfectly adapted to UK latitude
Difficulty
2 out of 5 -- patient sowing; the most rewarding onion from seed in the UK
Understanding Ailsa Craig
The Long-Day Requirement -- UK Latitude Advantage
Ailsa Craig is a long-day onion variety -- it requires a minimum of 13 hours of daylight per day to trigger the bulb-formation process. This requirement is a direct adaptation to the latitude band of 38-60° North, which includes the whole of the UK (50-58°N). At UK latitudes, day length exceeds 13 hours from approximately late April through to late August, providing the extended photoperiod that triggers bulb development. This makes Ailsa Craig perfectly calibrated for UK growing conditions: the seed is sown in the cool of late winter, grows as a seedling through spring, and begins active bulb formation precisely when UK summer day length crosses the 13-hour threshold. Short-day varieties, bred for Mediterranean or subtropical latitudes with shorter growing seasons, do not perform well in the UK for this reason.
Exhibition vs. Kitchen Sowing -- The January Difference
Two distinct sowing timings produce two different results from Ailsa Craig. A January-February indoor sowing at 15-20°C produces the longest possible growing season -- typically 30-35 weeks from sowing to harvest -- which allows the bulbs to develop to their maximum potential size. Show bench specimens of 2kg or more are achievable only from this early January sowing followed by careful indoor growing-on, hardening off, and planting into deeply prepared, richly manured ground in March-April. A March-April sowing produces excellent kitchen onions of 500g-1kg that mature in August-September -- very good results for general use, but without the show-stopping size potential of the January-sown crop.
Not a Storage Onion -- Plan the Harvest
Ailsa Craig's mild, sweet flavour comes with a significant practical caveat: it is not a long-storage onion. The mild flavour that makes it so pleasant to eat fresh is associated with a lower sulphur content than pungent storage varieties -- and sulphur is the compound that preserves onions in storage. Ailsa Craig bulbs cured and stored in cool, dry conditions last approximately 1-2 months before quality begins to decline; pungent storage varieties last 6-9 months. Plan accordingly: grow Ailsa Craig for summer and early autumn fresh eating and cooking, and use other storage varieties (Sturon, Red Baron, Stuttgart Giant) if long-term winter storage is the priority.
Sowing & Growing On
Sow January-February for Exhibition or March-April for Kitchen Crop -- 15-20°C -- 18-24 Days
Sow in a seed tray or modules indoors at 15-20°C from January (for show-bench results) or March-April (for excellent kitchen onions). Cover 6mm deep. Germination 18-24 days. Grow on in maximum light. Plant pencil-thick seedlings outdoors from March (exhibition) or April-May (general crop) at 10cm spacing (standard) or 15-20cm (large/exhibition bulbs).
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Sow indoors January-February (exhibition) or March-April (kitchen crop) at 15-20°C. Sow thinly in seed trays or 1 seed per module in a heated propagator, windowsill, or heated greenhouse. Cover 6mm deep. Germination 18-24 days. The seedlings emerge with a characteristic J-shaped loop (the seed still attached to the tip of the leaf) before straightening as the seedling grows.
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Prick out into individual modules or small pots when seedlings are 5-7cm tall. Grow on in maximum available light at 10-15°C -- a bright, cool greenhouse or cold frame is ideal. Reduce watering slightly as the weather warms. Clip the tops to 5cm if the foliage becomes very tall and floppy (this encourages stronger root development without harming the plant).
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Plant out when seedlings are pencil-thick, from March (exhibition) or April-May (kitchen crop). Prepare the bed well: deep cultivation, rich soil with compost or well-rotted manure incorporated the previous autumn, free-draining, full sun. Space 10cm apart for standard kitchen bulbs; 15-20cm for maximum exhibition size. Set the seedling with the white base just below the soil surface.
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Harvest when the tops fall over naturally and begin to dry. When approximately half the tops have fallen over (usually August-September), stop watering and allow the remaining tops to fall. Pull bulbs on a dry day. Cure outdoors in warm sunshine for 1-2 weeks (or under cover in a warm greenhouse if the weather is wet). Store in net bags or on slatted shelves in a cool, airy space. Use within 1-2 months of harvest.
Growing On & Care
For the Show Bench
Ailsa Craig has won more first prizes at village and county shows than arguably any other single vegetable variety in British horticultural history. For show preparation: sow in January under glass; grow on with the best available light; plant out in March into deeply dug, richly manured ground; water regularly during bulb development (once per week at minimum, more in dry weather); feed with a high-potash liquid fertiliser from June; allow the tops to ripen naturally; and cure carefully before show day. Presentation matters: the three onions in a show class should be as uniform in size and shape as possible, with the skins clean and tightly dried to a consistent golden straw colour.
Culinary Qualities -- When to Use It
The mild, sweet flavour of Ailsa Craig rewards specific culinary applications where onion pungency would be unwelcome. Raw in salads: sliced thinly, the mild flavour provides the onion note without the eye-watering quality of storage varieties. French onion soup: the large size and sweet flesh caramelise beautifully with long, slow cooking, producing the deep, rich sweetness that the dish requires. Grilled or roasted whole: the large halved or quartered onion benefits from direct heat that caramelises the natural sugars. Pickled: the mild flesh produces a delicate, subtly sweet pickled onion in white wine vinegar -- use within 2 months.
Companion Planting -- Carrot and Onion Fly Deterrence
The Allium family produces volatile sulphur compounds that act as a deterrent to several significant vegetable pests. Ailsa Craig grown adjacent to carrot rows repels carrot fly (Psila rosae) -- the carrot fly locates carrots by scent, and the overlapping Allium scent disrupts this location system. Conversely, planting mint near the onion bed deters onion fly (Delia antiqua) -- the mint scent masks the onion scent that the fly uses to locate its egg-laying sites. The traditional three-way companion -- onions, carrots, and mint -- provides mutual pest deterrence for all three crops simultaneously.
From Seed vs. From Sets
Ailsa Craig is one of the few onion varieties widely grown from seed rather than sets (small pre-grown bulbs). Onions from seed consistently produce better quality, longer-lasting, more disease-resistant results than onions from sets, though they require significantly more growing time and skill. The quality advantage is most significant with Ailsa Craig because the potential size of the bulb is so large -- sets-grown Ailsa Craig rarely achieves the full size potential that seed-grown plants reach with a full season's growth. The additional patience required for seed growing (sow in January, harvest in August -- a 7-month commitment) is rewarded with bulbs that sets cannot match.
Watering During Bulb Development
Consistent moisture during the bulb development phase (June-August, once the day length has triggered bulbing) is critical for maximum Ailsa Craig size and quality. Inconsistent watering during this period -- alternating between dry and wet soil -- causes the bulbs to develop irregular rings internally and can split the outer skins, reducing both quality and storage life. Water deeply (thoroughly soaking to 20-30cm depth) once a week in dry conditions rather than shallowly more frequently. As the tops begin to fall over (harvest signal), reduce and then cease watering to allow the bulb skins to dry and firm up properly.
Curing and Storage
Proper curing is essential for maximising Ailsa Craig's short storage window. Pull bulbs on a dry day when the tops have fully fallen and are beginning to dry. Lay in a single layer outdoors in full sun for 1-2 weeks if the weather permits, turning every few days. If wet weather prevents outdoor curing, use a warm greenhouse, conservatory, or any warm airy covered space. The bulb skins should become papery, dry, and pulled tight before storage. Store in net bags or on wooden slatted shelves in a cool (10-15°C), dry, well-ventilated space. Check weekly and remove any showing signs of soft spots or mould. Use within 6-8 weeks of harvest for best quality.
Sowing & Harvest Calendar
| Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | |
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| Exhibition sow (Jan-Feb indoor) |
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| Kitchen crop sow (Mar-Apr indoor) |
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| Plant out exhibition (Mar) |
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| Plant out kitchen (Apr-May) |
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| Bulb development (Jun-Aug) |
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| Harvest and cure (Aug-Sep) |
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Common Problems & Solutions
| Problem | Likely Cause | What to Do |
|---|---|---|
| Small bulbs; disappointing size | March-April sowing only; insufficient season | For large bulbs: sow in January-February for the longest possible growing season. March-April sowing produces good kitchen onions but not exhibition-size bulbs. January-sown plants have a 7-month growing season; March-sown plants have approximately 5 months. |
| Bulbs splitting or irregular shape | Inconsistent watering during bulb development | Water deeply once a week rather than shallowly more frequently. Consistent moisture through June-August is critical for even bulb development. Irregular watering causes internal ring irregularities and split skins. |
| Poor storage; rotting quickly | Not properly cured before storage; harvested too wet | Cure thoroughly before storage: 1-2 weeks in warm, airy conditions until the skins are papery-dry. Never store incompletely cured bulbs. Check stored onions weekly and remove any showing softness or mould. Ailsa Craig's maximum storage is 1-2 months even when perfectly cured. |
| Tops not falling over; won't ripen | Day length insufficient; waterlogged; planted south of UK | Ailsa Craig requires 13+ hours of daylight and UK latitudes for proper bulbing. In very wet summers or poorly-drained soil, tops may stay erect longer than expected. Bend tops over gently if they haven't fallen by early September to encourage ripening. |
Vegetable Specifications
The 1887 Scottish show bench legend -- from mild sweet kitchen bulbs to 2kg exhibition giants, grown from seed at UK latitudes
Sow January-February under glass at 15-20°C for exhibition-size bulbs, or March-April for an excellent kitchen crop. Germination 18-24 days. Plant pencil-thick seedlings into rich, deeply-prepared ground at 10-15cm. Water consistently June-August as bulbs develop. Harvest when tops fall over, cure for 2 weeks, and use within 1-2 months -- Ailsa Craig is a fresh-eating onion of the finest quality, not a storage variety.
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