You’ve probably seen those photos of gardeners cradling an onion like a newborn baby. Nine times out of ten, that "baby" is an Ailsa Craig. Honestly, if you're trying to grow a bulb that looks like it belongs in a cartoon or a state fair exhibit, this is the one. But there is a massive difference between just sticking some ailsa craig onion plants in the dirt and actually hitting that legendary five-pound mark.
Most folks treat them like any other yellow onion. That’s the first mistake. These aren't your supermarket storage onions that sit in a pantry for six months. They are a different beast entirely.
The Scottish Rock That Started It All
Back in 1887, a guy named David Murray—who was the head gardener for the Marquis of Ailsa at Culzean Castle—introduced this variety to the world. He named it after Ailsa Craig, that big hunk of volcanic rock sticking out of the Firth of Clyde in Scotland. If you've ever seen the island, it’s basically a giant, rounded dome, which is exactly what a perfect Ailsa Craig onion looks like.
It’s a British heirloom, and while it’s been around for over 130 years, it’s still the "gold standard" for exhibition growers. Why? Because it’s genetically programmed to get huge without becoming woody or flavorless. You get this beautiful, straw-colored skin and a flesh that’s so mild you can practically eat it like an apple. Well, maybe with a bit of salt.
Why Your Latitude Actually Matters
I see this happen every year: someone in Florida or Southern Texas buys a bunch of ailsa craig onion plants because they want giant onions, and then they’re disappointed when they only get golf balls.
Here’s the deal. Onions are photoperiodic. That’s just a fancy way of saying they count the hours of daylight. Ailsa Craig is a long-day variety. It needs 14 to 16 hours of sunlight to trigger the bulbing process. If you live south of the 35th or 36th parallel (roughly the Oklahoma-Kansas border), the days just don’t get long enough in the summer to tell this plant to start making a bulb. It’ll just keep growing green leaves until it gets confused and gives up.
If you’re in the North, you’re in the sweet spot. The longer those summer days, the more energy the plant pours into that bulb.
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The "Leaf-to-Ring" Secret
Most people don't realize that an onion is basically just a set of swollen leaf bases. Every single green leaf you see poking out of the top represents one ring inside the onion.
Think about that for a second.
If you want a giant onion with 13 rings, you need 13 healthy green leaves. If a leaf dies early because of thrips or drought, you just lost a ring. This is why you’ll see serious show growers obsessed with keeping those greens pristine. They’ll even build little supports for the leaves so they don’t kink or break. You don't necessarily have to go that far, but you do need to keep them fed.
Getting Them in the Ground
You can start from seed, but honestly, buying ailsa craig onion plants as started seedlings gives you a massive head start. These are usually shipped in the spring as "bundles" of bare-root plants. They look like pathetic little leeks when they arrive, but don't let that fool you.
Soil Prep is 90% of the Battle
These onions are heavy feeders. Like, really heavy. They want "rich" soil, but what does that actually mean? It means you should have dug in a mountain of well-rotted manure or compost the previous fall.
They like a pH between 6.0 and 6.8. If your soil is too acidic, they’ll just sit there pouting.
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The Planting Depth Trap
This is where most beginners mess up. They bury the plant deep like a tomato. Don't do that. You want to tuck the roots into the soil but keep the base of the plant right at the surface. If you bury the neck, the bulb won't be able to expand properly. As the onion grows, you should actually see the bulb sitting on top of the soil, not under it.
Keeping the Giants Happy
Watering is the next hurdle. Onions have ridiculously shallow roots. If the top two inches of soil dry out, the plant stresses, and the bulb stops growing. You want consistent moisture—roughly an inch of water a week.
And weeds? Forget it. Ailsa Craigs are terrible competitors. A single dandelion can steal enough nitrogen to shrink your harvest by half. Mulching with clean straw is a lifesaver here. It keeps the moisture in and the weeds down. Just make sure the straw isn't piled up against the onion necks, or you're asking for rot.
Feeding Schedule
- At planting: Use a balanced organic fertilizer (like a 5-5-5).
- Every 3 weeks: High-nitrogen side-dressing. Nitrogen builds those all-important leaves.
- Mid-summer: Once you see the soil starting to crack or the "shoulders" of the onion popping up, stop the heavy nitrogen. Too much late-season nitrogen leads to thick necks that won't dry out, which means your onion will rot in a week.
The Storage Reality Check
We need to talk about the "S" word: Storage.
Ailsa Craig is a "Spanish-type" onion. Because it has a high water and sugar content (which makes it taste amazing), it doesn't keep forever. It’s not a Red Wethersfield or a Stuttgarter.
Usually, you’ve got about one to two months of storage life after they're cured. If you try to keep them until Christmas, they’ll likely be a mushy, sprouted mess. Eat them fresh. Use them for French Onion soup. Caramelize them and freeze the jam. Just don't expect them to hang in your cellar all winter.
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When to Pull the Plug
Harvesting is all about timing. Around August or September, the "necks" of the onions will start to soften. Eventually, the tops will just flop over. That’s the plant saying, "I'm done."
Once about half the tops have fallen, you can go ahead and knock the rest over. Leave them in the ground for another week to let the outer skins toughen up, then lift them.
Crucial tip: If it’s going to rain, get them out of there. Sitting in wet mud after the tops have died is a one-way ticket to neck rot. Cure them in a shady, breezy spot (like a porch or a garage) for 2–3 weeks until the skins are papery and the roots are brittle.
Actionable Steps for Your Biggest Harvest Yet
If you're ready to try your hand at growing these Scottish giants, here's how to ensure you don't end up with stunted bulbs:
- Check your latitude: Ensure you live in a long-day region (roughly North of 35°N). If you're in the South, look for "Short-Day" varieties like Texas Legend or 1015Y instead.
- Order early: Most nurseries ship ailsa craig onion plants in March or April. They sell out fast because the exhibition crowd snaps them up.
- Prepare a dedicated bed: Don't just squeeze them between other crops. They need full sun and zero competition. Mix in 2 inches of compost and a handful of bone meal before planting.
- Space for greatness: If you want 3-lb onions, you cannot space them 3 inches apart. Give them at least 6 to 8 inches of space between plants so the bulbs have room to swell without hitting their neighbors.
- Monitor for Thrips: Keep an eye out for tiny, yellowish insects in the leaf folds. If the leaves look "silvery" or streaked, treat them immediately with insecticidal soap, or you'll lose those precious "rings" of growth.
Focus on the leaves, keep the water consistent, and don't bury the bulbs. Do that, and you'll be the one posing for the "baby" photo this summer.