White Castle

Onion

Zone 5a Save Garden

🌱 120d to harvest Bulbing

Sweetly mild and serenely aromatic, White Castle onions develop into crisp, snow-white bulbs with a clean, glassy skin that peels…

Crop Dates

Milestone	Date
Start Indoors	Direct Sow
Last Frost	May 8th
Transplant / Sow Outdoors	May 1st
Harvest Begins	Aug 29th
Harvest Ends	Sep 30th

Crop Details

Trait	Value
Days to Maturity	120
Sun Requirements	Full Sun
Growth Habit	Bulbing
Support Needed	None
Planting Depth	Normal
Germination Temp (°F)	65
Min Soil Temp (°F)	45
Min Night Temp (°F)	35
Harden Off (days)	7

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Frequently Asked Questions

What pest or disease commonly affects Allium cepa (white onions), and how can I control it?

Onions (Allium cepa) are commonly hit by purple blotch (Alternaria porri) and downy mildew, which show up as purple/gray lesions on leaves and can reduce bulb size. Remove and destroy infected foliage promptly, water at the base to keep leaves dry, and improve airflow by thinning so plants aren’t crowded. If problems persist, use an onion-safe fungicide labeled for purple blotch/downy mildew and start treatments at the first leaf symptoms.

How often should I water Allium cepa during the main growing phase, and what soil moisture target should I maintain?

During the main bulbing period, keep the soil evenly moist but not soggy—aim for about 1 inch (2.5 cm) of water per week, split into 1–2 deep waterings depending on rainfall. Water when the top 1 inch of soil feels dry, especially during dry spells, because inconsistent moisture can lead to stalled growth or split/poorly formed bulbs. Stop regular watering as leaves begin to yellow and fall over, so bulbs cure and store better.

How do I know when white onions (Allium cepa) are ready to harvest?

Harvest when about 50–75% of the tops have turned yellow and started to flop, and the bulb skin feels papery and firm when gently squeezed. Avoid harvesting too early, because underdeveloped onions won’t cure well for storage. Lift bulbs carefully (don’t pull hard enough to damage skins), then cure them in a dry, airy spot out of direct sun before storing.

Sweetly mild and serenely aromatic, White Castle onions develop into crisp, snow-white bulbs with a clean, glassy skin that peels away in satisfying layers. The flesh is tender and juicy, offering a gentle bite that shines in fresh applications and rewards slow roasting until mellow and golden. Grow White Castle for a dependable 120-day season and a harvest that stores with confidence for weeks of flavorful versatility.

Sowing Tips

Transplant Conditions

Wait until soil reaches 45°F and nights stay above 35°F before moving outdoors. Harden off for 7 days first.

Expert Note

For sweet caramelizing onions, keep soil evenly moist and avoid nitrogen-heavy feed once bulbs begin swelling.