Unzen Early Flat Onion is a short-day heirloom variety from Nagasaki, Japan, prized for its distinctive flat white bulbs and exceptional sweet, juicy flavor. Named after the volcano overlooking Nagasaki, this onion holds deep cultural significance in Japanese cuisine and has been grown for generations. Maturing in 100-110 days with small, uniformly flattened roots, it thrives in zones 3-10 and reaches harvest in under four months with proper care. Its crisp, crunchy texture and pure sweetness make it the ultimate raw onion for salads, slicing, and snacking straight from the garden.
Full Sun
Moderate
3-10
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Unzen Early Flat Onion delivers a crackling, juicy sweetness that feels almost more vegetable than pungent onion, making it luminous in salads and sandwiches where raw texture shines. The flat white bulbs are compact and uniform, producing small, orderly root systems that fit neatly into tight garden spaces or containers. As a short-day variety shaped by generations of Japanese growers, it carries authentic heirloom genetics while proving remarkably easy to cultivate across a broad hardiness range, from cold northern gardens to warm southern ones.
Unzen Early Flat Onion shines as a salad onion, its crisp, juicy flesh and delicate sweetness transforming simple greens into something memorable. Slice it thin and raw into composed salads, layer it onto sandwiches, or serve it as a crudités platter centerpiece where its mild flavor invites people to eat it almost like an apple. Its sweet character also makes it excellent for fresh salsas and quick-pickled preparations where you want onion presence without harsh bite. Some growers use it for grilling whole when small, the flat shape creating an interesting presentation as a cooked side.
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Start seeds indoors 8-10 weeks before your last spring frost in flats or cell trays. Maintain soil temperature between 50-70°F for consistent germination, keeping the medium consistently moist but not saturated. Seedlings will be thin and grass-like; this is normal. Provide bright light from a grow lamp once sprouted to prevent legginess.
Transplant hardened-off seedlings outdoors 2-3 weeks before your last frost date; Unzen Early is frost-hardy and tolerates cool spring weather. Space plants 4 inches apart in prepared soil with pH 6.0-7.0, setting the seedling base slightly above soil level so the developing bulb can flatten properly. Water gently after transplanting and keep soil consistently moist during the establishment period.
Direct sow seeds outdoors 2-3 weeks before your last spring frost, pressing them lightly into the soil surface or covering with just 1/4 inch of fine compost. Keep the seedbed consistently moist until germination occurs at 50-70°F soil temperature. Thin seedlings to 4-inch spacing once they reach 2-3 inches tall, using thinned greens in salads.
Unzen Early Flat Onion reaches maturity in 100-110 days from transplanting. Begin harvesting green onions (scallion stage) as soon as tops reach usable size, typically 4-6 weeks after planting. For mature bulbs, wait until the foliage naturally yellows and begins to fall over, usually mid to late summer depending on your planting date. Pull bulbs gently from the soil once the neck has dried and softened; the flat form makes them easy to extract. Store immediately in a cool, airy location to cure before long-term storage.
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“Unzen Early Flat Onion originates from Nagasaki, Japan, where it has been cultivated as a traditional variety of immense cultural importance for centuries. The name honors Mount Unzen, a volcano that dominates the landscape of this island city, embedding the onion deeply into local identity and agriculture. Preserved through careful seed saving by Japanese gardeners who recognized its exceptional qualities, Unzen was rescued from obscurity and introduced to the broader gardening world through heirloom seed companies. Its journey from a regional staple to international cultivation represents the vital work of seed preservationists who recognize that traditional varieties carry both flavor and cultural history worth protecting.”