Bulbing onion, scallion, and shallot, along with garlic and leek, are members of the genus Allium. The characteristic flavors of Allium family members come from sulfur-containing compounds produced by these plants.
The bulbing onion, Allium cepa, appears to have originated near the intersection of Europe and Asia, whereas the bunching onion or scallion, Allium fistulosum may have originated in China.
Types and Varieties
Bulbing onions form bulbs in response to day length. Only long-day and intermediate-day types are recommended for summer harvest in New England since these types require at least 13 hours of daylight. Short-day onions begin forming bulbs when days are 10-12 hours long and are typically grown in southern regions of the U.S. where they are planted in the winter. Recently, some researchers and farmers in New England have been planting onions in low tunnels in the fall for spring harvest. The University of New Hampshire research report on overwintering onions includes planting details and trialed varieties (see https://bit.ly/overwinteronions). Shallots form clusters of bulbs. While shallots have traditionally been propagated vegetatively, hybrid cultivars that can be grown from seed are now available. Scallions a member of the Allium genus that do not form a fully developed bulb. They are planted from seed and commonly sold in bunches.
|Onion, SHallot, and Scallion Varieties|
|Scallion/Bunching Onion||Onion - Yellow Storage|
|Evergreen Hardy White||Northstar (80)|
|Ishikura Improved||Bridger (90)|
|Southport White Bunching||New York Early (98)|
|Onion - Red Storage|
|Onion - Sets||Red Bull (104)|
|Ebenezer||Red Carpet (114)|
|Onion - Sweet, non-storing||Onion - Overwintering|
|Ailsa Craig (110)||Bridger|
|Onion - Sweet Spanish||Electric (red)|
|Super Star (100)||Desert Sunrise (red)|
|Yellow Sweet Spanish (120)||Red Rock (red)|
|The number in parentheses is the approximate number of days to maturity from seeding.|
Apply lime according to soil test results to maintain soil pH at 6.5-6.8. Onions and leeks do not tolerate acid soil, especially in early growth stages. If the magnesium level is high, a lime high in calcium (calcitic lime) should be used to maintain a high calcium level.
Less nitrogen fertilizer will be needed if legume sod was plowed down or if manure was applied (see Table 1 and Table 7). These sources may also result in high soil nitrogen levels late in the season. Excessive nitrogen late in the season from any source can delay maturity and reduce storability of onions.
Onion flavor is determined by cultivar type, temperature and irrigation, and sulfur fertility. Growers wishing to produce mild-flavored onions for retail sale should choose mild cultivar types, irrigate regularly up to harvest, and maintain soil sulfur levels between 35 and 55 lb/A. Excessive sulfur levels in soils or fertilizers will increase pungency.
|PLANT NUTRIENT RECOMMENDATION ACCORDING TO SOIL TEST RESULTS FOR ONION, Scallion, and Shallot|
|ONION, Scallion, and Shallot||NITROGEN (N) LBS PER ACRE||PHOSPHORUS (P) LBS P2O5 PER ACRE||POTASSIUM (K) LBS K2O PER ACRE|
|SOIL TEST RESULTS||VERY LOW||LOW||OPTIMUM||ABOVE OPTIMUM||VERY LOW||LOW||OPTIMUM||ABOVE OPTIMUM|
|Broadcast and Incorporate||80-100||150||100||25-50||0||175||150||50||0|
|Sidedress 4-5 Weeks after Planting||50||0||0||0||0||0||0||0||0|
Bulbing onion and shallot. Sow transplants in late February or early March, 10-12 weeks before setting in field. Direct seed onions in spring as soon as soil can be worked. Seeding should be completed by late April or mid-May at the very latest to allow time for adequate plant growth before bulb initiation occurs. Plant 2-4 rows per bed, 9-18" between rows and 3-4" apart within rows for transplants. Spacing will affect bulb size. For direct seeding, aim for a stand of 6-9 plants per foot. This can be increased to 9-12 plants per foot if double shoe precision seeders are used. This requires 5-6 lb of seed per acre (about 0.5 oz per 100 feet of row). Instead of transplants, some growers use sets to plant all or some of their crop. Sets are small bulbs (about 1/2") raised the prior year and stored over the winter. This provides for an early harvest (mid-summer), but larger sets can be prone to bolting (premature flowering). Sets should be planted as soon as the soil has dried and can be worked. Spacing is the same as for transplants. Some growers are finding success using black or white-on-black plastic mulch on raised beds for sets or transplants.
Scallion. In addition to early spring planting, mid-summer plantings can be used to produce fall harvests. Even moisture must be maintained throughout germination for direct-sown scallions. Seed in rows 2-3” wide, spaced 4” apart.
Harvest and Storage
Bulbing onion and shallot: Late, hard, pungent varieties with good skin retention are preferable for winter storage. Undercutting several days before harvesting can improve keeping quality. It is best to undercut when most of the plants have 2 leaves that are still partially green. Allowing leaves to completely dry down before undercutting can result in excessive loss of skin during harvest. To develop best skin color, onions should be cured for 2 weeks at 75-80ºF and 70-80% relative humidity. After curing, lower temperature gradually to as near to 32ºF as practical without freezing. Cooling too rapidly, followed by a few warm days, can cause moisture condensation, resulting in bulb staining, sprouting and decay. Maintain storage relative humidity at 65-70%, and maintain sufficient air flow to keep cool air moving around bulbs.
Scallion. Scallions, or fresh bunching onions, can be harvested anytime after they are pencil-sized. To maintain good post-harvest quality, they should be cooled to 39ºF within 4-6 hours of harvest. Scallions can be stored for 7-10 days at 32ºF.
For long-term storage of bulbing onions or shallots, select storage varieties. Pungent dry onions can be stored for 6-9 months at 32ºF. For very long-term storage, a sprouting inihibitor, maleic hydrazide (2 gal of Royal MH-30/A in a minimum of 30 gal/A), may be applied when about 50% of the tops are down, the bulbs are mature, the necks are soft and five to seven of the leaves are still green. See the label for details.