Rodale's Encyclopedia of Organic Gardening defines scallions by saying that most onions (Allium cepa) pulled young before the bulb forms qualify as scallions, but that the species A. fistulosum, a perennial bunching type, produces the most superior scallion, and is practically pest-free and disease-proof.
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Preparing The Soil
Scallions, like most onions, have a shallow root system, so keep the plants evenly watered and grow them in well-drained soil and full sun.
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Begin sowing scallion seed in early spring, either indoors under grow lights or directly into the garden.
Indoors, start the seed 5 to 6 weeks before the average last-frost date for your region. Sow 5 to 10 seeds in each cell of a cell pack. When the clumps of seedlings develop enough roots to retain the soil around them (usually 4 to 5 weeks), harden them off and plant out the clumps 2 weeks before the last frost, spacing them 8 inches apart in rows 1 foot apart.
Outdoors, sow the seed thickly, ½ inch deep in rows 1 foot apart, 3 to 4 weeks before the last frost. Germination can take up to a month and may be spotty if the soil dries out in the interim, so be sure to keep an eye on watering. Thin the seedlings to 1 inch apart (use the little leaves as you might young chives). Make successive sowings every 3 weeks through summer to ensure a regular harvest and consider growing a mix of white and red varieties, including Red Welsh, Tokyo Long White, and Deep Purple.
For winter crops, sow cold-hardy varieties like Beltsville Bunching and Evergreen White Bunching in summer and mulch them with 8 to 12 inches of straw before frost, which will keep them sound over winter.
If growing perennial scallions, apply a winter mulch to protect them and help them come into growth quickly as the soil warms.
In USDA Plant Hardiness Zones 7 and higher, scallions can be direct-sown in September for winter and spring harvests.
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Pull weeds by hand to prevent damaging the scallions' superficial roots. Water well after weeding, then mulch between the rows with grass clippings. Water spindly or slow-growing plants weekly with diluted fish emulsion (about ½ tablespoon per gallon of water) until they begin to thrive.
Rotating scallions to a different spot in the garden each year prevents most pest and disease problems.
Scallions become stronger tasting as they mature, so start harvesting as soon as they reach 6 inches tall. If seedlings were left as clumps, harvest the entire clump. When growing the perennial sort, harvest only the thinnings the first year, leaving a single plant every 12 to 18 inches. In the second year, a clump will form. To harvest, lift the clump with a shovel, then use a sharp knife to separate the clump into several divisions. Replant the divisions and water well. Washed and trimmed scallions will keep for a week in the refrigerator, or longer if wrapped in a moist paper towel and stored in a plastic bag.
Tip: When grilling scallions, place them crosswise on the grate to prevent them from falling through.