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A sunny, open site with fertile, well-drained soil is ideal for members of the onion family because they are prone to fungal diseases in damp conditions. Treat soil with a pH of less than 6.5 with lime, and don’t grow them in the same place year after year. Manure the ground a few months in advance to prevent too much soft growth.
All alliums, except garlic, can be grown from seed. Sow in modules in early spring under glass for early crops, or outdoors for later crops. Harden off seedlings grown indoors, and plant out at the desired spacing, or thin direct-sown rows. The final spacing dictates the harvest size of the bulbs. Transplant leek seedlings when they are pencil-size. Drop them into holes 6 inches deep and the width of a spade shaft. Water well, but do not backfill with soil. Succession-sow green onions. Onions and shallots can also be planted as sets (and garlic as cloves); these small bulbs mature faster and are less prone to disease than seed-grown plants. Place sets 4 inches apart in shallow drills. Water if the soil is very dry.
Water onions and shallots in very dry weather. Leeks respond well to regular watering and mulch. Keep the ground weed-free. All alliums are susceptible to fungal diseases, including onion white rot, downy mildew, and fusarium. Maintain good air circulation around plants and well-drained soil to minimize problems, and remove infected material. Seedlings are more susceptible to damage by onion flies than sets.
Harvest leeks and green onions when green, but allow the leaves of onions, shallots, and garlic to yellow and die down before lifting them. Store onions, shallots, and garlic on a wire rack until the leaves rustle; then hang them in a cool, dry place.
Planting Maincrop Onions (image 1)
From late winter to early spring, press sets into a shallow drill, so their tips just show when firmed in.
Harvesting Once Leaves Have Died Down (image 2)
Be patient and allow the leaves of onions, shallots, and garlic to die down naturally before harvesting them from late summer to early fall. If you intend to store the bulbs, don’t be tempted to fold the leaves down, which speeds up their withering, because it could damage the necks of the onions. Lift the harvest carefully with a fork to avoid any bruising that might allow rot to set in later.
Onions (image 1)
Small or heat-treated onion sets are less likely to bolt, so are a good choice for novices wanting a trouble-free crop.
Shallots (image 2)
A single shallot set will divide to produce a crop of several small, sweet bulbs, which are expensive in stores.
Garlic (image 3)
Do not plant supermarket cloves; you will achieve better yields using virus-free stock of cooler-climate varieties.
Green Onions (image 4)
A quick, easy onion, ideal for the gaps between slower- growing crops. Try one of the unusual red varieties.
Excerpted from Simple Steps: Vegetable Gardening
© Dorling Kindersley Limited 2007