Onions: Sow and Grow Guide

Onions: Sow and Grow Guide

Onion seeds should be started indoors (with the exception of the South) 10 to 12 weeks ahead of your average last spring frost, and transplanted out 4 to 6 weeks before your average last spring frost. Leeks and shallots also follow the onion rule- the bigger the transplant, the bigger the potential yield, so start these early (8 to 10 weeks before average last spring frost). Shallots are cold hardy and can also be transplanted out in the fall, and over-wintered from Alaska to Hawaii to S. Florida.

Growing onions from seed versus starter plants offers a wider variety, is less expensive, and gives you more control over growing conditions and inputs like fertilizer or pesticides. Plus, we are all itching to get our hands dirty again!

Tips for growing onions, leeks, and shallots

  • The ideal soil temperature range for best germination is 60°–85°F; although, they will germinate at 45°F, but it will take more time.
  • While growing indoors, the green tops can be trimmed to 3” to keep them upright and manageable. The trimmings are similar to chives and can be used in the same way.
  • Thinning while growing indoors is not necessary. Divide onions to single plants at transplanting. Sow bunching onions in groups to save time and space; they do not require dividing at transplant time.
  • Onions are heavy feeders. Amend your planting area according to soil test suggestions. Nitrogen is usually the first major nutrient to be used up, so usually a test suggests adding nitrogen and often, organic material (compost or ground leaves work well).
  • After hardening off, transplant onions 4” deep. This may leave only a tiny bit of green poking up through the soil, but don’t worry, leaves will quickly catch up. Scallions and bunching onions can be grown densely; all others should be separated into individual plants and transplanted 3”–4” apart.
  • Once greens are 8”–10” tall, beds can be heavily mulched or hilled with soil to reduce weed pressure, conserve water, and in the case of leeks, blanch the stems (keep white). Leeks should be hilled monthly to keep as much of the stalk white as possible.
  • Keep weeds in check, onions do not compete well with weeds and will be smaller.
  • With the exception of bulb-producing onions and shallots, the other onions discussed can be harvested whenever you like. Bunching onions and leeks are frost tolerant but should be harvested before a hard freeze. Many gardeners prefer to harvest leeks after a light frost, because plants produce sugars to avoid freezing, making them sweeter.

Bulbing Onions

Bulbing onions require special attention at sowing because their growth is triggered by day length (latitude). Understanding what varieties grow best in your area is the first step to success.

  • Long-day varieties grow well in the north (above the 37th parallel), as they need 14 to 16 hours of daylight to trigger bulb formation. Try Ringmaster, Yellow Sweet Spanish Utah, and Cipollini Borettana.
  • Intermediate-day varieties overlap the long and short day ranges a bit and cover the middle of the country (32nd to 42nd parallel). These varieties start the bulbing process when sunlight reaches 12 to 14 hours. Try Red Amposta.
  • Short-day varieties are best sown in fall in the south (below the 35th parallel), for a late winter/early spring harvest. These varieties need 10 to 12 hours of daylight to trigger bulbing. Depending on soil temperature, southern gardeners may choose to sow onions directly into the garden. In the north, short-day onions may be grown over the winter in a greenhouse, or transplanted out in the spring; this method produces early but smaller bulbs. Try Yellow Granex PRR (Vidalia type).

Harvest bulbs when tops fall over, and have turned yellow or brown. Cure bulbs in well-ventilated area for 3 to 7 days before cutting off tops and most of the root (be careful not to nick the bulb). Farmers generally cure bulbs in the field if dry weather is expected.

Once cured, trimmed, and any extra soil is knocked off (do not wash), bulbing onions and shallots are ideally stored in a cool (40°–55°F), dry place. As a general rule, the sweeter the onion, the shorter the storage time, but you can still expect a month or more if cured and stored properly.

Leeks and bunching onions store best with their roots trimmed close to the stalk, placed in a sealed container like a plastic bag, and stored in the refrigerator crisper. Leeks can be quite long, and you may choose to trim the tops. Leeks and bunching onions, fresh from the garden can store for a month, potentially longer.

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