• There are 2 different ways to begin growing onions; seeds or sets
  • Begin growing onion seeds in January or February; mild winter areas are able to begin planting in the fall
  • Plant onion sets (also known as seedlings or bulbs) outdoors in April or May or when soil temperatures reach a steady 55°F
  • The ability for the onion bulb to form is dependent on the length of day; longer, warmer days encourage bulbing
  • Bulb development will not take place in colder weather no matter how long the day
  • If you’re planning to start growing onions, there are three categories of onions for different climates:
  • Long-day: the best choice for Northern gardeners; longer summer days (15+ hours during the summer months) provide the needed day length for bulb development
  • Intermediate-day: the best choice for those gardening in the mid-section of the United States; 14 hours of daylight is needed to induce bulbing
  • Short-day:best for Southern climates which only need 8-12 hours a day to trigger bulbing
  • Your onion seed/set supplier is a good resource for other questions regarding onions in your region

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  • Onions grow best in full sun
  • In the heat of the summer, a little shade is desirable as it will delay the plant from going to seed too quickly
  • Plant your onions behind a taller crop (tomatoes are a good choice), as they can provide some shade on hot mid-summer days

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  • The fewer rocks in your soil, the better. Rocks may cause deformities in the onion bulb’s development
  • To improve heavy soil, loosen the soil at least 18” down and mix in well rotted manure, compost, shredded leaves or other organic matter
  • Onions prefer slightly acidic soil with a pH of 6.6-6.8
  • Sandy loam soil is good when mineral content is high
  • Avoid planting onions in clay soil
  • Soil should be non-crusting so bulbs don’t “cement in” and stunt growth. Keep your garden’s soil moist
  • If your soil is potassium deficient, your onions will store poorly
  • If your soil is phosphorus deficient, your onions will have thick necks and smaller bulbs
  • For sweeter onions, avoid gypsum fertilizers (which contain sulfur)
  • High nitrogen levels delay bulb development unless you have extra-long days
  • Discontinue supplementing nitrogen once the bulbs begin to form

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  • Starting onioins from seed offers the most variety, though it can take up to 5 months to mature and the plants are more susceptible to disease than sets
  • Using fresh onion seeds is important; if not kept cool and dry, they lose viability quickly
  • Onion seed viability: 1 year in cool, dry storage
  • Germination occurs at temperatures 50-95°F
  • Optimum temperature for the quickest germination of your onion seeds is 75°F

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GETTING STARTED INDOORS (and transplanting)

  • Start your seeds indoors at least 8 weeks before the last frost
  • If indoor growing space is limited, plant seeds ¼” apart in flats; leave onions in flats until outdoor planting
  • In the early growth stage, young plants grow best in cool temperatures around 60°F
  • Keep temperatures below 70°F during the day and around 50°F at night

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PLANTING / GROWING (Planting seeds directly into the garden)

  • Onion sets are easier to plant than seeds, although the availability of varieties may be limited
  • When planting onions in spring or fall, loosen the soil well
  • Push onion bulbs into the soft dirt just far enough to secure in place but don’t cover the tops
  • Early weed control is vital to the health and development of onion starts
  • Be careful not to disturb the onion’s roots when pulling weeds
  • Pulling weeds too close to the bulb could pull the onion start from the ground
  • Consistent weeding is advised to keep the weeds from growing large; if you find a large weed, clip it at ground level if it’s too close to a bulb
  • For direct-to-ground seeding, the maximum seed depth should not exceed ½” (¼” in heavy soil)
  • When planting in rows, thin the onions to 4” apart
  • As top spears become thick like chives, harvest early shoots for green onions
  • Another option for planting is to transplant the onion sets in a grid formation with enough room for easy cultivation (think of a tic-tac-toe grid)
  • While the day length remains consistent from year-to-year, air and soil temperatures will vary. This may affect your crop quality
  • Jenny’s Tip: This year we discovered a liquid organic leaf spray fertilizer called Organic Garden Miracle™. Spraying a light mist over your onions every couple of weeks increases the plant sugar in your onions, in turn creating sweeter and more robust onions that are up to 50% larger than they’d be otherwise.
  • On a Personal Note:

    At On The Green Farms we’ve successfully grown our own sets from the seeds of rare Italian varieties such as Yellow of Parma, Red of Florence, and Bianca Di Maggio. Perpetuating these old world onions preserve a history and a flavor well worth preserving!

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  • Keep your soil moist, but not soggy, from planting through harvest
  • Once outdoors, onion plants need 1” of water weekly with occasional heavier applications throughout the season
  • Onions are susceptible to disease when their tops are wet
  • If you are able to, use a soaker hose to keep the water on the ground and off the leaves
  • If overhead watering is your only option, water in the morning so that the leaves have time to dry out completely before evening

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  • Onions planted among carrots have been observed to repel carrot flies
  • Some gardeners have reported an enhanced taste in their onions when planted with summer savory and chamomile
  • Most issues with onions can be minimized (both pests and diseases) by 1) regularly rotating your crop ( not planting in the same location as the previous year), 2) applying row covers immediately after planting to keep onion maggots from accessing plants, and 3) by controlling the weeds, which also reduces the breeding ground for pests

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  • Onions can be harvested early for green onions
  • Harvesting mature onions: Once the tops of most of your onions begin to fall over, mark the day on your calendar and stop all watering; after two weeks, harvest your onions
  • Be very careful not to bruise or damage bulbs with your shovel. It shortens the shelf-life of onions, causing them to rot more quickly (like an apple or a peach would)
  • Spread your onions in rows in the sun, layering them so tops of one row cover the previous row of bulbs, allowing them to dry, but avoiding “sunburn”

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  • After your onions have cured, brush (don’t wash) any excess dirt you can of the onions
  • If you braid your onions, do it when the tops are still flexible
  • Hang in a warm, dark, ventilated area to allow the onions to fully dry

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  • Most onion issues can be minimized (both pests and fungi) by regularly rotating your onion crops, providing well-drained soil, and not allowing onion leaves to be damp for long periods of time
  • The use of row covers right after planting will help to keep onion maggots out
  • Keeping the weeds under control will eliminate habitats for pests to hide and breed in
  • Corn-gluten meal is a safe, natural by-product of corn processing which will prevent weed seeds from germinating
  • Spread meal around well-established onion plants then mulch with 2” of “seed free” straw (we like barley straw if it’s available)
  • Humid conditions are a breeding grounds for fungal diseases, the onion’s biggest enemy

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    How to grow your own onion sets from seed:

  • Set aside a few square feet of garden bed in early spring
  • Scatter about an ounce of seeds in a 4 square foot area
  • Don’t thin plants; crowding keeps the bulbs small
  • Smaller sets give you larger bulbs and are less likely to bolt (go to seed) the following year
  • Pull plants in late July before the bulbs reach a ¾” diameter
  • Use larger bulbs for small onions in soups, stews or pickling
  • Cure sets in sun until tops thoroughly dry (7-10 days in sunny, dry weather)
  • Braid (for hanging) or cut off tops and place in a ventilated box or crate (see storage)
  • Store in a dark, dry, and airy location with a temperature maintaining as close to 50°F as possible

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