Growing Organic Sweet Onions

WHEN TO PLANT

There are 2 different ways to begin growing onions; seeds or sets.

Plant onion seeds in January or February; mild winter areas are also able to begin planting in the fall.

Plant young sets (also known as seedlings or bulbs) outdoors in April or May or when soil temperatures reach a steady 55F.

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.

There are 3 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 middle-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.

Note: Your seed/set supplier is a good resource for other questions regarding onions in your region.

WHERE TO PLANT

Onions grow best in full sun, although in the heat of the summer, a little shade is desirable as it will delay the plant from seeding 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.

PREPARING THE SOIL

The fewer rocks in your soil, the better. Rocks 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 in clay soil

Soil should be non-crusting so bulbs don’t “cement in” and stunt growth. Keep your soil moist for best results.

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; and too much nitrogen will delay the bulb's development, and discontinue supplementing nitrogen once the bulbs begin to form.

For sweeter onions, avoid gypsum fertilizers (which contain sulfur).

SEEDS AND GERMINATION

Growing onions from seed offers the most variety but 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 is about 1 year in cool, dry storage.

Germination occurs at temperatures 50-95F, with the optimum temperature around 75F for quickest germination.

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 60F

Keep temperatures below 70F during the day and around 50F at night.

SOWING AND GROWING (Planting seeds directly into the garden)

Onion sets are easier to plant than seeds, and availability of varieties may be limited

When planting in spring or fall, loosen the soil well. Push 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 growing onions is to sow your transplants or sets in a grid formation with enough room for easy cultivation (think of a tick-tack-toe grid).

While the day length remains consistent from year to year, air and soil temperature will vary. This may affect your crop quality.

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!

WATERING

Keep 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.

Growing 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.

COMPANION PLANTING / ROTATION

Onions planted among carrots have been observed to repel carrot flies.

Some gardeners have reported an enhanced taste in onions when planted with summer savory and chamomile.

Most issues with growing 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.

WHEN TO HARVEST

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”.

STORAGE

After 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.

COMMON PESTS AND PROBLEMS

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.

SAVING SEEDS

How-to-Grow 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 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 50F as possible.