Onions are one of those crops that you should be growing on your market farm. 

Onions are widely consumed by most families and can be sold throughout the whole market season.

Mind you, they are not the most profitable crop around, but they are popular, and you can set yourself apart from other market farmers by growing interesting, diverse varieties. 

For example, JM Fortier grows shallots and cipollinis that aren’t necessarily found in grocery stores, but his customers know they can find them at his farmer’s market stand.

We will be learning from JM Fortier (the market gardener), as well as Pam Dawling (Sustainable Market Farming), and their experience growing onions for market.

Bulb Onion Phases

Onions are cool-season plants. They have three distinct phases of growth: vegetative, bulbing, and blooming.

The switch from one to the next is triggered by environmental factors. 

It does not work to plant onions at a random date in the year without taking account of these environmental factors.

Success depends on understanding what the crop needs during each of the three phases. 

Vegetative Phase

The first stage is vegetative growth (production of roots and leaves). 

To grow large onions it is important to produce large healthy plants before the vegetative stage gives way to the bulbing stage. 

Each onion leaf represents one ring of the future onion bulb: more leaves mean more rings. 

The larger the leaf, the fatter the ring becomes. If plants are small when bulbing starts, only small bulbs are possible. 

Cool, but not cold, weather and adequate irrigation encourage heavy leaf growth. 

It’s important to grow varieties that are suitable for the latitude of your farm.

The further north you are, the more hours of daylight you have in summer. Onion varieties are often described as “Northern / long day” or “Southern / short day.”

There are also “intermediate day” types and a few genuinely “day-neutral” varieties. 

The names refer to the relative day length needed before the plant will start to make a bulb. 

The dividing line between short day (south of 35°N) and long day (north of 38°N) varieties leaves a gap where neither type is ideal. 

Your latitude can be found on maps or on weather forecasting websites. 

Bulbing Phase

Bulbing is initiated when the daylight length reaches the number of hours critical for that variety. 

Temperature and light intensity also determine when vegetative growth stops and bulbing starts.

It takes a daily average temperature of 60°F (15.5°C), or even 70°F (21°C), to trigger bulbing (depending on the variety).

The rate of bulbing is more rapid with high light intensity and increase temperature.

The optimal temperature for rapid bulb development is 75-85°F (24-29°C).

Long-day onions are bred to start bulbing at 14 to 16 hours of daylight, depending on the particular variety.

The pungent storing onions tend to be long-day onions, so if you live north of the 38°N, you can happily grow these kinds.

In cooler northern regions, the day length trigger may be reached before the temperature trigger. In these places, bulbing is delayed until warmer weather.

Onions then bulb during the summer and are harvested in the fall.

Further south, the temperature trigger is reached before the day length trigger, so bulbing starts as soon as the days are long enough and finishes early in summer.

South of their ideal growing region, long-day onions don’t start bulbing until day length triggered near the summer solstice, and the bulbs are exposed to hot conditions as they mature.

Soils dry out fast under hot conditions, and if the water supply is insufficient, growth will be stunted. The leaves may die, and the bulbs can get scalded or even start to bake if temps are 90°F (32°C).

Short-day onions start to bulb at 10-12 hours of daylight, provided temps are warm enough. 

In the south, below 35°N, they are sown in September or October, grown through the winter, and are harvested in May.

If short-day onions are grown too far north (where they can’t be overwintered and must be started in spring) they will bulb before much leaf growth has occurred.

Only small bulbs can result from this.

One way to deal with this is to start the seedlings in the late fall/early winter, let them make some vegetative growth, and keep them alive indoors over winter, to continue growth in the spring.

Onions can survive at 20°F (-7°C), but not under colder conditions.

Short-day onions are mild flavoured and do not store.

What makes an onion sweet is the same as what makes it not store well–high water content.

Where they can be grown, short-day varieties can provide early season onions for immediate use.

Cipollini, also known as pearl or boiling onions, are varieties of short-day onions sown in spring, planted at high density, which form small bulbs and mature in a couple of months.

Intermediate day onions start to bulb at 12-14 hours of daylight.

They are relatively sweet (not as pungent as long-day onions) and mostly do not store well. 

This is not a concern for growers selling all their crops soon after harvest, but for growers who wish to provide onions for as much of the year as possible, these are not ideal.

Day-neutral onion bulb independently of day length. Most of these varieties do not store well.

Blooming Phase

Onions are biennial plants, which means that they normally grow leaves and produce a bulb in the first year.

In the second year, they send up more leaves, followed by a thick central flower stem. 

When onion plants experience an extended period of cooling temperatures, such as winter, they go dormant. 

When temperatures rise, they start growing again. 

After being exposed to cold temperatures, smaller seedlings with a diameter less than pencil thickness and fewer than six leaves will resume growth and not usually bolt (bloom). 

The trigger for a transition from bulbing to flowering is temperatures below 50°F (10°C) for 3-4 weeks, after the plants have six or more leaves.

This can happen, if you are unlucky, after an unexpected cold period in spring.

Bolting is to be avoided because the flower stems are tough and inedible, and the bulbs start to disappear to feed the growing flower stems.

Bolted onions will not dry down to have a tight neck and so will not store.

It is possible to sow onions in the fall and plant the seedlings out in early spring, for bigger vegetative growth and therefore the chance of bigger bulbs. 

The temperature and size trigger limits how early in fall seeds can be sown.

If seedlings have made lower stems larger than a pencil in diameter when winter closes in, the plants are likely to bloom in the spring rather than forming bulbs. 

For Pam, starting seedlings in a hoophouse in early November works well. 

The hoophouse works well because the plants get much better airflow, are protected from very cold temperatures and can be easily seen and cared for. 

As well starting onions in a hoophouse helps reduce the rate of bolting as you can control the climate somewhat.

In colder zones, a slightly heated greenhouse might work better for overwintering. 

 

Seeds

Use fresh seed of a variety suitable for latitude and the time of year (fall or spring sowing).

Store seeds in an airtight container in a cool place. Seed from the previous year is as old as you should go. 

Yellow onions are easier to grow than whites or reds, and have tougher skins. 

Whites are more susceptible to sunburn, and to green streaks if rained on near harvest.

Reds are slower-growing and therefore tend to make smaller onions. 

Timing and Choice of Variety

North of 38°N: Use only long day or intermediate day varieties, as short-day varieties will bulb before reaching a good size.

Sow indoors in late winter (12 weeks before likely suitable weather for transplanting).

If you can keep seedlings above 20°F (-7°C), sow indoors in late fall. 

In mild coastal locations, it may be possible to sow outdoors in late summer or fall. Transplant in spring, but avoid having transplants thicker than a pencil. 

Between 35 and 38°N: Sow suitable long day varieties (ones which bulb at 14 rather than 16 hours of daylight) indoors from November to early January. Use row cover.

Sow intermediate day varieties in mid to October to late November, or January if necessary. 

If temps don’t go below 0°F (-18°C), some intermediate day varieties will survive outdoors (e.g. Walla Walla or Juno, which have a good cold tolerance).

Sow short day varieties in fall, but keep seedlings from getting too cold (e.g. sow in greenhouse and transplant in February). Choose only the most cold-tolerant short-day varieties.

Short-day onions will harvest May-June, long-day varieties will harvest June-July.

South of 35°N: Sow in the fall (Sept-Oct), using only short or intermediate day varieties.

Long-day varieties will likely get scorched by hot weather before finishing bulbing, and may never make decent bulbs as the longest day is still too short for that variety.

Sowing

Sow seeds 1/4 to 1/2 inch deep, a quarter to a third of an inch apart if you plan to transplant later. 

Sow every two inches if direct sowing.

Keep the soil damp, as the first roots will be very near the surface. Use a temperature of 57-86°F (14-30°C) to germinate. 

The ideal temp is 75°F (24°C).

Use row covers if there’s a very cold snap. If you sow too late you will get small bulbs; if too early your onions will bolt.

Onion seedlings take 3 days to emerge at 77°C (35°C)and 7 days at 59°F (15°C).

Onions will still germinate at 95°F (35°C), but take 12 1/2 days and only produce 73% normal seedlings. 

JM and team start all of their onions at the same time, at the end of March.

They sow the seeds by broadcasting onto open flats, which saves space. 

As the onions grow, they clip their leaves once or twice to a length of 4-5”, to stimulate their development. 

They transplant onions in early May into soil fertilized with nitrogen-rich chicken manure.

When transplanting, it is important to ensure that both the soil mix and the ground are wet enough, as moisture helps the tiny onion plants get established.

They should be planted as close to the surface as possible, as they do not like being put too deep in the soil. 

Soil

Rotate your onion beds with at least three years between alliums.

A pH of 6 to 6.5 is ideal. Onions need fairly rich soil, moist but well-drained, with plenty of organic matter and a loose crumb structure. 

Waterlogged soil promotes disease, so make raised beds if your soil does not drain well.

Around 145 lbs/ac of nitrogen during the growing season may be needed.

The pungency of onions relies on soil sulfur. Ensure adequate potassium or the onion neck will not dry down well, and the onions won’t store. 

Onion plants compete poorly with weeds, so choose an area with low weed pressure. 

Transplanting Onions

Transplant fall sown onions as early in spring as possible, and those sown after New Year once they have at least three leaves, but four or five is better. 

The final bulb size is affected by the size of the transplant as well as the maturity date of that variety.

The ideal transplant is slightly slimmer than a pencil but bigger than a pencil lead. 

Onion seedlings are slow-growing, even in spring they can take 10 weeks to reach a size suitable for transplanting. 

Overly large transplants are more likely to bolt.

If seedlings are becoming thicker than a pencil before you can set them out, undercut 2” below the surface to reduce the growth rate. 

Transplant 4” apart for single seedlings or 12” for clumps of three or four. 

Set plants with the base (stem plate) 1/2 to 1 inch below the soil surface. Don’t plant too shallow.

Give plenty of water to the young transplants: keep the top 3 to 4” of the soil damp for the first few weeks to prevent the stem plate from drying out.

JM prefers to transplant his onions at wider spacing(10″), but clumps three or four together instead of one at a time. 

This can provide more yield, is a space-saving technique, and makes the long job of transplanting onions go much faster.

Planting onions three to a hole also makes the crop much easier to hoe later on. 

For early harvest, they cover many of their onion beds with row cover immediately after transplanting.

This protection makes an enormous difference in the early stages of growth and really makes the vegetables take off. 

With onions, the secret to success is establishing the plants well and encouraging vigorous leaf growth first and foremost.

Caring for Onions

Keep weeds under control!

Onions do not compete well with weeds as their leaves have a relatively small surface area and easily get shaded out. 

Yield is reduced 4% per day by weeds, or 50% in two weeks. 

Some growers use plastic mulch.

Those growing on bare ground cultivate frequently, and shallowly, as the roots are near the surface. 

In colder areas, some growers add straw or hay mulch after the soil has warmed a bit in spring to combat weeds. In warmer areas, this would encourage too many fungal diseases, and is not helpful. 

As well, the better you manage weeds on the plots that precede the onions (the year before), the easier it will be to keep weeds in check the following year.

The ideal is to grow at least 13 leaves before bulbing starts.

Give one inch of water per week until the leaves start to die back. 

If needed, side-dress to provide extra nitrogen in spring while the plants are still growing.

When the leaves begin to brown, consider drawing some soil away from the bulbs to help them dry out. 

Do this gradually, once a week, two to three times a day. 

If the weather is very hot, drawing soil away from the bulbs might not be a good idea as it could lead to sunscald. 

Pests

Row cover can be used to exclude pests. The most likely problems are due to maggots, aphids, and thrips. 

To control thrips, attract ladybugs and lacewings by planting flowers they are attracted to. Mow the field edges to reduce thrips habit. 

Onion maggots (larvae of the onion fly) are attracted by the smell of rotting alliums, therefore do good crop clean-up. 

The small grey-brown onion fly lays eggs at the base of the onions. When the larvae hatch, they feed on the onion roots.

To disrupt the onion maggot life cycle, consider an early spring allium-free period (if you don’t grow garlic) or a July / August gap (if you don’t grow leeks).

Diseases

Onions are vulnerable to many fungal diseases including downy mildew and botrytis.

A weekly application of copper and sulfur in alternation at the first sign of fungal disease can help control fungal disease, especially during excessive wet weather or when the leaves have been damaged by hail.

Harvest

Be patient and watchful, if harvested too early, the neck area will not be softened enough to allow shrinkage and tightening and bacteria will enter, causing the centre of the bulb to rot.

In the South, when 30 to 40% of the tops of the planting have died, lift the onions gently and put them in partial shade to cure. 

Leaving bulbs in the ground too long risks sunscald as well as fungal and bacterial diseases. 

Bruising can seriously damage the bulbs, as can stressing the neck, so do not pull the onions. 

If they don’t lift easily from the ground, use a spading fork to raise them.

One key to organic production is to handle the bulbs in ways that minimize physical damage. Handle and place onions gently.

Think eggs, not tennis balls!

In the North, where intense sunlight and humidity when onions mature is not such an issue, it is fine to wait until most or all of the tops have fallen, then lift the onions and dry in the field for two to three days, as long as heavy rain is not expected. 

Compost all onion crop residues. Don’t leave any residue in the onion bed. 

You can start eating the onions fresh right away and cure the rest. 

Curing

Curing takes two to three weeks.

In the South: spread the untrimmed harvested bulbs in a single layer in a warm dry place, and check every few days.

The ideal conditions are 85 to 90° F (27 to 32° C) with a constant air movement and no strong direct sunlight. 

In the North, you can simply cure in the outside sun.

Storing

Rub the neck between finger and thumb to detect any remaining slidey slipperiness, which would indicate that the onion is not yet fully cured. 

When the necks are dry, clip the tops to 1-2” and gently brush off dirt and loose scales. 

Minimize the removal of skin, though, as skin serves to protect the onion in storage.

Remove any onions that are not curing properly, for immediate use or processing.

Bag the cured, trimmed onions and label with the variety name, to ensure less storable varieties are used first and to compare storage quality.

Store in a dry place at either 60 to 90°F (15 to 32° C) or 32 to 40°F (0 to 4.5°C). Avoid 45 to 55°F (7 to 13°C), and do not move onions from cold storage into warm storage or you will promote sprouting. 

You could use a barn, shed or basement at first, until the temperature there drops to close to 55°F (13° C), and then move the onions to a refrigerated cooler.

If at all possible, do not store onions with fruits, including squash, as these exude ethylene, which also promotes sprouting. 

Bottom Line

While onions may not be the hardiest of all crops and require quite a bit of attention and care to grow, it is still worth it to add them to your market garden repertoire.

I for one cannot resist amazing looking onions at the farmer’s market.

I must have them! And I know I am not alone in that sentiment.

We are giving onions a shot this year on our market farm, so stay tuned for updates on our progress!

What is your favourite type of onion? Let us know in the comments below.

Stay Local,

Kathy & Jon

your friendly neighbourhood growers