How To Grow Onions in Canada | Complete Growing Guide

Download | How to Grow Onions in Canada | Complete Growing Guide (PDF)

Onions are one of the most useful vegetables Canadian gardeners can grow from seed. Whether sliced into salads, sautéed for everyday cooking, roasted, or pickled, homegrown onions bring fresh flavour to the kitchen and make productive use of garden space.

Growing large, healthy onion bulbs in Canada requires more than simply planting seeds in spring. Day length determines when many onion varieties begin forming bulbs, making variety selection and planting time especially important. Starting seeds early allows plants to develop strong roots and plenty of healthy leaves before the long days of summer trigger bulb formation.

This complete Canadian onion growing guide explains how to choose suitable varieties, start onion seeds indoors, transplant seedlings, manage watering and fertilizer, recognize bulb development, prevent common problems, harvest, cure, and store onions. We'll also look at two varieties available from Caribou Seed Company: Ailsa Craig and Red of Florence.

Onion Growing Guide at a Glance

Feature	Growing Requirement / Detail
Botanical Name	Allium cepa
Plant Type	Biennial, generally grown as an annual for bulbs
Sun Exposure	Full sun, ideally 6–8 hours or more daily
Soil	Fertile, well-drained soil; pH approximately 6.0–7.0
Start Indoors	Approximately 10–12 weeks before the last expected spring frost
Direct Sow	Early spring when soil is workable; indoor starting is preferable for large bulbs in many regions
Sowing Depth	Approximately 6 mm (¼ inch)
Germination	Usually 7–14 days at approximately 18–24°C
Plant Spacing	10–15 cm (4–6 inches) for mature bulbs
Row Spacing	30–45 cm (12–18 inches) in traditional rows
Water Needs	Consistent moisture during active growth; reduce irrigation as bulbs mature
Difficulty Level	Easy to moderate
Container Friendly	Yes, in wide containers approximately 20–30 cm deep or deeper

Choosing Onion Varieties for Canadian Gardens

Onion varieties differ in day-length requirements, bulb shape, flavour, colour, and storage potential. For most Canadian gardens, long-day varieties are a dependable starting point, but always check the particular variety's growing requirements.

Ailsa Craig Onion

Ailsa Craig is a traditional long-day onion known for producing exceptionally large, mild, sweet yellow bulbs under favourable growing conditions. It is an excellent choice for slicing, fresh eating, and cooking.

Because Ailsa Craig is a mild, relatively high-moisture onion, it is not a long-term storage variety. Grow it for generous fresh harvests rather than expecting it to last all winter.

Ailsa Craig Onion Seeds

Red of Florence Onion

Red of Florence is a traditional Italian heirloom onion recognized for its elongated, red-skinned bulbs and mild, sweet flavour. Its distinctive shape makes it useful for slicing, salads, pickling, and cooking.

Unlike the typical round storage onion, Red of Florence is primarily valued for fresh use. It is not generally considered a long-keeping storage variety. Its day-length response can vary by seed strain, so follow the supplier's growing recommendations rather than assuming it behaves exactly like a standard northern storage onion.

Red of Florence Onion Seeds

Caribou Growing Note: Fresh-Eating vs. Storage Onions

A large, sweet onion is not necessarily a good storage onion. Both Ailsa Craig and Red of Florence are excellent choices for fresh kitchen use, but gardeners who want onions to last through winter should also grow a variety specifically selected for long storage.

Understanding Long-Day, Intermediate-Day, and Short-Day Onions

Bulb onions respond to the number of daylight hours they receive. Once day length reaches the variety's approximate threshold, the plant begins directing more energy into bulb formation.

These thresholds are approximate and vary among varieties. Temperature and plant maturity also influence how successfully onions develop bulbs.

For most Canadian gardeners, long-day onions are a sensible choice because summer daylight hours are sufficiently long. However, the best variety also depends on your latitude, growing season, and intended use.

When to Plant Onions in Canada

Onions need a long period of healthy leaf growth before bulb development begins. Starting seeds indoors during winter gives plants time to establish before they are transplanted into the garden in spring.

Start onion seeds indoors approximately 10–12 weeks before your average last spring frost. Transplant hardened-off seedlings outdoors when the soil is workable and weather conditions are suitable. Onions tolerate cool conditions better than tomatoes or peppers, but very young plants should still be protected from severe freezing.

Canadian Onion Planting Calendar

These dates are approximate. Coastal, inland, northern, and high-elevation locations can differ substantially. Use local frost dates and soil conditions to adjust your schedule.

Read More | When to Start Seeds Indoors in Canada

How to Start Onion Seeds Indoors

Growing onions from seed offers a wider choice of varieties than relying on commercially available sets or transplants. It also gives Canadian gardeners greater control over the timing of early growth.

1. Prepare containers: Fill clean seed trays or containers with a fine, well-draining seed-starting mix.
2. Sow seeds: Plant approximately 6 mm (¼ inch) deep and cover lightly.
3. Maintain warmth: Keep the growing mix around 18–24°C for reliable germination.
4. Keep evenly moist: Water gently without saturating the mix.
5. Provide strong light: Once seedlings emerge, supply approximately 14–16 hours of bright grow-light exposure daily, with a suitable dark period.
6. Maintain airflow: Provide gentle ventilation to discourage damping-off and weak growth.
7. Feed appropriately: Once seedlings are established, use a diluted balanced fertilizer if the growing medium does not already supply adequate nutrients.

Onion seedlings initially resemble thin blades of grass. Their early growth can appear slow, but healthy plants gradually develop more leaves and stronger roots.

Read More | Seed Starting 101: Beginner Guide for Canadian Gardeners

Onion Germination and Troubleshooting

Onion seeds usually germinate within 7–14 days under favourable conditions, although cooler temperatures can delay emergence.

Why Onion Seeds May Not Germinate

• Old seed: Onion seeds lose viability relatively quickly compared with many other vegetable seeds. Fresh seed is especially valuable.
• Dry growing mix: Seeds need consistent moisture during germination.
• Excess moisture: Saturated conditions encourage decay and damping-off.
• Deep sowing: Seeds planted too deeply may struggle to emerge.
• Unsuitable temperatures: Cold conditions slow germination, while excessive heat can reduce success.

Use fresh seed, maintain even moisture, and provide suitable temperatures before assuming a sowing has failed.

Read More | Why Seeds Don't Germinate: 10 Common Causes

Should You Trim Onion Seedlings?

Young onion seedlings sometimes develop long leaves that bend or become tangled. Some gardeners trim them for easier handling, but routine trimming is not necessary.

Healthy onion leaves carry out photosynthesis and support later bulb development. Excessive or repeated cutting can reduce the leaf area available for growth.

If seedlings are weak or unusually stretched, first check their light intensity, watering, and temperature. If trimming is needed to manage tangled foliage, remove only a modest amount and avoid cutting seedlings severely.

Caribou Growing Note: Every Healthy Leaf Counts

Onion bulb size is strongly influenced by the number and health of leaves the plant develops before bulbing begins. Give seedlings plenty of light and time to grow rather than repeatedly cutting them short.

Hardening Off and Transplanting Onion Seedlings

Indoor-grown onion seedlings need gradual exposure to outdoor sunlight, wind, and temperature changes before transplanting.

Begin hardening off approximately 7–10 days before planting, starting with sheltered outdoor conditions and gradually increasing exposure.

How to Transplant Onions

1. Choose a sunny location with fertile, well-drained soil.
2. Remove weeds and prepare a loose planting bed.
3. Space seedlings approximately 10–15 cm (4–6 inches) apart for full-sized bulbs.
4. Plant at approximately the same depth they grew in their containers, keeping the growing point above the soil.
5. Gently firm the soil around the roots and water thoroughly.
6. Protect recently transplanted seedlings if severe frost or unusually cold weather is forecast.

Do not bury onion seedlings deeply or mound soil over developing bulbs. Onions naturally form bulbs near the soil surface.

Read More | Hardening Off Seedlings: 7-Day Canadian Plan

Growing Onions from Seeds vs. Sets vs. Transplants

For gardeners interested in growing particular varieties such as Ailsa Craig or Red of Florence, starting from seed provides flexibility and control over the growing season.

Choosing the Best Location and Spacing

Onions grow best in full sun with loose, fertile soil and good drainage. Because they have relatively shallow root systems and upright leaves, they compete poorly with weeds.

• Sunlight: Aim for at least 6–8 hours of direct sun daily.
• Soil: Use well-drained soil with a pH around 6.0–7.0.
• Spacing: Allow approximately 10–15 cm between mature bulb onions.
• Rows: Space traditional rows approximately 30–45 cm apart.
• Weeding: Remove weeds while small to avoid disturbing shallow onion roots.

Large varieties such as Ailsa Craig may benefit from the wider end of the spacing range or slightly more room when growing for exceptionally large bulbs.

Read More | Vegetable Garden Spacing Guide: How Far Apart to Plant Vegetables

Watering and Mulching Onions

Onions need consistent moisture during leaf growth and bulb formation. Their shallow roots make them vulnerable to dry soil, especially during hot weather.

• Early growth: Keep the root zone evenly moist while plants establish.
• Bulb development: Provide approximately 2.5 cm (1 inch) of water weekly as a starting point, adjusting for rainfall, soil, and weather.
• Mulching: Apply a light layer of clean organic mulch to conserve moisture and reduce weeds, keeping it away from plant necks.
• Approaching maturity: Reduce supplemental watering as foliage naturally falls and bulbs finish maturing.
• Before harvest: Avoid unnecessary irrigation when bulbs are mature and conditions are dry enough for harvesting.

Excess moisture in poorly drained soil can encourage root disease, while repeated drought stress can reduce bulb size.

Read More | How Often Should You Water a Vegetable Garden?

Fertilizing Onions for Healthy Bulb Development

Onions need adequate nutrients to produce strong foliage before bulb formation. Healthy leaf growth is especially important early in the season.

• Before planting: Improve soil with finished compost where needed.
• Early growth: Adequate nitrogen supports leaf development.
• During bulb formation: Maintain balanced fertility without excessive feeding.
• Late season: Avoid heavy nitrogen applications, which may delay maturity and reduce storage quality.
• Soil testing: Use a soil test to determine whether additional phosphorus, potassium, or other nutrients are necessary.

Avoid applying large amounts of fertilizer simply to produce bigger onions. The correct variety, early planting, healthy foliage, suitable spacing, and consistent moisture are equally important.

Read More | How to Fertilize Vegetables Naturally

How Onion Bulbs Form

During early growth, onions develop roots and leaves. As the days become long enough for the variety, bulb formation begins and the bases of the leaves swell into the layered structure of an onion.

This explains why starting onion seeds early is so important in Canada. Plants that establish strong foliage before the bulbing threshold is reached generally have greater potential to form larger bulbs.

Late-planted onions may begin bulbing while still relatively small, resulting in smaller mature onions even when watering and fertilizing are adequate.

Why Are My Onions Small?

Small bulbs are often the result of several growing factors rather than a single problem.

• Late seed starting: Plants have insufficient time to produce leaves before bulbing begins.
• Unsuitable day-length variety: Bulb formation occurs at the wrong stage for local conditions.
• Overcrowding: Closely spaced plants compete for light, nutrients, and moisture.
• Weeds: Onion roots compete poorly with established weeds.
• Water stress: Dry soil during active growth limits development.
• Low fertility: Nutrient deficiencies can restrict leaf and bulb growth.
• Insufficient sunlight: Shaded plants may develop smaller bulbs.

For larger onions, begin with the correct variety, start seeds early, maintain healthy foliage, and avoid competition during spring and early summer.

Why Are My Onions Flowering?

Onions are biennial plants, normally producing flowers during their second growing season. When onions grown for bulbs produce a flower stalk prematurely, the process is called bolting.

Bolting is often triggered when sufficiently developed plants experience prolonged cold followed by warmer growing conditions. Onion sets, especially larger ones, may be particularly susceptible.

Once an onion bolts, the bulb may develop a hard central stalk and its storage quality declines. Harvest bolted onions and use them relatively soon rather than storing them with sound mature bulbs.

Growing Onions in Containers

Onions grow successfully in containers when given adequate sunlight, root space, drainage, and moisture.

• Depth: Choose containers approximately 20–30 cm (8–12 inches) deep or deeper.
• Width: Allow enough room to space full-sized bulbs approximately 10–15 cm apart.
• Drainage: Use containers with drainage holes.
• Growing medium: Select a quality, well-draining potting mix.
• Watering: Monitor frequently, particularly during warm and windy weather.

Onions grown for green tops can be planted more closely together than those intended to form full-sized bulbs.

Read More | Best Plants for Container Gardening: Vegetables & Flowers

Common Onion Problems, Pests, and Diseases

Not every yellow leaf indicates disease. As onions mature, their foliage naturally begins to yellow and collapse. Diagnose problems according to the plant's growth stage and other visible symptoms.

Read More | Natural Pest Control for Vegetable Gardens

When and How to Harvest Onions

Most mature bulb onions are ready to harvest when their foliage naturally softens, yellows, and begins falling over. The necks lose firmness as the plant completes its growing cycle.

Signs Onions Are Ready

• A substantial proportion of the tops have fallen naturally.
• Leaves are yellowing and drying.
• Bulbs have reached their expected mature size.
• Necks are beginning to soften and dry.

When most tops have fallen, harvest during a suitable dry-weather period. Carefully loosen the soil with a garden fork and lift bulbs without bruising or cutting them.

Do not bend or force onion tops over to accelerate maturity. This can damage the necks and interfere with normal drying.

Caribou Growing Note: Let the Tops Fall Naturally

Onions tell you when they are finishing their growing season. Allow the tops to soften and fall naturally rather than forcing them down. Harvest carefully and separate damaged or bolted onions for earlier use.

Read More | When to Harvest Vegetables: Beginner Timing Guide

How to Cure Onions After Harvest

Curing allows onion necks and outer skins to dry, helping reduce moisture loss and decay during storage. It is especially important for varieties intended for long keeping.

1. Harvest carefully: Lift mature onions without damaging the bulbs.
2. Choose a dry location: Use a sheltered, well-ventilated area protected from rain.
3. Spread bulbs out: Arrange in a single layer with space for airflow.
4. Allow adequate drying: Cure until the necks are thoroughly dry and the outer skins become papery, often approximately 2–3 weeks.
5. Trim and sort: Once dry, remove excess roots and tops if desired, and separate damaged or soft onions.

Do not wash onions intended for curing. Avoid damp, poorly ventilated locations that encourage rot.

Ailsa Craig and Red of Florence are primarily fresh-use onions. Although they may benefit from surface drying after harvest, curing does not turn them into long-storage varieties.

Storing Onions for Winter

Properly cured storage onions can keep for months under suitable conditions, but storage life depends heavily on the variety.

• Temperature: Store in a cool location, ideally around 0–4°C for suitable storage varieties, without freezing.
• Humidity: Keep the storage environment relatively dry compared with conditions used for many root vegetables.
• Airflow: Use mesh bags, ventilated crates, or other breathable storage containers.
• Light: Keep onions in a dark location.
• Inspection: Check periodically and remove any bulbs that become soft, mouldy, or damaged.

Sweet onions generally do not store as long as firm, pungent storage varieties. Use Ailsa Craig and Red of Florence relatively soon after harvesting, and grow a dedicated storage onion if winter keeping is your priority.

Read More | How to Store Fresh Vegetables After Harvest

Frequently Asked Questions About Growing Onions

Are onions easy to grow from seed?

Yes. Onion seeds germinate readily under suitable conditions, but producing large bulbs requires early starting, appropriate varieties, and consistent care.

When should I start onion seeds indoors in Canada?

Approximately 10–12 weeks before your average last spring frost. In Southern Ontario, this commonly means February or early March.

What onions grow best in Canada?

Long-day onions are generally well suited to most Canadian growing regions. Check the specific variety's day-length requirements and maturity period.

How deep should I plant onion seeds?

Approximately 6 mm (¼ inch) deep.

How long do onion seeds take to germinate?

Usually 7–14 days under favourable temperatures and moisture conditions.

Can onions tolerate frost?

Established onions tolerate cool weather and light frosts, but young plants may be damaged by severe or prolonged freezing.

Should I trim onion seedlings?

Routine trimming is unnecessary. Strong light and healthy growing conditions are more important than keeping foliage short.

Why are my onions not forming bulbs?

Possible causes include unsuitable day-length requirements, insufficient plant maturity, poor growth, or incorrect planting schedules.

Why are my onions small?

Late planting, overcrowding, water stress, weeds, inadequate fertility, and unsuitable varieties can all reduce bulb size.

Why are my onions flowering?

Premature flowering, or bolting, can occur when sufficiently mature plants experience temperature conditions that trigger their reproductive cycle.

Can I grow onions in containers?

Yes. Use a wide, well-draining container approximately 20–30 cm deep or deeper and allow sufficient spacing for bulb development.

When should I stop watering onions?

Reduce supplemental irrigation as tops naturally fall and bulbs approach harvest. Avoid waterlogging mature bulbs.

How do I know when onions are ready to harvest?

Look for naturally falling tops, yellowing foliage, softening necks, and mature bulb size.

How long should onions cure?

Often approximately 2–3 weeks, until the necks are dry and the outer skins are papery. Conditions and varieties affect the time required.

Can Ailsa Craig onions be stored all winter?

Generally not. Ailsa Craig is a large, mild, sweet onion best used relatively soon after harvest.

What is special about Red of Florence onions?

Red of Florence is a traditional Italian heirloom with elongated red bulbs and a mild flavour, especially useful for fresh eating, slicing, and pickling.

Grow Onions from Seed Today

Growing onions from seed is a rewarding way to produce flavourful bulbs suited to your kitchen and garden. Start seeds early, choose varieties appropriate for your region, transplant into fertile soil, and maintain healthy leaf growth before bulb formation begins.

Caribou Seed Company offers two distinctive onion varieties for Canadian gardeners: Ailsa Craig, valued for its large, mild yellow bulbs, and Red of Florence, prized for its elongated red bulbs and sweet flavour.

Featured Onion Seeds

	Ailsa Craig Onion Seeds A traditional long-day onion producing large, mild, sweet yellow bulbs. Excellent for fresh eating and everyday cooking.
	Red of Florence Onion Seeds A distinctive Italian heirloom with elongated red bulbs and a mild, sweet flavour. Ideal for salads, slicing, pickling, and cooking.

Download | How to Grow Onions in Canada | Complete Growing Guide (PDF)

See You in the Garden!