- Most seeds germinate best between 18–24°C; cold soil is the most common reason seeds fail.
- Planting depth matters: many small seeds need light and should not be buried.
- Overwatering causes more seed failure than underwatering, especially indoors.
Seed germination is often treated as a mystery, but it’s a predictable biological process governed by a few key factors: temperature, moisture, oxygen, and light. When one of these elements is missing or out of balance, seeds fail to sprout — leading to frustration and wasted seed.
This guide breaks down how germination works, why different seeds have different requirements, and how Canadian growers can optimize germination indoors and outdoors.
The Science of Germination
Germination begins when a seed absorbs water, activating enzymes that trigger growth. The seed coat softens, roots emerge first, and shoots follow once conditions are right.
Each species evolved to germinate under specific environmental cues that signal survival conditions.
Temperature: The Most Important Factor
Temperature determines whether seeds remain dormant or begin growing.
- Cool-season crops germinate in cooler soil
- Warm-season crops require consistently warm temperatures
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Too much heat can prevent germination entirely
Typical Germination Temperature Ranges
| Crop Type | Ideal Soil Temperature |
| Lettuce | 10–18°C |
| Peas | 7–16°C |
| Tomatoes | 21–27°C |
| Peppers | 24–30°C |
| Basil | 20–25°C |
Cold Stratification
Some seeds require a period of cold, moist conditions before they will germinate. This process, called cold stratification, mimics winter. In nature, these seeds fall to the ground in autumn, experience freezing and thawing over winter, and sprout in spring when conditions are favourable.
Without this cold period, certain perennial flowers, trees, and native plants may remain dormant for months — or fail to germinate entirely.
Cold stratification works by softening the seed coat and breaking internal dormancy mechanisms that prevent premature sprouting.
How to Cold Stratify Seeds
There are two common methods:
1. Natural outdoor stratification
Sow seeds outdoors in late fall. Winter weather will provide the required cold exposure naturally.
2. Refrigerator method (controlled stratification)
• Moisten a paper towel or small amount of seed-starting mix
• Place seeds inside and seal in a labelled plastic bag or container
• Refrigerate at 1–5°C for 2–6 weeks (depending on species)
• Check periodically to ensure the medium remains slightly moist
• After the required period, sow seeds as directed
Do not freeze seeds unless specifically recommended. Most stratification requires cold, not deep freezing.
Examples of Seeds That Require Cold Stratification
Many perennial and native species benefit from or require stratification, including:
There are also annual flower seeds that benefit from cold stratification. Here are some examples:
- Bells of Ireland
- Bupleureum
- Larkspur
- Nigella (Love In The Mist)
- Poppies
- Snapdragons
- Sweet Peas
- Verbena
Always check seed-specific instructions, as stratification time varies by species. Some seeds benefit from 2–4 weeks of cold exposure, while others may require 8 weeks or longer.
Light vs Darkness
Some seeds need light to germinate, while others require darkness.
- Light-required seeds: Lettuce, petunia, snapdragon
- Darkness-required seeds: Beans, peas, corn
This is why seed depth matters — planting too deep or too shallow can prevent germination.
Moisture and Oxygen Balance
Seeds need moisture but also oxygen. Overwatering can drown seeds by displacing air in the soil.
Best practices:
- Keep soil evenly moist, not wet
- Use well-draining seed-starting mix
- Avoid compacting soil
Indoor vs Outdoor Germination in Canada
Indoor seed starting offers temperature control, while outdoor sowing relies on seasonal timing.
Indoor sowing works best for:
- Long-season crops
- Heat-loving plants
- Early spring starts
Outdoor sowing suits:
- Cold-hardy vegetables
- Fast-growing greens
- Crops that dislike transplanting
Common Germination Problems (and Fixes)
- Seeds rot before sprouting → soil too wet
- Slow or uneven germination → temperature too low
- Seedlings fail after sprouting → insufficient light
Final Thoughts
Understanding germination removes guesswork from gardening. When seeds receive the conditions they evolved for, success rates improve dramatically.
FAQs
Why do some seeds take longer to germinate than others?
Germination time depends on seed type, temperature, moisture, and light exposure. Hard-coated seeds and cool-season crops often take longer than warm-season vegetables.
What is the ideal temperature for seed germination?
Most vegetable seeds germinate best between 18°C and 24°C. Cool-season crops may sprout at lower temperatures, while heat-loving plants like tomatoes require warmer conditions.
Do seeds need light to germinate?
Some seeds require light, some require darkness, and others are indifferent. Always check seed-specific instructions, as planting depth plays a major role in successful germination.
Can I start seeds indoors too early in Canada?
Yes. Starting seeds too early can result in leggy, stressed plants if they outgrow their containers before outdoor conditions are suitable. Timing should align with your local frost dates.
Why do seeds sprout but then die?
This is often caused by damping-off disease, inconsistent moisture, poor airflow, or insufficient light. Clean containers and proper growing conditions greatly reduce risk.
