Seed starting indoors: a frost-date approach
Starting seeds indoors exists to solve one problem: in most of Canada the outdoor growing season is shorter than many crops need. A tomato or pepper sown directly outdoors after the last frost often will not ripen before the first fall frost. Growing the plant indoors for several weeks first closes that gap.
Start from your frost date, then count back
The single number that drives indoor sowing is the average last spring frost for your location. Seed packets list a recommended sowing time as a number of weeks before that date. You estimate your frost window from regional climate data, then count backward.
Frost dates are averages, not guarantees. A late cold snap can occur after the average date, which is why hardening off and watching the short-range forecast matter as much as the calendar.
As a working example of how the counting works, not as fixed advice for your site:
| Crop | Typical indoor lead time | Notes |
|---|---|---|
| Tomato | 6–8 weeks before last frost | Warmth-loving; needs strong light early |
| Pepper | 8–10 weeks before last frost | Slow, warmth-dependent germination |
| Lettuce | 3–4 weeks before last frost | Tolerates cool conditions; transplants young |
| Basil | 4–6 weeks before last frost | Very frost-sensitive; wait for settled warmth |
Confirm the lead time on your own seed packet. Suppliers test for the conditions they sell into, and the figure on the packet is more specific than any general table.
Medium and containers
Seeds are usually started in a light, sterile seed-starting mix rather than garden soil, which is heavy and may carry pathogens that damage seedlings. The mix holds moisture while staying open enough for fragile roots and air.
A basic indoor setup
- Fill clean cell trays or small pots with a moistened seed-starting mix and firm it lightly.
- Sow at the depth printed on the packet; many small seeds need only a shallow covering.
- Cover loosely to hold humidity until germination, then remove the cover once shoots appear.
- Move seedlings under bright light immediately so they do not stretch.
Light is the usual bottleneck
Indoor seedlings started in late winter face short days and a low sun angle. A south-facing window is rarely enough on its own at that time of year, and seedlings respond by stretching toward the light with thin, weak stems. Many growers add a supplemental light placed close above the seedlings and run it for a long daily period.
Leggy, pale seedlings almost always signal too little light rather than too little fertilizer. Address the light first.
Common reasons seedlings fail
- Damping off: a fungal collapse at the soil line, encouraged by overwatering and stagnant air. Good drainage and airflow reduce the risk.
- Stretching: too little light, as above.
- Stalling after transplant: often a sign seedlings were moved out without hardening off.
Once seedlings are sturdy and the outdoor frost risk is passing, the next step is the gradual transition outdoors, covered in the hardening off and transplanting note. If you are growing on a balcony, container choices are discussed in the balcony container gardening note.
References
- Agriculture and Agri-Food Canada — climate and growing-season information: agriculture.canada.ca
- Environment and Climate Change Canada — Canadian climate normals: climate.weather.gc.ca