Step 1: Know Your Last Frost Date
Everything starts with your local last frost date. In much of Ontario, that falls somewhere between late April and mid-May, depending on where you live.
The general rule is simple:
Count backward from your last frost date.
Most seed packets will say something like “start indoors 6–8 weeks before last frost.” That’s your guide.
For example:
-
Tomatoes: 6–8 weeks before last frost
-
Peppers: 8–10 weeks before last frost
-
Broccoli & cabbage: 6–8 weeks
-
Lettuce: 4–6 weeks
If your last frost date is around May 10th, that means:
-
Tomatoes get started mid-March
-
Peppers start early March
-
Lettuce can wait until early April
Starting too early is the most common mistake I see. Bigger isn’t better. Healthy and sturdy is what you’re after.
Step 2: Choose the Right Seeds to Start Indoors
Not everything needs a head start.
Seeds that benefit from indoor starting:
-
Tomatoes
-
Peppers
-
Eggplant
-
Broccoli
-
Cabbage
-
Cauliflower
-
Most annual flowers
Seeds that usually do better direct-sown outdoors:
-
Carrots
-
Beans
-
Peas
-
Corn
-
Radishes
Root crops especially don’t like being disturbed. I’ve learned over the years that it’s better to let them grow where they’ll finish.
Step 3: Use Proper Seed Starting Mix
Garden soil is too heavy for seedlings. It compacts, drains poorly, and can introduce disease.
Use a light, sterile seed-starting mix. It holds moisture without becoming soggy and allows delicate roots to develop properly.
Fill your trays or containers loosely. Don’t pack the soil down. Seeds need oxygen as much as they need moisture.
Step 4: Plant at the Right Depth
Here’s a simple rule I’ve followed for decades:
Plant seeds at a depth about twice their size.
Tiny seeds? Press them gently into the surface.
Larger seeds? Cover lightly.
If you plant too deep, they struggle to emerge. Too shallow, and they dry out.
After sowing, mist lightly or water gently from the bottom so you don’t wash the seeds around.
Step 5: Light Is Not Optional
This is where many beginners go wrong.
A bright window is often not enough in late winter. Seedlings stretch toward light and become tall and weak — what gardeners call “leggy.”
If you’re serious about seed starting, invest in a simple grow light. It doesn’t have to be fancy.
-
Keep lights 2–3 inches above seedlings
-
Provide 12–16 hours of light daily
-
Raise the light as plants grow
Strong light equals strong stems.
Step 6: Keep Moisture Consistent (Not Soaked)
Seeds need consistent moisture to germinate. Once they sprout, the goal shifts slightly.
You want soil that feels like a wrung-out sponge — damp, not dripping.
Bottom watering works best:
-
Place trays in a shallow pan of water
-
Let them absorb moisture from below
-
Remove once the surface is damp
This encourages roots to grow downward and reduces fungal problems.
Step 7: Maintain Proper Temperature
Most seeds germinate best around 20–24°C.
If your house runs cooler, a simple heat mat can improve germination rates dramatically, especially for peppers and tomatoes.
Once seedlings emerge, slightly cooler temperatures help prevent legginess.
Step 8: Thin Your Seedlings (Even Though It Feels Wrong)
When multiple seedlings pop up in one cell, it’s tempting to let them all grow.
Don’t.
Crowding leads to weak plants competing for light and nutrients. Snip off the extras at soil level and leave the strongest one.
It feels ruthless the first few times. After a few seasons, you’ll understand why it matters.
Step 9: Harden Off Before Moving Outdoors
Indoor-grown plants can’t go straight into the garden.
About a week before transplanting:
-
Place seedlings outside for an hour or two
-
Increase exposure daily
-
Protect from strong wind and harsh sun
This process, called hardening off, prevents transplant shock.
After forty years, I still follow this step carefully. It’s the difference between plants that stall and plants that thrive.
Common Seed Starting Mistakes to Avoid
Over the years, I’ve seen the same errors repeated:
-
Starting too early
-
Not enough light
-
Overwatering
-
Skipping hardening off
-
Using garden soil instead of seed mix
Keep it simple, follow the timing, and resist the urge to rush the season.
Final Thoughts
Starting seeds indoors isn’t just about getting a jump on spring — it’s about participating in the season before it officially begins.
There’s something grounding about watching those first green shoots push through the soil while snow still covers the ground. It reminds you that growth is always happening, even when it’s quiet.
If you respect timing, provide strong light, and keep conditions steady, your seedlings will reward you with stronger plants and a more productive garden come summer.
After decades in the garden, I can tell you: the best gardens don’t begin in May. They begin on the kitchen table in March.