Every year, gardeners make the same mistake: they start seeds too early, end up with leggy seedlings sitting in the window for weeks, and wonder why their transplants struggle. Or they start too late and lose half the growing season.
The fix is simple. You need two pieces of information: your last frost date, and how many weeks each crop needs indoors. Everything else follows from there.
Why Frost Dates — Not the Calendar — Are What Matter
March 1st means something completely different depending on where you garden. In Zone 7b southern England, your last frost might already be behind you. In Zone 4a northern Canada, you might still have ten weeks to go.
Seed starting dates are always calculated backwards from your last spring frost. If tomatoes need 6–8 weeks indoors before transplanting, and your last frost is May 15th, you start tomatoes between March 20th and April 1st. Simple math — but only once you know your actual frost date.
Frost Dates by Zone
These are approximate last spring frost dates. Your specific city may vary by one to three weeks depending on elevation, proximity to water, and local microclimates.
| Zone | Approximate Last Frost | Example Locations |
|---|---|---|
| Zone 3 | Late May – Early June | Northern Prairie Canada, Northern Minnesota |
| Zone 4 | Mid to Late May | Calgary, Winnipeg, Vermont |
| Zone 5 | Early to Mid May | Toronto, Denver, Chicago |
| Zone 6 | Mid April – Early May | Vancouver, Philadelphia, London (UK) |
| Zone 7 | Late March – Mid April | Victoria BC, Tennessee, much of UK/Ireland |
| Zone 8 | Mid February – Late March | Pacific Northwest coast, Southern UK |
For your exact frost dates, GrowSmart calculates them automatically based on your city — start for free here.
What to Start Indoors and When
Here's when to start the most common crops, working backwards from your last frost date.
10–12 Weeks Before Last Frost
- Onions and leeks — slow growers, need a head start
- Celery and celeriac — very slow germination, needs warmth
- Peppers — all varieties need a long indoor season, especially in cold zones
8–10 Weeks Before Last Frost
- Eggplant — needs warmth and time
- Lavender — slow from seed
- Lisianthus — extremely slow, start early
6–8 Weeks Before Last Frost
- Tomatoes — the most common mistake is starting these too early; 6–7 weeks is usually right
- Tomatillos and ground cherries — same family, same timing as tomatoes
- Broccoli, cauliflower, cabbage — cool-season brassicas, can go out early
- Brussels sprouts — start at 8 weeks; they need a long season
4–6 Weeks Before Last Frost
- Basil — starts fast but is frost-tender; don't rush it outdoors
- Cucumber, zucchini, squash — fast growers, don't need a long head start
- Melons and watermelon — same as cucumbers; starting too early makes them root-bound
- Kale, Swiss chard, lettuce — can also be direct sown, but indoor starts give you a jump
2–4 Weeks Before Last Frost
- Pumpkins — fast growing, start late to avoid overgrown transplants
What to Direct Sow Instead
Not everything needs to be started indoors. These crops actively dislike transplanting and should go straight into the ground:
- Carrots, beets, parsnips, radishes, turnips — tap roots don't transplant well
- Peas and beans — direct sow as soon as soil can be worked (peas) or after last frost (beans)
- Corn — needs to be sown in blocks, direct in the ground
- Dill, cilantro, fennel — bolt quickly when transplanted
- Nasturtiums and sunflowers — easy direct sow, no benefit to starting indoors
The Mistake That Costs the Most Time
Starting seeds too early is more damaging than starting too late. Seedlings that outgrow their containers before it's safe to transplant become rootbound and stressed. A rootbound tomato started 12 weeks early will often be overtaken by a healthy seedling started at 7 weeks.
When in doubt, start on the later end of the recommended window.
Making It Easier
Calculating all of this manually — cross-referencing zones, frost dates, and per-crop timing windows — is exactly the kind of work that GrowSmart handles automatically. Enter your city, add your plants, and the app generates a planting calendar specific to your location and zone.
Try GrowSmart free — no account required to start.
Seed starting windows are based on average last frost dates. Always check your local extended forecast before moving seedlings outdoors.