Ontario · Growing Zone 4a
Gardeners in Thunder Bay, ON can expect the average last spring frost around May 28, and the first fall frost around September 25, based on historical Environment Canada frost data for the area. That gives Thunder Bay a frost-free growing season of roughly 120 days. Actual frost dates shift by a week or two year to year, so use this as a planning baseline rather than an exact guarantee, and always check your local forecast before moving tender plants outside.
You have your Thunder Bay frost dates. Now put them to work: GrowBlock counts back your indoor seed-starting weeks, marks your safe planting day, warns you before a frost, and lays your whole season out on one timeline.
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Most warm-season crops (tomatoes, peppers, squash) need transplanting only after the last frost has passed, and are usually started indoors 6 to 8 weeks before that date. Cold-hardy crops (peas, lettuce, spinach) can often go in the ground a few weeks before the last frost. As Thunder Bay's first fall frost approaches in September, it's time to plan the last harvests for anything frost-sensitive still in the ground.
Approximate dates for common crops in Thunder Bay, worked out from the local last frost around May 28. Warm crops go out after frost, cold-hardy crops a few weeks before it, and anything with an indoor start is counted back from its sow date.
| Crop | Start indoors | Sow / transplant out | Approx. harvest |
|---|---|---|---|
| 🍅 Tomato | April 23 | June 11 | August 30 |
| 🌶️ Pepper | April 9 | June 11 | August 25 |
| 🥒 Cucumber | — | June 4 | August 1 |
| 🥬 Zucchini | — | June 4 | July 24 |
| 🥬 Lettuce | — | May 7 | June 21 |
| 🥬 Spinach | — | May 7 | June 24 |
| 🫛 Peas | — | May 7 | July 8 |
| 🥕 Carrots | — | May 7 | July 14 |
| 🫘 Bush beans | — | May 28 | July 17 |
| 🥬 Kale | April 9 | May 7 | July 1 |
Guideline dates from Thunder Bay's average frost window, not a guarantee. GrowBlock tunes these to each variety you plant and adjusts as you build your beds.
This calendar is a starting point. In GrowBlock every date is tuned to the exact varieties you plant and updates as you lay out your beds, and you can share the plan with whoever you garden with, changes show up for both of you.
Build your Thunder Bay garden planAround May 28 on average, though it can vary by a week or two either way depending on the year.
Thunder Bay is in zone 4a, with a frost-free season of about 120 days.
Now that you have your dates, here is how to put them to work through the season:
More how-to on the GrowBlock gardening blog.
See frost dates for all Canadian cities covered by GrowBlock.