Indoor Seed Starting for a Bountiful Fall Garden
As summer heats up, gardeners who want to maximize their harvest should already be looking ahead to fall. While it might seem counterintuitive to talk about the fall harvest while you're still sweating through July, getting the timing right for planting fall crops starts now.
Indoor seed starting plays a crucial role in the success of a fall garden. Unlike spring, fall growing conditions are affected by shortening daylight hours, decreasing temperatures, and the looming threat of frost. These variables mean one thing: timing is everything.
To get started, determine your average first frost date. In most areas, this is your countdown clock. If you don’t get frost, use the date when your daylight hours dip below 10 hours per day. From there, apply the fall buffer: add extra time to your seed packet’s “days to maturity” to account for slower growth in cooler conditions.
Planting Window Before Frost —> DTM Adjustment
> 11 weeks —> +10% buffer
8–10 weeks —> +20% buffer
5–7 weeks —> +40% buffer
3–4 weeks —> +70% buffer
For example, spinach labeled at 45 days might actually need 63 days to mature in fall. That’s because as daylight dips below 10 hours (known as the Persephone period), and the weather gets colder plant growth significantly slows.
45 days = about 6.5 weeks. That’s 5–7 weeks zone → 40% buffer. 45 + (0.4 * 45) ≈ 63 days
So, plan your transplant dates and indoor seed starting accordingly!
Short-season gardeners (zones 3–4) should start even earlier and rely on season extenders like row covers and cold frames. For everyone else, success lies in the countdown:
First frost date —> subtract days to maturity (with buffer)
Subtract hardening-off time (yes, you still have to do this in summer!)
Subtract time to grow seedlings indoors
By layering in succession sowing and choosing fast-maturing crops, your fall garden can be just as productive—and more enjoyable—than your spring one.
👉 Ready to dive deeper? Listen to the full episode here.
Need help with indoor seed starting? My Seed to Sprout course is just the help you need!
Your Friend in the Garden,
