March is that transitional month where we in the Northeast start easing into deceptively springy weather, tempting us to launch right into all of our indoor sowing. But sometimes we jump the gun, sow too early–or all at once– and end up with leggy, spindly seedlings come planting time.

Germination and growth rates vary plant to plant, which is why you'll find specific sowing instructions on most seed packs. Slow down, read the instructions, and create a schedule for your sowings. Of course, you'll have to know ahead when you'll be planting out, so that you can count backwards and determine the sow date.

If your seed pack says something like, “Sow indoors 4-6 weeks before your last frost,” you’ll need to know: 1) your last frost date and 2) how to create a sowing schedule.

Know your last frost date.

First, find your average last frost date (The Farmer’s Almanac has a calculator here).

Using this tool, our average last frost falls around April 24. But, of course, “average last frost” does not mean “absolutely final frost.” Frosts can and will continue to fall after the average last frost. For cold climate regions like ours, adding at least three weeks to the average last frost date will give a more conservative estimate for safely planting out frost-vulnerable crops. 

Accord, NY: April 24 (average last frost date) + 3 Weeks = May 15 (frost much less likely)

Larry Hodgson (aka “The Laidback Gardener”) calls this calculation the “spring frost-free date” and gives an excellent explainer here.

Hodgson suggests adding one week if you’re in a relatively mild climate, and 2-3 weeks for colder regions. In our area, many gardeners shop for seedlings on Mother’s Day weekend (May 8-10), and plant out soon after. Even so, rogue frosts can sneak past this "frost-free" date–so always check your weather forecast and protect plants with row cover when necessary. 

Create a sowing schedule. 

Using the variety-specific sowing instructions on your seed packs, count backward from the “frost-free" date.

For example, Butterfly Weed includes these sowing instructions: “Sow 8-10 weeks before last frost.” This means, if May 15 is the target planting date, then our sowing date will be between March 7-21. 

For Silverleaf Sunflower, on the other hand, the instructions sound more like this: “Direct sow after threat of frost has passed, or start indoors up to 2 weeks earlier.” If we were to indoor sow our Silverleaf Sunflowers as early as our Butterfly Weed, by May 15 our sunflowers would likely be spindly and weak, maybe not even worth planting out.

So, for healthy seedlings, make sure to read the seed pack instructions and time accordingly.

Okay, but what about outdoor sowing in March?

More March Tasks