We based our illustrated planting guides on “first” and “last” frost. While we've had a few frosts at the seed farm, last night we had our first freeze. What's the difference?
If you live in a climate with a freezing season, you have the pleasure of experiencing light frosts, hard frosts, and freezes. A light frost is when moisture is pulled out of the air due to falling temperatures and forms crystals on the surface of the plants. This can kill some tender plants but other plants are unfazed. When this sparkling magic happens we call it our “first frost” and I go out and take lots of photos and feel dazzled.
The difference between a “light” frost and a “hard” frost is often the length of time that the frost lasts. Light freezes are ephemeral. Hard freezes are enduring. When hard frost happens I practice accepting impermanence but secretly hope that the squash blanketed in row cover live to cure another day and there’s one more week of picking dahlias.
But last night we had a “freeze." That means that temperatures were cold enough to actually start freezing water, not just on the surface of the plants, but within the plants themselves and in the top layer of soil. With a freeze, including a quick freeze, the cells within the plants can burst in a very short amount of time. Some people refer to this as a "killing frost." When this un-sparkly, less than ephemeral, and harbinger of winter happens I turn around, go back inside, make tea, and mumble into the steam rising into my face about not being ready for winter.
How do frosts and freezes affect seeds? Frosts don't usually kill most seeds. This is partly because seeds are higher off the ground and frost tends to settle lower. Freezes can kill seeds from species that originated in warmer climates, like tomatoes. Our goal is to get as much of the seed crop in as possible before the first real freeze.
How do frosts and freezes affect tubers? We let our dahlias continue to bloom as long as possible. We just can't bear to dig them up while they are still sharing their gorgeousness. But as soon as we get a killing frost it's time to let go, say goodbye to the riot of color in the dahlia field, and dig before the freeze gets down into the soil and damages the tubers. We recommend the same for you!
We'll have an upcoming post about digging and storing your dahlia tubers for the winter unless you're lucky enough to live somewhere where they overwinter all by themselves.
To learn how to organize and plan your sowings around the first and last frost, visit our collection of Planting Guide Posters. They're pretty enough to frame and provide week-by-week inspiration for the entire growing season.
