Protecting Sprouts from Frost

Early this morning, I saw frost on my neighbors’ roofs, and a light smattering of it on my lawn. And my birdbath water had a very thin layer of ice.

Should I be worried?

Icy Birdbath

Ice in the birdbath

If I have planted any vegetable seeds outdoors that are now breaking through the soil, yes, I should be concerned.

A bit of frost (see more about this fascinating weather feature at the end of this post) like we’re having now is not a big deal for established plants, and it’s certainly no problem for vegetables under season extension devices like cloches or cold frames. But if you’re like me, you get excited about the warm days — hey, it must be planting time, it’s in the 50s! — and you get out there sprinkling seed onto the soil. But then reality hits.

It’s simple enough to protect your freshly-seeded bed, even at the last minute on a Sunday night at dusk. If you don’t want to mess with the cloche, you can even just roll out a layer of floating row cover right on the soil. (And if you don’t have floating row cover, please get some. It’s one of the prime tools of vegetable gardening in our climate, along with a soil thermometer.)

Protecting the seedlings

Your goal is to cover the plants when they first poke through the soil, when they are the most vulnerable.

But consider also that if you put something over them like a cloche or cold frame, it may have the revese effect of a frost if not treated carefully. If you don’t vent the device on one of these warm, sunny days, the soil around the seedling may dry out too fast, or the air under the device may get too warm, either of which could kill the baby plants.

FRC on peas

Floating row cover over a row of peas will protect from frost. It is laid loosely on the bed with the edges tucked under a bit of soil.

This effect will not happen with floating row cover, which lets air and water through, but does not dry out the soil.

FRC closeup

Floating row cover is a very thin, spun-bonded (not woven) polyester cloth.

Whether it’s a bout of frost from surprisingly cold nights, or an equally surprising bout of solar energy from warm, sunny days, give your seedlings a stress-free start with a little blanket of appropriate protection.

 

Frost date calculations

I shouldn’t be surprised that we might have a bit of frost here in Seattle in March. After all, our last frost date this year is not until April 6. I like the frost-freeze calculator at Dave’s Garden.com, which also tells me that “almost certainly, you will receive frost from November 13 through March 19.” Put in your Zip code and see what your frost dates are.

 

What is frost?

I dug into the science of frost and freeze when I was researching Cool Season Gardener, and it is fascinating.

The common type of frost that dusts our lawns and gardens is “radiational” frost, also called ground frost or hoar frost. It occurs through condensation of moisture when the ground cools faster than the air temperature and ground temperature drops quickly below the freezing level of 32 degrees F. (The other type of frost, advection frost, occurs with lower ground temperatures and cold winds. This is also known as akilling frost.)

Frosty Lawn

Coming out of winter, our garden soil is pretty cool. In my raised beds it’s in the 40s, and under the cold frame it’s in the 50s, but in cold microclimates or low-lying areas, the ground is cooler.

When the sun does come, it often does not stick around all day, and it takes consistently warmer days to eventally warm the soil. When that sun goes away and the temperature plunges, as it does on clear nights around here, the soil temperature sinks back to its wintry levels, with cold radiating up into the warmer air.

Moisture on the soil’s surface is caught in the middle, between the cold soil and warmer air, and water droplets turn to frost crystals.

Although this description only skims the surface of the topic like a morning dew, I recommend a book like Cliff Mass’ “The Weather of the Pacific Northwest” or weather websites to dig more deeply into this engaging topic.

 

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