How to protect your outdoor plants in the winter

Close up of frost on a plant

Winter is here, and you’re cold. But you’ve got your fuzzy pajama pants, your favorite hoodie, space heater, and YakTrax. You’re settled in for the blustery winds and freezing rains. But what about your outdoor plants?

During these cold months, the most vulnerable part of a plant is the root system. The roots, whose job it is to absorb water and nutrients from the soil and supply the plant with the hydration, vitamins, and minerals it needs to grow stronger, are most susceptible to freezing temperatures and the cellular destruction they cause.

To understand why freezing causes cellular destruction, we first need to understand what happens to water when it freezes.

Something fascinating you didn’t know about water

I know, I know. You didn’t sign up for a physics lesson. But here’s something fascinating you didn’t know about water. It behaves like no other liquid on Earth, and that’s what’s keeping all of us (and our plants) alive.

When most liquids freeze, they shrink because the crystalline structures formed during the freezing process are tightly packed. But when water starts to freeze, it goes through a contraction/expansion cycle that ends in expansion.

That’s because when water molecules start to lock into their crystalline lattice structure, their hydrogen bonds keep them spaced apart. That leaves open spaces in the structure.

That’s nice, but what does this have to do with anything?

If your plant’s roots freeze, it means that the water around and inside them has locked into a jagged crystalline structure. That slices them up inside and out, compromising their ability to take in nutrients from the soil and maintain the plant’s health.

Freezing one time might not damage them enough to kill the plant, but freezing and thawing repeatedly will.

If you want to keep outdoor plants alive during the winter, you’ve got to keep some heat at the roots.

How to defend your plants against the freeze/thaw cycle

It’s all about slowing down the loss of heat accumulated from sunlight. Heat takes the path of least resistance, so the more obstacles you put in its way, the slower it travels. To keep more heat trapped at the ground level longer, you have to create many layers of air pockets separated by a multitude of barriers.

Think about it like this. If you need to get to a destination 15 miles away, drive through the city, and get stopped at every light, it’s going to take longer to get there than if you took the freeway. Even if the freeway route is a greater distance.

The air pockets in the insulating material are like the stretches of road between stops, and every place the pieces of material come into contact with each other are the traffic lights.

How to insulate potted plants

The first thing you need to do is figure out which kinds of pots will be appropriate. You’ll want to use those made from materials that won’t crack. Here is HGTV’s breakdown on all-weather gardening pots if you could use a few tips to make your selection.

Push them together so they help shield one another. Like penguins. Penguins even take turns in the middle, so maybe your plants would like it if you occasionally rotated them.

Wrap them in old sheets, towels, or clothes. Some people suggest bubble wrap, but others insist on avoiding plastic, as it will retain too much moisture and could cause mold to grow. Something that breathes and will hold layers of air to trap heat is ideal.

Soil is also a good insulator. You could bury the plants, pots and all.

Make sure the pots have enough drainage and water them adequately. If the soil is dry, frost will penetrate more deeply into it.

What to do if you have planters

Here’s a post from Lindsay Stephenson’s blog about using foam core to insulate roots against the surrounding cold air. Then again, just because I like playing devil’s advocate, not every expert gardener agrees that polystyrene necessarily keeps plants warmer.

Some, like Larry Hodgson of Laidback Gardener, argue cold will make it to the roots anyway and that the foam’s real purpose is to slow down the loss of heat long enough to prevent freeze/thaw cycles.

It’s true that the purpose of any insulation is to create lots of air spaces separated by barriers that slow down loss of heat and foam certainly does that. But there’s another thing worth considering.

Polystyrene off-gases chemicals that get absorbed by whatever’s in contact with it. If the chemicals leech into the soil and the plant’s roots, that’s not great for your plants. I wouldn’t eat polystyrene packing peanuts. And I wouldn’t give them to my dog. So I wouldn’t want to feed them to my plants, either.

How to insulate plants in the ground

Some people go all out and build insulating structures filled with material around each plant. Here’s an example if you like making things and want to check that out.

For others, simply covering the ground to protect the root system suffices. Straw, mulch, and burlap all work well for that.

There are enough simple and inexpensive solutions that you can find something that works for you, whether you have a little space and time or a lot.

As thorough as we try to be, we could have left something out. Have a favorite insulating technique I haven’t mentioned? Share it with your fellow gardeners and us in the comments.

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