Keeping plants warm during cold snaps with Instant Heat Paks

anney(Georgia 8)May 9, 2007

I think the cold weather is over in my part of the US (Georgia) for this season, but last week we had a temp drop to the low 40s two nights in a row. I'd already set out my tomatoes, squash, cukes, and melons the week before, since our last frost date is supposed to be around April 15.

Though many people on the Tomato forum said the tomato plants would be okay, I was still nervous about the others. I covered the small plants with large cups and larger ones with an old curtain, and they did okay, though the squash looked a little wilted. They perked up in the daylight sunshine and are fine now.

But I did some thinking about how to protect plants when we have these unexpected temperature drops, and came up with a couple of ideas I'm laying out here, not sure whether they're workable or not.

First, Mylar. It reflects both light and heat, but I was interested in its heat-reflecting properties. You can get cheap Mylar Emergency Blankets on Amazon, ten for $10.99 plus shipping. They measure 52" x 84". Sporting goods stores should carry them, too. Ten of them would make a LOT of plant covers in the form of hot-caps or even boxes lined with the material. You'd need to put the Mylar on the inside of the protector with the heat-reflecting side toward the plants. It's so light-weight that in a pinch you could probably also just toss it over the plant and hold it down with rocks or anything heavy enough to keep it from blowing away.

So, okay, this might protect the plants somewhat. But what about a heat source for the plants? Sometimes, these cold snaps are likely to defeat gardeners' efforts to protect plants by just covering them. Some folks run lights to their plants on cold nights and use the warmth of the lights to protect their covered plants.

So I thought about a heat source that didn't involve running electric lights out to the plants and suddenly remembered Body Warmers or heat packs! Those at the link are made in China but the site gives more information than some other sites. You can also get them on Amazon. They come in various sizes to warm hands and feet, and I think aching backs now. They also last anywhere from ten to 18 hours.

Here's how they work for the scientifically-inclined. The "freezing" referred to here is not the freezing of water at 32 degrees F but sodium acetate's "freezing" at 130 degrees F -- that's when it begins to solidify and lose its 130 degree heat. That's when it gradually cools down. Here's how they explain it.

From the link: "Our magic heating pads contain sodium acetate and water. It turns out that sodium acetate is very good at super cooling. It "freezes" at 130 degrees F or 54 degrees C, but it is happy to exist as a liquid at a much lower temperature and is extremely stable. Clicking the disk, however, has the ability to force a few molecules to flip to the solid state, and the rest of the liquid then rushes to solidify as well. The temperature of the solidifying liquid jumps up to 130 degrees F or 54 degrees C in the process."

The disk referred to is contained in reusable heating packs. To activate the heat packs, you manipulate the disk. To reuse the packs next time, all you do is boil them.

When you boil the solid you melt it back to the liquid state. You have to completely melt every crystal or the liquid will quickly re-solidify. You can repeat this cycle thousands of times, just as you can freeze and melt water as many times as you like."


Not all body-warmers/heat packs are the same. I haven't been able to find any reusable ones that emit heat for 12 hours, which is what the gardener would probably want. The 12 hour ones are cheaper but work only one time as far as I can tell. IOW, you can't reactivate them to use again.

NOW! All that how-to and information aside, how do you think a heat-pack placed near one's tender plants and then the whole plant and heat pack covered with Mylar would work?

Remember, the heat packs activate at 130 degrees F and then gradually cool down during the ensuing 12 hours. If placed in the vicinity of but not touching the plant in cold weather and then covered with some sort of Mylar "tent" to prevent heat-escape, the initial 130 degrees would disperse into the space and earth beneath and warm the surrounding area, though not to 130 degrees F. With that much to heat, the entire enclosed space certainly wouldn't be anything like 130 degrees at any time and would gradually cool down.

What do you think? Anybody in the Northern states that might still have cold snaps want to try it?

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yukichan(z9 FL)

It sounds very intriguing. I am considering growing orchids outdoors permanently attached to tree limbs. Your idea sounds like it could work for the 2-3 nights a year when the weather dips below 40 degrees in my part of Florida. I could hang the heat pack from a hook under the mounted orchids and tent the whole area with the mylar blanket overnight. Unfortunately, I'll have to wait seven or eight months to try it.

    Bookmark   June 28, 2007 at 1:33PM
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