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Waking at 1 am, I get my shoes on to run out to my greenhouse. I've had a heck of a time getting my garden in this year. I've lost quite a few plants to 'pop' freezes and I'm probably getting paranoid.

The weather report said the low would be 40 degrees F but the sky is clear and it feels very cold. In the greenhouse the thermometer reads 39 degrees. My emergency heater is low on propane. I've got no row cloth left. I check the temperature again and now it says 37 degrees.

There are squash, sweet potatoes, bean plants that are very vulnerable. There are artichokes, salad stuff galore, cabbage, cauliflower, broccoli that are huge and beautiful but a freeze will set them back. What should I do?

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    Do some jumping jacks in the greenhouse? If it's small enough it could warm the temperature a few degrees. – Philip Jun 19 '14 at 8:17
  • How about floating row covers? They will retain some heat. – kevinsky Jun 19 '14 at 10:04
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    How large is your greenhouse and does it have/can you get electricity out to it? I'm wondering whether you could use light bulbs to add some heat... – michelle Jun 19 '14 at 14:18
  • This is will be made into a heated greenhouse by next year. We've got very little electric service on this property and I've got one little heated greenhouse for my warm season starts and that gets the heater for now. We live in a yurt with wood heat...Light bulbs would be a great idea! Oh and it's 540 sq. ft. right now. By the way, it DID freeze that night and last night as well... – stormy Jun 19 '14 at 16:45
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Even if you cannot heat your greenhouse, there is a lot you can do to prevent freezing. On the other hand, though, there is a point after which effort is useless without a heat source. If it is just "pop" freezes, as you call them, and not generally not going below for more than a couple hours, there are several ideas you might like to try:

  • Frost Patrol® Plant Protector is a food-plant safe product I've used in the past, on late season tomatoes and peppers. You spray it over the plant, which absorbs it and put it to work within 24 hrs. The spray basically causes a change within the plant, which lowers the temperature at which the plants cells become damaged by 1 or 2 degrees. Spray it 48 hours before you expect frost (maybe every other day for you).

  • Lightweight cotton bed sheets are insulating and catch frost well. I've used these in emergencies where a sudden unforcast frost slips up on me and my row cloths are on the row crops. They are also nice because they lay fast and stay where you put them.

  • In an emergency, if you have a long enough electric cable to get power into the greenhouse, you can try using a small inexpensive electric heater. I know a guy who did this, and he said you have to distribute the air quickly so you don't overheat the plants. He placed the heater directly behind a large fan, and aimed that at the ceiling. The entire thing was put on a small timer and set to go 15 minutes to the hour.

  • As michelle said, high wattage incandescent bulbs will produce some heat. Do not install these within 3' of the plants, or damage may occur from inconsistent temperatures.

  • Don't know if this is an option, but I knew someone who would put a slow cooker full of water in their greenhouse on some nights, and simmer it all night.

You can use a combination of these if you feel it would be helpful. Growing tomatoes, sweet potatoes, and beans in an unheated greenhouse in your climate is something of a gamble. As long as you catch each freeze as it comes, though, you'll probably pull through. Also, the healthier a plant is, the better it will cope with cold stress. Keeping your plants in top condition (which it sounds like you're doing) is something to keep in mind. Also, you mentioned being out of row cloth. That is something you can fix right away. Row cloth is something you never can get too much of (unless you're a kleptomaniac).

  • Excellent answer!! Can you use the Frost Patrol on edibles? How exactly does this stuff work? – stormy Jun 19 '14 at 21:11
  • I used newspaper...I had to use the little stuff similar to Little Nickel Ads. Newspaper is an incredible insulation properties. I use newspaper before row cloth for freezing. But row cloth would be easier. A little burn on my beans and basil but that was it after 2 freezes. I will be redoing this greenhouse with automatic everything! I want to be able to go fishing someday!! – stormy Jun 19 '14 at 21:20
  • @stormy Yes, frost patrol was designed partly with edibles in mind. It works partly by changing the temperature at which the plant becomes damaged, and partly with frost resistant properties that remain on the plant's exterior. 1 20$ bottle makes like 5 gallons of spray. – J. Musser Jun 19 '14 at 21:23
  • @stormy newspaper is harder to lay, and shouldn't get wet, but yes, that's another good insulator. – J. Musser Jun 19 '14 at 21:25
  • When I use it outside in the wind, I do wet it. Works great, once. – stormy Jun 19 '14 at 22:03
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It might be a bit late, but in a pinch you can use a few candles to bump your internal temperature just a bit in a pinch. You would need to protect things from the open flame, but rigging a system with an unused clay pot with a hole in the top (so the candle gets air) and keeping it on a flat surface should work.

Also, you can consider trying to make use of the daytime heat that's generated by saving bottles of water in your greenhouse and letting them act as a small buffer. It won't make a huge difference, but could help you make it over a snap freeze.

  • I actually liked this answer...this person is thinking. I would never have learned that candles produce a wax vapor that... – stormy Jun 21 '14 at 7:11
  • @jmusser do you have any publications to back that up? As a one-off in a sudden freeze I can't imagine you'd get too much vapor build-up and the clay pot or some other type of covering should help collect any vapor coming from the candle. However, I find the reference to house plants intriguing, as my wife does burn candles in the house somewhat often and I'd never thought of it. – Laughing_Jack Jun 23 '14 at 4:37

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