What to do about mild overnight freeze

Denver in October. Usually, nights are in the 40s F. But just tonight (actually early tomorrow morning) the temperature dips below freezing, then goes right back to 40s at night. Also, there may be some rain off and on tonight and maybe tomorrow morning.

I have stuff that I would like to leave a little longer: sweet potatoes, butternut squash. I would like to leave tomatoes a little longer, I doubt that can happen.

If it wasn't for the rain, I would throw my old towels and t-shirts over the plants. But having them wet from rain might make it worse.

Any ideas?

Update: after checking weather again, I don't think there will be rain. I guess I will try covering with the old t-shirts and towels. Unless I shouldn't.

If you had some water proof options like a tarp or plastic of somekind I would probably use that if there was rain called for. Otherwise, yes, like you said old t-shirts, towels, sheets, etc are generally good enough to keep frost off the plants for a small freeze like you are describing.

On a different note, I recall reading in the book "Farmer Boy" where they were out in the middle of the night pouring water over the plants before sunrise. There was something about the frost plus sunlight. Not sure how much of that was story and how much was science, but it is an interesting anecdote.

How many plants and how big an area?

If its just a few, sheets and towels are great. Big areas need the large row covers made for this application.

Frost works by radiating heat into cold outer space on clear nights. It worse at higher elevation (like CO), still air, and low humidity (also CO). Air temps below freezing are obviously a problem - blankets and row covers block air flow.

So we counter it by blocking the radiation to space and blocking cold air movement. Multiple layers are better than one. Quilts and thicker blankets are better, because the top side can get colder while the inner layer stays warm. The ground and the plants are radiating heat up the blanket. So do everything you can to increase the heat storage, conductivity and emissivity of the ground and plants...

- You might try removing light, fluffy, dry mulch from under the plants - this blocks the heat from the ground.
- Water thoroughly - moist soil conducts heat much better than dry, and the heat capacity of the water adds to your energy reserve. Wet foliage is bad for mold and fungus, but adds a slight measure of protection from frost - the plants can't freeze until the liquid water freezes first.
- Old milk jugs or buckets of warm water next to the plants, under the cloth covers, will release heat and keep the important parts above freezing.
- Suspend the blankets above the plants so there is a gap, otherwise the parts of the plants touching the blanket will get colder faster.
- Make sure the space between the warm moist soil and the plants is open so heat can convect and radiate up to the plants.

Hope that helps, but its a lot to get done this afternoon before the dark and cold arrive!

Sweet potatoes I honestly wouldn't worry about, the soil does enough to protect the tubers. Butternut too. I'd just throw something over the tomatoes and call it a night.

Sweet potatoes I honestly wouldn't worry about, the soil does enough to protect the tubers. Butternut too. I'd just throw something over the tomatoes and call it a night.

I suspect the tubers and squash will be alright. But, I would like to keep the leaves from dying off. After the one freezing night, the nights will be in 40s and the days in 60s and 70. I would like to let things grow and/or ripen a little more, if I could.

I suspect it will only go below 32 for an hour or two. At worst it will hit 31. Some forecasts are saying 34. I also suspect the freeze will in the early morning 5AM to 6AM.

We used to use old sheets for insulation and old plastic tarps over those to keep the insulation dry. I recall using oil lamps and incandescent light bulbs in tin cans inside improvised greenhouses. (I had more to prove back then.)

Now I am lazier and just toss a tarp over the tomatoes. The top leaves may get a frost kiss but the rest of the plant keeps on trucking for a while longer, and then I'll just pick the green fruit and throw it in a cardboard flat to ripen. The squash are on their own.

Matt McSpadden wrote:On a different note, I recall reading in the book "Farmer Boy" where they were out in the middle of the night pouring water over the plants before sunrise. There was something about the frost plus sunlight. Not sure how much of that was story and how much was science, but it is an interesting anecdote.

It certainly does work! If you can keep the frost crystals from forming on the leaves, the plants survive. I recall being out in the garden at 4 am with a garden hose, spraying everything down, going inside to warm up, and doing it again. (This reflects the general bloody-mindedness required to garden in Zone 3.)

If someone has a sprinkler system that's a good time to use it.

In the fruit orchards of the Okanagan, British Columbia, they hire helicopters to mix up the air layers and prevent a killing frost from wiping out their blossoms and fruit. This is a consideration when looking for a quiet homestead property.

I have a lots of painter's plastic left over. It's very thin and light and easy to spread. Works fine for preventing frost.
I have used it to make temporary greenhouses, just a few sticks to hold it up. It kept sensitive plants alive an extra month, and we got greens into mid December. A few things that don't usually survive our winters made it through to spring.
In an emergency, an old bedsheet tossed over the plants works well.

I think I may have been semi-successful.

I covered butternut and sweet potatoes as best I could with old towels and so on.

Around 8AM, it was 40F, I took off the old towels, and sprayed with water to remove frost.

Some leaves look okay, so far. Some are turning black, I expect they will dye off.

I found a few cucumbers in the AM. I picked them and put them in water. I wonder if they will be alright?

We covered our tomatoes and sweet peppers with plastic tarps the last two nights when temps were forecast to drop into the low 30s. Our uncovered beans, squash and cucumbers took a hit and the leaves are wilted and dark green. Now we can see the few unharvested beans, squash and cucumbers that were hiding out in the dense foliage. Now we have at least a week ahead of us with lows in the mid 40s and highs in the upper 60s so we can take our time harvesting the last of the crop and won't need to worry about covering up with the tarps.

We've kind of lucked out this year. After a devastating late frost in May that killed almost the whole garden (even the peas), we replanted late, and now our first frost is a month later than usual, so in the end our harvest has been similar to previous years.

Like Douglas, we also like to pick the green tomatoes just before the real hard frost and store them in the garage. They'll slowly turn red and we can have "fresh" red tomatoes for Thanksgiving and beyond.

Steve Mendez, your weather sounds like Denver.

We had a few late frosts, and snows, as well. The plants were still sprouts, at the time. So we went over and covered the sprouts with jars. I think we had to do that 3 times. For us, it worked very well.

BTW: highs are supposed to be 79F today. No freezes for the next ten days. So frustrating to lose so much because the temperature drops - just barely - below freezing for about an hour, then go right back to moderate weather.

If you know a big freeze coming the night. Water the patch and area real good. Soak it real good. The water will act as an insulator. Compost also helps by creating heat, old hay or straw etc.

I remember hearing about Florida getting some freak freezes around citrus harvest time.  They actually used some sort of sprinkler system to protect their crops.

Walter Byrd wrote:I think I may have been semi-successful.

I covered butternut and sweet potatoes as best I could with old towels and so on.

Around 8AM, it was 40F, I took off the old towels, and sprayed with water to remove frost.

Some leaves look okay, so far. Some are turning black, I expect they will dye off.

I found a few cucumbers in the AM. I picked them and put them in water. I wonder if they will be alright?

Good job, Walter!  Glad you saved your foliage!

I don’t follow the logic of watering the plants in the morning after temperature was well above freezing… what was that about?  Thanks

Colorado always has unpredictable weather. My spouse is a native Coloradoan and I lived there over 30 years. Wonky weather is the norm. Hopefully you covered your tomatoes. Living here in Western Montana gives us weird weather too. The state doesn't know if it should be Pacific Northwest or arid Rocky Mountain, and settles for both and neither. Frost that required me to cover the tomatoes one day and temperatures in the 80s the next. Got to love it. 🤷‍♀️🤷‍♀️🤷‍♀️

Slightly off topic, but here in mid-Michigan I had to cover a few of my potted fruit trees overnight to protect them from frost on my deck.  The tropical ones were brought inside already, but Mediterranean ones like fig, olive and pomegranate need chill hours in the 40s to fruit the next year, so it's a touchy time in the fall trying to leave them out long enough but not too long!  I've gotten nice figs and pomegranates, but haven't gotten the right combination for olives yet.

I cover them with contractor trash bags, then have to be sure to remove them before the sun gets too hot the next morning.

Is there any way to tell, other than "wait & see" which tomatoes are most likely to ripen off the vine?

I guess the yellow ones are more likely to have the potential?

I have had just about every one I have tried ripen, regardless of color…. and I have had them last into February if picked really really green.

I have always been glad to have them in deep winter, but they don’t have the same flavor as August tomatoes.

One year I brought so many in, I was canning tomato juice in December as they ripened.  When summer came, I could really taste the difference.  Like the difference between a tree ripe peach and a grocery store peach😊.

What's the best next step ..

I would get enough shallow cardboard boxes to lay them out one tomato thick.

I don’t know if they will ripen when stacked, probably, because commercial tomatoes are packed in boxes.  The point of making them one layer deep is that each one receives light, and you can see them all.  You can remove the occasional tomato that begins to rot.  You get the tomatoes as they ripen.

I think as long as they are green there is so small amount of photosynthesis.  
To encourage ripening, put an apple in a loosely closed paper bag with the tomatoes.  The loose closure is so that the tomatoes can breathe… they are alive, and need to carry on respiration.  And, to contain the ethylene gas released by the ripe apple.

I keep the boxes of ripening tomatoes at “room temperature”, which at my house is closer to 60 than 70 F.

Walter, I have done this a few times with a light frost in the forecast, and your towel idea is a good starting point. I water the soil in the late afternoon so it is damp, not soggy, because damp soil holds warmth better overnight than dry soil. Then I put a couple of stakes or upside down buckets around the plants so the cover does not sit right on the leaves. I drape an old sheet or towel over the stakes, then add a light tarp on top if rain is possible, and I weigh the edges down with bricks so cold air cannot slip underneath. I take everything off in the morning once the temperature is back above freezing and the sun is on the plants, usually around 9 or 10. For sweet potatoes and squash, I would also check the forecast for a second cold night, because one mild frost is survivable, but repeated cold nights are harder on the vines.