I'm Confused (Frost Damage)

[SOB]

About a month ago I planted the following cool weather crops:

Cabbage (direct seed), Cauliflower (direct seed), broccoli (direct seed), peas, potatoes, and more that don't really matter for this thread.

About two weeks ago I put my tomatoes in the ground. Well, we got that frost earlier in the week and I covered my tomatoes with large buckets and pots. I didn't cover any of the cool weather crops.

The next morning when it warmed up to about 50 I uncovered the tomatoes. That night I could tell about half of my tomatoes (5) were showing some real signs of damage (leaves dark and wilted). It's been a few days now and 2 are completely dead and the other 3 are half dead.

First question, what did I do wrong when covering the tomatoes?!

The peas and potatoes show some real signs of damage but I think they will pull through. The cabbage, broccoli, and cauliflower are now completely dead.

Second question, why are these considered cool-weather plants when they can't take one night of light frost?? Carrots, onions, lettuce, garlic, asparagus all did just fine uncovered...

Thanks!

[JayPoc]

Something is not right here. I can just about guarantee that it was NOT the cold that killed your broccoli, etc. Mine have survived temps in the teens before. Peas can easily go well below 32 without showing ANY ill effects. I'd look to other causes....

[digitS']

I have lost tomatoes to frost when they were under buckets.

The problem was that the buckets were fairly small and the leaves touched the buckets. The freezing occurred because the plastic itself, didn't provide much insulation for the leaves. Growing tips are particularly sensitive and a plant pushed down under a bucket that isn't tall enough is in danger. The part of the frost-bitten plant that is dying should be removed and there is some chance that the other parts of the plant will recover.

No garden plant benefits from freezing, that I can think of. It is hard on all of them even tho' some can get through it without obvious damage. Dehydration is part of it and I have found that it really helps to have adequate soil moisture. I think that many plants that are otherwise frost-hardy are more likely to be killed/damaged if they are in dry soil.

Steve

[JayPoc]

I have lost tomatoes to frost when they were under buckets.

The problem was that the buckets were fairly small and the leaves touched the buckets. The freezing occurred because the plastic itself, didn't provide much insulation for the leaves. Growing tips are particularly sensitive and a plant pushed down under a bucket that isn't tall enough is in danger. The part of the frost-bitten plant that is dying should be removed and there is some chance that the other parts of the plant will recover.

No garden plant benefits from freezing, that I can think of. It is hard on all of them even tho' some can get through it without obvious damage. Dehydration is part of it and I have found that it really helps to have adequate soil moisture. I think that many plants that are otherwise frost-hardy are more likely to be killed/damaged if they are in dry soil.

Steve

Good points...moist soil holds heat better, for sure. I always water heavily before covering when I find myself in that situation. The other thing I try to do is once I get the buckets in place, I hill up mulch all around and over the bucks. I've had the ambient temps get down to about 29 with zero loss in my tomatoes, altough the cukes died. I try to avoid that at all costs...gotten more patient in my old age. Killing and replanting stinks...

I've often wondered about throwing those iron based hand warmers under a bucket to see if that can buy me a few degrees.

[DownriverGardener]

This past frost earlier this week, I covered as much as I could in my garden (see here: https://i.imgur.com/4mSa72V.jpg)

However, some of my tomato plants that were covered with tall buckets still didn't make it. Sometimes, even when covered, some plants just don't make it I guess.

[applestar]

Ouch. That's really a shame.

Frost can occur above freezing temp, but if the temp had fallen below freezing, any and all condensation on the covering material would have frozen along with any leaves touching the surface. It sounds like you got freezing temps.

Also, often the danger is the morning after -- these freak frosts are often followed by blistering high daytime temp next day, and the sun is already getting hot. If they were under the solid covers in the sun, they would have frozen then baked all in the matter of hours. The frost damaged or stressed areas of the leaves would have immediately turned to mush (brown).

You have to take the covers off the moment (or as close to) the sun starts to shine on them and the tem starts to rise. The tighter the cover (needed to protect from the sub-freezing temps) the more critical timing is needed.

For tomatoes and such, I let the sun warm them up to about mid-50's then take off the covers, but create ventilation of some sort -- e.g. rock under the bucket rim, loosen the plastic sheeting, etc. You said you uncovered after temp rose to 50, but was it official temp, your own thermometer in the shade (where they are usually located), or did you have a thermometer measuring the tomato foliage in the sun (yes I can be that obsessive )?

As for the cool weather crops, if you have had the weather we've been having, it's been like summer temps for a while, then all of a sudden this frost. When acclimated, young cabbages, etc. can take temps down to mid 20's but the older tender leaves grown in warmer temps are more vulnerable. I didn't really think about covering my broccoli, etc. but I did cover the taller potatoes with floating cover (lightest summer weight only though). My brassicas were already under the insect netting to keep out the white and yellow butterflies (which is sufficient to keep light frost off of the leaves) that and the way they are planted to catch afternoon shade probably meant they were in a more protected area.

...although I have to admit the combination of insect cover plus the afternoon shade is looking like a little too much shade... Still tweaking.

P.S. I agree that watering if they need it before tucking them in for the night of possible frost is a good idea. Something to do with water's thermal reactions and also to make sure the plants are not in distress.

[rainbowgardener]

Lot of good information here. I have to say that broccoli, cabbage, cauliflower ARE cool weather crops and would not be frost killed unless they were seedlings recently brought out from indoors. Once hardened off, they are extremely cold hardy and frost tolerant. I always put my broccoli in the ground a month or more before my average frost date. They go through frosts and snows just fine and I don't cover them. If yours were well hardened off, then I'm thinking something else killed them.

Tomatoes and peppers and cukes are a whole different story though. In general, if they freeze, they die. A bucket may not keep them from freezing.

It has been a somewhat difficult spring for many of us, with lots of late cold snaps.

[SOB]

Wow...where do I start

I think the point about the plastic bucket freezing and the leaves touching the sides is a good point that I didnt think of. I don't think it got to freezing temps - just frosty. When I got up at 530am it was only 37 degrees which is about what they were calling for. I was also worried about "cooking" the tomatoes in the buckets because I did put them on the tomatoes pretty early in the evening with the sun still out. I'm starting to think the tomatoes may have been a combination of those things.

As for the broc/caul/cabb...they were direct seeded so there was no hardening off period. They were, however, still fairly small but had at least the first true leaf. Would being young make them not handle the frost as much?

The peas are about a foot tall and just yellowed a bit at the top so I think they are OK. The potatoes were a good 6-8" tall and all but the bottom inch or two of leaves are completely black. I think they will pull through.

Tonight maybe I'll take pictures so you can see what I mean. I already pulled out and replanted the tomatoes that were real bad. I also had cabbage worms each through two of my tomatoes - which is also a first - so my garden is hurting this year. At least the garlic and onions look great!!

[Dillbert]

SOB -

one does not get "frost" without "freezing" temperatures.
"frost" is a thin - or not so thin - coating of ICE - ice is frozen water, so no 32'F or less, no frost....

so what the heck happens in the garden?

the plants were not wet. they were covered. frost came.
so first humidity from the air condenses onto the cover and then temps continue to drop and the condensation turns to ice = frost.

in a light frost & atmospheric conditions, the condensation sits onto top of the cover, freezes, the freezing temps do not physical "reach in so far" to affect the plants under the cover.

heavy frost and/or temps more than a tad under 32'F - toast.

you may have heard of the Florida citrus groves wildly spraying water on the orange blossoms when a late freeze comes through.... why? because the blossoms _can_ take 32'F, but not much under that. by 'coating' the blossoms with ice, the ice insulates the blossoms (one hopes...) from even colder air temps.

so a featherbed comforter will protect better than an old sheet curtain. tad 'pensive, them feather things tho....

the heat from the ground trapped by a cover will "protect" the plants up to a point. when "outside the cover" gets too cold, or water soaks through the cover & turns to ice,,, toast.

stuff like cabbage/brocc/cauliflower/peas are hardy - sounds like yours were still a bit on the too tender side to take what I'm thinking was multiple degrees less than 32'F

[mattie g]

Dillbert...you absolutely can get frost when the measured temperature is above freezing.

Objects can lose heat more rapidly than the air does, especially on cool, clear nights via something called "radiational cooling." When this happens, and the surface of the objects facing towards the sky meet a dewpoint that is below freezing, frost forms. This can happen while the air temperature is above freezing. Basically, the surface temperature of the plants is below freezing because they lost heat more quickly than the air did.

Another way this can happen is because the air temperature reading at ground level is often lower than the level at which official air temperature is generally taken (usually two meters above ground level), since cold air sinks. Since most plants reside much lower than 2m AGL, frost can form on them while the air only a couple meters above them is above freezing.

[JayPoc]

Dillbert...you absolutely can get frost when the measured temperature is above freezing.

Objects can lose heat more rapidly than the air does, especially on cool, clear nights via something called "radiational cooling." When this happens, and the surface of the objects facing towards the sky meet a dewpoint that is below freezing, frost forms. This can happen while the air temperature is above freezing. Basically, the surface temperature of the plants is below freezing because they lost heat more quickly than the air did.

Another way this can happen is because the air temperature reading at ground level is often lower than the level at which official air temperature is generally taken (usually two meters above ground level), since cold air sinks. Since most plants reside much lower than 2m AGL, frost can form on them while the air only a couple meters above them is above freezing.

I think you're arguing semantics here. I think you and Dilbert both agree that water doesn't freeze above 32...

he's right. If you see frost on a leaf, then there is water there on that leaf that is @ or below 32 degrees...and the laws of physics say that it is 100% impossible to have frozen water that is WARMER than 32 degrees (under anything resembling normal pressures)...

[applestar]

But, don't overlook Mattieg's expert description of the relationship between atmospheric temp vs. dew point temp on the surface of the leaves that can result in frost despite the predicted and measured above freezing temp. ( That dew point is still a bit f a mystery to me )

And that the official temp is measured 2m above ground which can be *quite* significant.

[Dillbert]

you can overlook the comments.

dewpoint is the temperature at which the (given level of) moisture in the air condenses from water vapor to water liquid. dewpoint is dependent not only on temperature but also relative humidity.

dewpoint is not related to nor dependent on frost temperatures. in any way shape fashion or form.

there's a law of thermodynamics that suggests heat energy moves from higher temperatures to lower temperatures.

so perfecting a technique whereby a warm plant can radiates its heat into an even warmer above freezing environment to produce a frost on the leaf surface is a topic many physicists would be keen to examine.

in fact, one could book passage to Sweden for the Nobel prize on that one.
just be sure it's a refundable ticket.

and so far as where to measure temperatures in regard to frost on the tomato - let me suggest that the -40'F measured at 40,000 feet above sea level _at the exact spot of the tomato_ is not actually relevant.

[JayPoc]

But, don't overlook Mattieg's expert description of the relationship between atmospheric temp vs. dew point temp on the surface of the leaves that can result in frost despite the predicted and measured above freezing temp. ( That dew point is still a bit f a mystery to me )

And that the official temp is measured 2m above ground which can be *quite* significant.

I'm not trying to be contrary, but I don't think dew point and temperature are related...at least not the way you mean. As far as I understand it, dew point is simply the calculated temperature at which dew will form based on the temperature and humidity at the given time.

Depending on all the variables...its possible to have frost on a night where the low is 31.9 degrees, and no frost on a night where the temp is 27 degrees. By and large, I don't think thats all that relevant. I think for most plants, its the freezing of the tissue in the plants that is deadly....not whether or not actual frost forms.


A metal bar will FEEL cooler than a piece of wood at the same temp to you and me, but they'll still be the exact same temp...

[applestar]

In case you haven't noticed yet, I'm not a science or math person. Precision is NOT my thing. I operate more on accumulated personal experience, pattern/relationship recognition, (some sort of internal/background analysis), and gut feeling.

You guys lost me several posts ago

All I can say is in my experience, there have been patches of frost on the ground when the reported official weather station temp (supposedly for my zip code/local station) is 37°F and my own remote sensor thermometer on a bench on the front porch reads 34°F. The frost is generally patchy, dewey to light in some areas and "hoary" in others with grass crunching underfoot.

My personal perception has been that frost patches occur depending on exposure (more on open areas, less on less exposed areas; more on lower lying areas, etc.). I can tell you almost exactly where on my property frost tends to form and where they tend not to form. I protect plants in garden areas that get frost more carefully than garden areas that are usually free of frost. I automatically categorize areas without really thinking out the reasons, but could probably list the reasons why I think or don't think one group of plants should be protected and to what degree.

That's not to say I get it right every time, but I've gotten better at it over time.

[jal_ut]

Dew point:

Air can hold water vapor , this is measured as humidity. Warm air can hold more water than cool air. As air cools, a point is reached where the air cannot hold any more water, and if it cools more, some of the water condenses out of the air and becomes liquid water. This is the dew point. The liquid water that collects on cool objects in the area is dew. The dew point (temperature) can be different depending on what the relative humidity of the air was. This has nothing to do with frost nor freezing.

Freezing:
By definition this is the temperature where liquid water turns to solid water, that is ICE. Frost = ice.
It is 32 degrees F where this happens. You cannot have frost unless the temperature gets that cold. Absolutely impossible.

So your plants got blackened? It sounds like they got frozen. To freeze the temperature at the level of the plants had to be 32° F or lower. That is just a simple law of physics.

Sorry you got hit. The potatoes will recover. I am surprised the peas took a hit. They usually have pretty good resistance.

A cover like a blanket is better than a bucket or jar as it has better insulating qualities.
It is good to wet things down well if expecting a frost. Water is interesting in that it must give off quite a little heat even after reaching 32° before it will change state (freeze). So more water = more heat.

It soon becomes obvious that there are cold pockets and six thermometers in six locations will all read different. Best to expect frost any time the forecast is for under 40°.

[applestar]

I see, so, and I think we are all talking about the same thing in the end -- but at ground level in "cold pockets" the temperature fell to -- or below -- freezing (32°F) where frost formed.

...and perhaps in the OP's garden, the temperature perhaps fell significantly and for a prolonged period? Damage seems too severe to be light or even simple frost -- more consonant with hard frost or freeze.

Thank you for elaborating the dew point definition. I think I get it now, especially after re-reading the different posts referencing dew point.

Here's an interesting correlation I've often noticed: the forecast temp is often wrong (too high by 2-3 degrees), but the dew point in the forecast is often the temperature I get on my porch thermometer.

[SOB]

For the record the recorded low temp for my area was 35 degrees. I understand that my garden could have experienced temps lower than this. Also, FWIW, the dew point was 32. I do know that we experience a recorded temp of 33 degrees the following day and there were no frost warnings because it was cloudy and breezy. It's my understanding that the cloudy breezy nights don't allow the moisture to settle on the plants. I did cover that night too just to be sure.

Is there a possibility I covered the plants too early and the sun warmed inside the buckets causing condensation which then froze causing extra damage?

The main point I took from the conversation above was that buckets are not the best covers. What I don't get is I see people covering things with thin sheets that have very little insulating properties and they seem to have no problems. I guess next time I'll break out some old sheets.

I looked again last night and the potatoes look like they will make it. They definately lost a out 75% of their foliage though. Peas look fine now. The broc/caul/cabb weren't a total loss but pretty close. I'm not too worried about that as I will probably grow a fall crop. What I will do in the future is start those crops indoors so they can get bigger and then be hardened off. Maybe that will give them a better chance.

Thanks all!

[cynthia_h]

Freezes aren't that common in the part of the Bay Area where I live, so the garden advice is always repeated: "Do not cover your plants with plastic sheeting because any part of the plant that touches the plastic can freeze." Then they go on to recommend cloth, or plastic sheeting up on sticks away from the plants, etc.

Even in one's own yard, whether a large property, a "standard" suburban quarter-acre lot, the 50'x100' lot DH and I have now, or the 25'x80' (I think 80', might have been 100' from the street) lot we had in Berkeley, there are microclimates (sometimes referred to as micro-zones) where the temps can vary by as much as 5 degrees F (approx. 3 deg C). In December 1990 or 1991, whenever The Big Freeze that destroyed the California citrus crop hit, DH and I kept our cymbidiums and a few other plants alive on the patio in Berkeley in 28 deg F weather for several days, including one night at 25 or 26 deg, by:

1) clustering all the cymbidium pots under the avocado tree,
2) watering them with warm water right before going to bed (the planting medium only, not the leaves or plants proper),
3) propping old sheets over the orchids and other container plants using sticks so we could get "just a couple more" plants under shelter, and
4) running a heavy-duty extension cord and attached household heater (meant for use under a desk or table) under the sheet covering the plants while we were asleep. We later added a string of twinkling Xmas lights so that more plants would have something in the way of warmth.

It was quite a bed-time ritual for that week or so: first, lugging warm water out to the plants in watering cans over multiple trips (that house had only a shower, not a tub, so the kitchen was called into service; the kitchen was farther from the back door than the bathroom was--of course); second, setting up the sticks-and-sheet "superstructure," once we'd figured out just how many of our plants we could fit under it, and third, turning the heater on and plugging the lights in.

We saved all but one of the plants, and all of the cymbidiums. Nothing in the front yard died, though, so I think the front yard must have been 30 or 31 degrees with very little humidity. Certainly not nearly as cold as the back yard, a postage-stamp sized thing surrounded by fence, house, etc. with almost no air circulation.

Maybe some of these tips will help when the temps are threatening again.

And, believe it or not, I had to do Snail Patrol each of those freezing nights! The little beggars were trying to sneak onto the patio for the heat, no doubt, or to eat the other plants. Just made them more visible to me....

Cynthia H.
Sunset Zone 17, USDA Zone 9

[SOB]

UPDATE: I replanted most of the tomatoes that got killed in the frost. Everything else snapped back. I have a total of 10 tomatoes and since then I planted my 6 bell peppers, 2 hot peppers, 2 eggplant and various herbs. Also, some of my beans and cucumbers came up.

Then...Friday night I knew it was going to get on the cold side (they were calling for 38) but there were no frost/freeze warning so I figured I was OK. Boy was I wrong! I walked out to the garden Saturday morning and EVERYTHING I had just planted was dead! I felt like puking.

I spoke with my neighbor (her garden is probably a good 1000 feet from mine, roughly the same elevation and just as open meaning no trees or structures) and she had ZERO damage to her veggies?! She didn't cover them or anything.

Sunday I bought a couple tomatoes so we have SOMETHING this year and I still had one eggplant left over that survived under the patio. I'm not buying anything else! I guess next year I have to wait until Memorial Day to plant!

[JayPoc]

Did you cover yours? This is so odd. Maybe something in the cover itself? Low of 38 and the neighbor is 100% fine, but you're all dead again? You got a mortal enemy with a Round-Up fetish?

[SOB]

No, I did not cover mine. I think it got colder than 38 actually. In fact, I know it did but I don't know exactly how cold it got. That morning my thermometer read 36. I have since moved it out to the garden to get a more accurate reading out there.

Funny you ask about the enemy...my other neighbor and I did get into a spat just before all these frosts hit From my experience though round-up makes the vegetation turn yellow and die. Everything turned almost black and wilted before dying.

It also dawned on me that I did use my round-up sprayer (which I clean after each use) to spray Dipel for controlling cutworms. But, again, things didn't yellow. Also, I sprayed all my onions and they are still growing just fine. I will, however, buy a new sprayer that is just for garden spraying and will never have round-up in it.