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: http://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.