View Full Version : Why does frost kill plants?


origin
10-25-11, 10:29 AM
At first blush I know that this sounds like a stupid question but I am curious about the mechanism that causes the damage.

The thing that I find particularly odd is that you can save a plant even after it is covered with frost if you spray the plant with water before the sun rises and starts to warm the air.

So it seems that it is not the freezing but the gradual thawing that causes the damage.

I have tried the trick of spraying the plants so I know it works, I just don't know why.

adoucette
10-25-11, 10:43 AM
Water expands when it freezes.

So when the water inside a plant cell freezes it can expand and bust the cell wall, causing cell death.

You only notice it on thawing, because that's when it appears limp.

Plants which can tolerate temps below freezing have evolved essentially "anti-freeze" that lowers the temperature at which the fluids in the cells freeze.

Arthur

wynn
10-25-11, 10:47 AM
The thing that I find particularly odd is that you can save a plant even after it is covered with frost if you spray the plant with water before the sun rises and starts to warm the air.

I've never heard of that, and it doesn't seem right to me.

On what plants did you do that, and how often?

origin
10-25-11, 10:48 AM
Water expands when it freezes.

So when the water inside a plant cell freezes it can expand and bust the cell wall, causing cell death.

You only notice it on thawing, because that's when it appears limp.

Plants which can tolerate temps below freezing have evolved essentially "anti-freeze" that lowers the temperature at which the fluids in the cells freeze.

Arthur

Then why, even after the plant has already experienced the frost, does spraying the plant with water save it. I have sprayed all but one tomato plant and the unsprayed one dies but the others live.

scheherazade
10-25-11, 10:52 AM
In my experience, there are a number of hardy varieties of plant that can endure a light freeze, down to -2C.The occasional petunia can tough it out to -4C and pansies and violas can handle even a few degrees colder, to -7C or 20F.

Healthy, well watered plants can tolerate lower temperatures better.

The damage is caused by the expansion of freezing water within the confines of the plant cell walls, these cell walls being of variable tolerance to expansion before bursting, depending on plant variety.

The water spraying trick will work with a few degrees of frost on certain plants, but once you reach -7C we call that a 'killing frost'. Unless you have had your sprinkler running prior to the frost setting in, it's likely to be 'curtains' for the garden.

At least, that has been the experience of the gardeners I know in this northern climate.

origin
10-25-11, 10:52 AM
I've never heard of that, and it doesn't seem right to me.

On what plants did you do that, and how often?

I have done it to many types of plants. I read that it would work and I tried it one time when we got an unexpected frost and amazingly it worked - I don't get it either.:shrug:

Me-Ki-Gal
10-25-11, 10:52 AM
Then why, even after the plant has already experienced the frost, does spraying the plant with water save it. I have sprayed all but one tomato plant and the unsprayed one dies but the others live.

ice is an insulator . Just like snow . It is the theory behind ice caves in survival situations . It only works in a light frost . You get deep freeze and forget it . Dead .

origin
10-25-11, 10:55 AM
The water spraying trick will work with a few degrees of frost on certain plants, but once you reach -7C we call that a 'killing frost'. Unless you have had your sprinkler running prior to the frost setting in, it's likely to be 'curtains' for the garden.

At least, that has been the experience of the gardeners I know in this northern climate.

Agreed that this will not work with what we here call a hard freeze. But it does work with even a heavy frost. All you have to do is spray before the frost starts to melt on it's own.

Like I said I know it works, but I don't know why.

origin
10-25-11, 11:04 AM
Here is the scenario.

It is 6:00 am and the sun is about to rise, the yard and garden are covered with a frost. Just before the sun peaks over the horizon I spray 1/2 of the garden with water so that the frost is rinsed off.

I go to work and when I come home the unsprayed half of the garden will be dead and the sprayed half of the garden will be fine.

So why the heck does this save the plants?

adoucette
10-25-11, 11:15 AM
Agreed that this will not work with what we here call a hard freeze. But it does work with even a heavy frost. All you have to do is spray before the frost starts to melt on it's own.

Like I said I know it works, but I don't know why.

Well the phase change for water takes a lot of energy and you've already admitted that it doesn't work if it is much below zero so my guess is that by spraying the leaves with water you are coating them with essentially a warm liquid and probably preventing freezing of sufficent internal cells to kill the plant.

scheherazade
10-25-11, 11:15 AM
Here is the scenario.

It is 6:00 am and the sun is about to rise, the yard and garden are covered with a frost. Just before the sun peaks over the horizon I spray 1/2 of the garden with water so that the frost is rinsed off.

I go to work and when I come home the unsprayed half of the garden will be dead and the sprayed half of the garden will be fine.

So why the heck does this save the plants?

One can have an exceedingly heavy coating of frost because there is high humidity in the air. The visual effect is dramatic, yet the air temperature is not cold enough to damage the cell wall. That kind of frost also has the slight insulating effect as mentioned by Me-Ki-Gal.

I suggest that plants survive the water the water thawing better because there is not the disparity between the temperature of the inside and outside cell wall. When the sun rapidly heats the expanded to the max cell wall, the difference between inside/outside temperatures likely causes the cell wall to snap as when you take a glass jar from the freezer and set it on a bare counter.

The cell walls can handle a certain amount of expansion, but not the sudden change of temperature. The freezing itself usually comes on over a somewhat longer period of time, allowing some option to adjust. Sunrise, on a clear day, presents to quick of a change. Spraying the plants both warms them gradually and the evaporation of the sprayed water provides a buffer while the air temperature changes.

billvon
10-25-11, 11:19 AM
It is 6:00 am and the sun is about to rise, the yard and garden are covered with a frost. Just before the sun peaks over the horizon I spray 1/2 of the garden with water so that the frost is rinsed off.

I go to work and when I come home the unsprayed half of the garden will be dead and the sprayed half of the garden will be fine.

So why the heck does this save the plants?

You've warmed up the plants just before they experience the coldest part of the day (just before sunrise.)

origin
10-25-11, 12:13 PM
None of the ideas so far hold water so to speak. So I googled for a bit and there is no answer out there as to why this works. Actually I could find no site that even addressed it. I did find plenty of anecdotal data from people who use this technique. Interesting...

I have saved plants by spraying, after the sun is up and melting the frost, because the plants were in the shade.

Trippy
10-25-11, 12:56 PM
None of the ideas so far hold water so to speak. So I googled for a bit and there is no answer out there as to why this works. Actually I could find no site that even addressed it. I did find plenty of anecdotal data from people who use this technique. Interesting...

I have saved plants by spraying, after the sun is up and melting the frost, because the plants were in the shade.

My understanding is that it's the same problem that stands in the way of cryogenics.

To some extent, at least, it's the thawing that causes the problems. My dim dark recollection from many moons ago is that at the temperatures being discussed, and at the scales being discussed ice forms sharp and pointy crystals (IIRC they're acicula, as the thawing occurs, these crystals start to move around and jostle against each other in the cell, which has the net effect of slicing up the cell wall, the nucleus, and so on, killing the cell.

As has been suggested already in so far as the timing goes, there will be a number of factors at play, but basicaly, you're racing against the rate at which the crystals grow, because as you can imagine, to be damaging they need to be of a minimum size - too small, and they just sorta 'bounce' off the various membranes without doing any harm (the implication being that for any given degree of frost, for identical leaves, there is some critical time after which the plant has been cold enough for long enough that the crystals have grown large enough to cause damage to the cells, but before which spraying will prevent fatal amounts of damage).

Verifying this experimentally could be done by having a series of plants, and spraying them at different times (on the same night - ideally you'd repeat it on the same night with multiple plants to give added robustness) and noting which ones lived and which ones died, but doing this would also require some kind of continuous record of ground and/or air temperature, from which a correlation between temperature and critical time could be infered (IE you'd need to repeat it across multiple nights) - alternatively, id you have access to a cool store/meat locker...

origin
10-25-11, 01:23 PM
My understanding is that it's the same problem that stands in the way of cryogenics.

To some extent, at least, it's the thawing that causes the problems. My dim dark recollection from many moons ago is that at the temperatures being discussed, and at the scales being discussed ice forms sharp and pointy crystals (IIRC they're acicula, as the thawing occurs, these crystals start to move around and jostle against each other in the cell, which has the net effect of slicing up the cell wall, the nucleus, and so on, killing the cell.

As has been suggested already in so far as the timing goes, there will be a number of factors at play, but basicaly, you're racing against the rate at which the crystals grow, because as you can imagine, to be damaging they need to be of a minimum size - too small, and they just sorta 'bounce' off the various membranes without doing any harm (the implication being that for any given degree of frost, for identical leaves, there is some critical time after which the plant has been cold enough for long enough that the crystals have grown large enough to cause damage to the cells, but before which spraying will prevent fatal amounts of damage).

Verifying this experimentally could be done by having a series of plants, and spraying them at different times (on the same night - ideally you'd repeat it on the same night with multiple plants to give added robustness) and noting which ones lived and which ones died, but doing this would also require some kind of continuous record of ground and/or air temperature, from which a correlation between temperature and critical time could be infered (IE you'd need to repeat it across multiple nights) - alternatively, id you have access to a cool store/meat locker...

That makes sense because the water spray should almost instantly melt the ice where the sunshine would be slower.

Me-Ki-Gal
10-25-11, 03:24 PM
I don't know . If spraying the next morning after the freeze works ? Maybe it is the heat exchange between the frost and the warmer water . Draws the ice crystals outward instead of driving the frost into the plant .