Question
I left my geraniums out last night and we had a frost.  I heard you could spray with water and it might salvage them.  Is this correct?

Answer
It's that time of year again, gardeners (like me) head out for a game of Russian Roulette with their tender potted plants.

You're not alone. I exposed my late mother's cherished 20-year-old bonsai to an unpredicted cold wave that fell late into the night, one October.  By morning, all was lost.  It happens.

Let's talk about your Geraniums.  First, let's confirm what plant we mean here: You call all Pelargoniums 'Geraniums'.  Everyone else does.  Even me.  But let's make sure:

en.wikipedia.org/wiki/File:Pelargon.jpg

Just checking.  Technically, these are Pelargoniums.  But we will use the term 'Geranium' for what picky people would instead refer to as 'Pelargoniums'.  No one calls them that outside of science circles, but we have to be clear.  Let us continue.

Now, this may come as a surprise to you, but these Geraniums/Pelargoniums are actually hardy to 28 degrees F.  Under normal conditions, a sudden cold snap is no big deal to a Geranium.  However, sub-freeze survival is only possible if it has been exposed to DAILY dips in air temperature, over several weeks -- the process we call 'hardening off'.

Not even your toughest Oak can take a Big Chill without weeks of prepararation. This phenomenon is described beautifully by writer/botanist William Cullina, Plant and Garden Curator at the Coastal Maine Botanical Gardens, who graciously posts online a draft chapter on 'Plant Hardiness' from his book 'Understanding Perennials':

www.williamcullina.com/Page72.html

Cullina explains: 'During the Summer, even very Winter-hardy species that can withstand -50 degrees F in January may be killed by just a touch of frost.'

Short days, cool nights trigger hormones that order hardy plant cells to pour natural antifreeze -- Abscisic Acid and Ethylene -- into their bloodstream. Their leaves stop making Chlorophyll. Carbohydrates retreat, heading down underground for storage in roots. Some begin a countdown of sub-freezing temperatures; if these plants don't get their quotas of cold Winter days, they won't bloom.

But NONE of this will happen if growth is not completely halted, first. Writer Cullina points out that this is why it's bad to fertilize and water this time of year: 'Cell dehydration is an important part of the acclimation process.' All that water and fertilizer sends the wrong message. That's asking for trouble.

By now, you have a good idea of how much, if any, damage your potted plants incurred. If your Geraniums have been outdoors, and had time to harden off, the cold wave is not too bad.

However, you used that F word in your question.  FROST.  And that changes everything.

Does it help to rush out and spray your plants with water?

What kind of problem are you dealing with when you deal with FROST damage, anyway?

One of the most obvious signs of frost exposure is wilting.  But watering won't help.  Tissue
damage in a plant exposed to frost comes from severe dehydration, which accounts for the wilting.

What is frost?

Take a look at a frosted window some Autumn morning.  The tiny ice crystals won't form without certain conditions.

First, you need a very humid night.

Then, you have to chill the humid air. The temperature plunges.  As it cools, the icy air condenses.  If a window is warmer than 32 degrees F, you see drops of water on the glass.  If the drops freeze, you see frozen dew -- not frost.

If the glass is 32 degrees F or colder, you have a setup for frost. Now it's possible for the water vapor to crystallize right onto the windowpane, without turning to dew first.  That instant crystalling, straight from ice cold air onto the glass, is frost -- BUT you STILL need something else: An ICE NUCLEUS. You can't make frost without it.

An Ice Nucleus can be a speck of dust on the window.  It can be bacteria on leaves. It just takes one, and poof! a chain reaction. One ice crystal forms, then another, and another.  Frost.

Just as a matter of record here, by the way, I have to tell you something that's very, very hard to believe.  And you probably will look this up over and over again in disbelief.  But for your information, Water does not freeze at 32 degrees F.

Yes, you read that right.  At 32 degrees F, pure Water DOES NOT FREEZE.  No fuzzy math here, no scientific sleight of hand.  Just the facts, m'am.

Pure Water SUPERCOOLS at 32 degrees F.  But it's still liquid.  Something like a piece of ice has to come along and causes a long chain reaction of frozen water molecules, like that frost on the window.  Witness this quick YouTube demonstration of literally ice-cold water in a bottle:

www.youtube.com/watch?v=V3R4VAdCbTg

See what happens when the guy puts an ice cube in the bottle?  Instant crystallization throughout the container, right before your very eyes.  Try that one at home.

Dust on the window, specks of soil, bacteria -- ESPECIALLY bacteria, the ubiquitous Pseudomonas syringae, discovered in the 1970s to be a key to rain and snow -- become the ICE NUCLEUS that ice-cold air is looking for.  Icy water vapor molecules come in contact with uniquely shaped Pseudomona cells and erupt into faceted crystals that grow and multiply. VOILA! you have frost in the morning. On your window. On your lawn. On your Geraniums.

Scientists have been trying to understand frost damage for hundreds of years. Even today, this is an economioc problem. Millions are lost each year to frost damage.

Research 20 years ago by two plant pathologists at University of California noted that 'probability of frost injury, which is proportional to the number of ice nuclei on leaf surfaces, is strongly determined by the particular bacterial strains that are present on a leaf surface,' a reference to those Pseudomonas microbes.

'Many frost-sensitive plants supercool and avoid damage from ice formation at temperatures above -5 degrees C,' they note.  But that depends on an absence of those frost-forming bacteria. 'When present, these bacteria limit the supercooling of water in plant tissue and incite frost injury.'  

You can read their article, 'Effect of Plant Species and Environmental Conditions on Ice Nucleation Activity of Pseudomonas syringae on Leaves' published in the September, 1988 issue of Applied Environmental Microbiology here:

www.ncbi.nlm.nih.gov/pmc/articles/PMC202850/

Yes, frost -- not temperature -- causes all this trouble with alfresco pots this time of year.  As you know.

Experts at Cornell University have issued a helpful 2-page broadside for homeowners titled 'Understanding Frost', a guide for gardeners about what to do when Jack Frost shows up by surprise.  One of their approved retaliative remedies: Spray frosted leaves with water.

'Watering plants helps raise their temperature and the air around them,' Cornell declares.

BUT...

This is useless once the damage is done.  Think of it as pouring water on a fire after the house burns down.  Because 'Once frost damage occurs,' they note, 'watering does not help.'

Have a look:

www.gardening.cornell.edu/weather/frost.pdf

It would work, you see, if you woke up at 5 o'clock in the morning and realized you forgot to bring in the Geraniums.

Even if frost was already forming, you could avoid damage.  Standing there in the dark with a watering hose, spraying your plants in the middle of the night, would be a stroke of genius.  Wait to do that until morning, however, and the house has hours to burn.  Could be too late.

That said, there is yet another option that I have written about so many times I feel like the official spokesman for a product called Messenger, a spray-on CPR-type treatment based discovery of Harpin Protein at Cornell Ag.

Cornell patented and licensed its discovery to specified companies, giving them permission to make the product and pay Cornell for the right to do that.  Messenger is Harpin Protein, a plant-made product that accelerates repair to damaged plant tissues.  I used it on a nearly dead tropical plant just last week and I was mighty glad I had it, because that plant would have kicked the bucket.  It's on life support right now, but with the right care, the patient will survive, and bloom again next year.

You will have to obtain it from your local garden center. The original manufacturer, Eden Bioscience, declared bankruptcy last spring.  Assets and licensing now belong to Plant Health Care, Inc., a subsidiary of Europe-based Plant Health Care, plc.  If you see any Messenger at the store, stock up -- it may be a while before you can get your hands on any more Harpin Protein.

Lastly, a botanist down South has developed a proprietary formula for 'plant antifree' it claims builds plant resistance, of sorts, to cold.  It's been met with skepticism, but you can draw your own conclusions.  Here's the article about it on Science Daily's website:

www.sciencedaily.com/videos/2008/1105-antifreeze_for_your_plants.htm

For future reference, Cullina's color-illustrated 'Understanding Perennials' is available, nicely discounted, from Amazon.com:

www.amazon.com/Understanding-Perennials-Favorite-Frances-Tenenbaum/dp/06
18883460/ref=sr_1_1?ie=UTF8&s=books&qid=1256009109&sr=1-1

Did I miss anything here?  Let me know.  Stay warm,

THE LONG ISLAND GARDENER