Geeking Out: Why does a well-hydrated plant survive colder temperatures than a thirsty one?

As I prepare for my upcoming Chemistry class, I thought it would be good practice to do a little bit of research into what happens to a plant at the cellular level when it freezes. Having no idea how complex the subject would be, I promised to share this information with any of you other geeks who might be interested (I know there are at least a couple of you out there). 

There are a lot of online articles covering the practical things you need to know about protecting your plants from frost damage, including a great publication by Linda Reddick, published by the Arizona Cooperative Extension.  While some people wouldn't consider it to be practical knowledge, this post is about how the plant's cells are damaged.  Why does a well-hydrated plant resist freezing better than one that is thirsty?

I've read this fact in numerous publications, and I've witnessed it to be true, but I had little understanding of exactly why it's true.  The answer is much more complex than I had thought. Beyond the fact that a well-hydrated plant simply functions better than one that is thirsty, there's a lot to know about how hydration relates to frost damage.

There are several ways that plant cells can be damaged by colder temperatures, but they do have a few tricks up their...uhh...cell membranes.  One of these tricks takes advantage of osmosis to decrease the temperature at which its insides can freeze.

This happens when ice crystals begin to form in the spaces between plant cells.  The solutes that were previously suspended within the water molecules are left behind when these crystals form.  Water molecules outside of the cells continue to form crystals, until the concentration of solutes (sugars, etc) in the water left behind lowers the solution's freezing temperature enough to keep a small amount of (highly concentrated) solution lingering between plant cells.  *Phew, that was a long sentence*

This extracellular solution begins to draw water from inside of the cell because its osmotic pressure is higher than the solution within the cell.  In turn, the solute concentration increases within the cell and lowers the freezing temperature.  

With this in mind, it’s easy to see how a well-hydrated plant cell would be better prepared for freezing temperatures than one that has little water to begin with.  A well-hydrated cell can give up more water without suffering damage associated with severe dehydration.

Of course, there are other ways that freezing temperatures can damage plant cells…but we’ll have to save that for another edition of Geeking Out.  For now, we've answered the question of exactly why a well-hydrated plant has better chances than a thirsty one when the temperatures get low.  

I'll leave you with one parting thought that should make a little bit of sense if you've read through this post.  Plants that are resistant to freeze damage are often also fairly tolerant of drought.

Protecting your plants from the cold

Being prepared isn't pretty...

Ideally, a post about frost protection should come before we have our first freezing temperatures of the season, but better late than never, right?  This year, I was determined not to be scrambling for old sheets at the last minute, covering "the lucky ones" by flashlight.  Preparation for the first freeze was meticulous and time consuming, but now I'm ready for anything this winter throws at me!

Unfortunately, this means that most of my potted plants are crowded together in one small nook of the patio, and others are getting leggy in the house; but we all know that the most practical choices aren't always the sexiest.  Regardless of how the yard looks in winter, effectively protecting plants from freezing will ensure that they still look attractive in spring.

I call this "The stairway to heaven",
because the plant on top is pretty hard to reach...

I live in a warmer pocket of Tucson, but I grow a lot of tender plants and succulents in containers, so my first chore in preparing for the mercury to drop is to group my potted plants according to their protection needs.  Since I don't have a greenhouse, my wimpiest plants come inside the house for the winter.  If you plan to do the same, you'll want to locate them in a non-drafty spot that receives bright light without getting direct sun through the window.  Mine go on a spiral stand in a west-facing window with a light shade to block direct afternoon sun.  It's easy to overwater in the winter, especially with plants that are accustomed to being outdoors, so check for moisture before watering, and only water when the top inch of soil is dry.  I tend to water every 10-14 days, unless a droopy plant tells me otherwise.  Sometimes, plants can get fussy and drop some leaves when they're moved.  Check the spot's draftiness and temperature and make sure you're watering correctly.  The plant will likely put on new growth when it gets moved back to its regular spot.

This group gets covered when the temperatures
drop below 45 F.

I group the remaining tender plants in the warmest nook of my patio, so it's easier to throw a frost cloth over them when they need it.  Areas that are protected from winds will typically stay warmer, and walls or covered patios can help to reflect some heat in the day and retain some through the night.  The sunniest spot of the patio or yard is often the warmest, but be careful to protect your plants from sunburn if you move them from a shady home to a sunnier one for the winter.  Again, check plants for moisture before watering so that you don't over-do it.

Frost cloth is porous and lightweight

 Now, it's time to talk covers!  Frost cloth is the best choice, but there are other things you can use in a pinch; newspaper, cardboard, wood, burlap, sheets, buckets, plastic bottles cut in half...the list goes on.  Frost cloth is best because it's lightweight and doesn't absorb water, so it still works in the rain or (dare I say it...) snow.




 


Sheets, cardboard and burlap don't insulate as well when they're wet, and they can become heavy when soaked with water.  Plastic can be used with a frame to make a mini greenhouse over a plant (I've seen this done very elegantly with a lawn chair and a mulch bag), but don't use plastic to drape over the plant like you would a sheet, or you'll get some frozen/burned leaves.

This poor Manfreda got burned when a dark colored
frost cloth was left on in the full sun. 
Don't worry though, it'll recover!

Covers work by trapping radiant heat from the ground, so make sure that whatever cover you use touches the ground all around the plant to trap the heat inside.  When using frost cloth or sheets, I like to have some clothes pins and a few big rocks or bricks handy to hold the covers in place.  If you're covering something that will be in direct morning sun, it's a good idea to pick a light color, or you might have some burned leaves by the time you uncover the plant (especially if you tend to sleep in on cold, wintry mornings, like some people I know...).


On the subject of uncovering, it's best if you do this chore daily.  It's OK to leave plants covered for a couple of days if freezing temperatures are expected, but they'll be a lot happier if you allow them some fresh air and sunlight in the daytime when temperatures are warmer.  Since covering works by trapping heat, it helps to let the ground around the plant soak up as much sunlight as possible throughout the day.

Celebrations C9 Light set at Acehardware.com

If radiant heat from the ground isn't enough, or if you're trying to protect something too large or tall to completely cover (such as a citrus tree), a string of old-school fluorescent Christmas lights can help generate some extra warmth that can be trapped under whatever cover you have.  LED lights are much easier to find, but they don't generate nearly as much heat.  I've seen some other "clever" heat generators do the job (garden lighting directed at the plant, flashlights hanging from branches), but fluorescent Christmas lights seem like the safest, most effective, and most festive option to me.


Lastly, I'd like to note that a thirsty plant will be damaged from the cold much faster and easier than one that is well-hydrated.  Try to check on moisture levels a day or so before a freeze is expected; it's best if a plant isn't too soggy or completely dry when the temperature drops.  Check out my next blog post for some geeky science behind how and why thirsty plants freeze faster than well-hydrated ones.

Here are some of my artsy photo extras from this post:

Philodendron hederaceum (aka Philodendron scandens)

I can't wait for this guy to grow up.  He lives on one of the lowest stairs,
where he's easy to see and reach.

Mr. Chomps lives near the bottom shelf, too.