Announcements first, then a lesson in farm physics!
We have four more weeks of produce this season!
Please let me know if you will be attending the Garlic Planting Gala on Saturday October 6th from 2:00 until we have dinner and a bonfire. This is a fun event for all ages. Come see your farm and enjoy a beautiful fall day in the country!
We are taking orders for the 2013 season. Get your order in early to reserve your spot and lock in this year’s prices for next year’s produce. We are having a great year this year with lots of veggies. I would caution anyone leaning toward purchasing a smaller share for next year. Next season is guaranteed to be unlike this season just like this season was unlike any previous season. It could be another banner year or it could be an average year. In any case get your orders in soon!
We had frost this past week so the produce offering will change dramatically. No more beans, edamame (though that was due to run out anyway), cherry tomatoes and tomatillos. Fewer regular tomatoes as we switch to only hoop house grown tomatoes. Fewer peppers. In their place we will have winter squash, pie pumpkins and carrots. We’ll also have cabbage, onions, garlic and maybe kale this week. The fall salad mix is getting watered as we speak. Hopefully it will be big by next week. The fall brassicas and fennel are looking good; I’m hoping these mature in the next few weeks as well. There are also a few beets left but we are waiting for them to grow up a bit more.
No FruitShare this week.
CheeseShare for Wednesday, Thursday and Friday folks.
WinterShares are sold out.
Time for this week’s lesson in Farm Physics with your farmer physicist.
Today’s question: How the heck does ice keep plants from freezing?
What a great question! However to answer this question we need to answer another question first: Why do plants die when they get too cold? We humans don’t die when the temps drop below freezing. Why do some plants?
Well the first and obvious reason is we wear clothes and plants don’t. The second reason is we don’t live outside our whole lives like plants do. The third part of the answer is our feet are not firmly planted in the ground so we can move to a warmer spot if we get too cold. The fourth reason…
Wait, wait wait. We can figure out why we humans survive below freezing temps. Let’s focus on why some plants can’t!
Oh, yes. Sorry about that. Plants are made out of cells. Inside cells there is dihydrogen monoxide — a chemical that is crucial to life on earth.
TOO TECHNICAL! How are we suppose to understand words we can hardly pronounce? You sound like the ingredient label on the back of a box of processed, chemically-grown food!
Oops, sorry again. Sometimes I don’t remember we are not all farmers or physicists. Plants are made up of cells. Inside cells is a chemical called dihydro…, oops, er, water. Water is an incredible molecule in many regards. First, it quenches my thirst on all those hot, sunny days we had this past summer.
Hey, keep to the subject!
Again, sorry. Sometimes my mind wanders. One particular feature of water that is unlike most other known substances is that when water turns from liquid to solid its volume increases; it expands. It becomes less dense. You might say it becomes more educated (dumb joke I know).
Stop with the humor and focus on the answer!
Ok, sure. One of the benefits of this phenomenon is that it allows us Minnesotans to go ice fishing in the winter time! And who doesn’t like ice fishing with the ice and the cold and the snow and the freezing water and the lack of fish. If water became more dense like virtually every other substance, it would sink to the bottom of the lake. Ice fishing from the bottom up would be problematic, not to mention awfully cold.
Ok, got it. When water turns to ice it gets less dense. So how does this protect the plant from dying?
I’m getting there. Hold on. When water in the plant’s cells freezes it expands. The expansion causes the cell walls to rupture. Once the plant warms above freezing the cell’s walls can no longer keep the now liquid water inside – it leaks out. This causes the cell to die, the leaf to die and if severe enough the whole plant to die. To prevent cold from killing the plant we need to keep the plants cells from freezing. And we do this by adding ice.
Ok. I understand the part about how the plant dies. Water expends, bursts the cell walls, killing the cell and the leaves. But how does adding ice stop this from happening?
Well we don’t exactly add ice. We add water that eventually turns to ice. One other interesting feature of water is that it takes a lot of energy to melt solid water — also known as ice — into liquid water — also known as water. Conversely it gives off a lot of heat to go from liquid water to solid water.
Wait a minute. You’re saying as water turns from liquid to solid it gives off energy? What kind of malarkey are you trying to feed us? Couldn’t we power the earth by making ice in our freezers?
Um, no. Just because the water has all this energy doesn’t mean it wants to give it to you to power your new iPhone. Water wants to reach an equilibrium with its surroundings. So if the temperature is hot, the water will absorb energy until it and its surroundings are at equal temperature. Same thing when the temperature of its surroundings is cold. The water will lose energy to the surrounding air and its temperature will drop — until it gets colder than the temperature where water freezes, which is 32 degrees Fahrenheit at standard pressure.
So if I understand what you are saying, the temperature of the water will drop right along with the temperature of the air until it gets to freezing. But if the temperature of the air continues to drop won’t the temperature of the ice also drop? Ice can be colder than 32 degrees!
That’s right. But at the point when water is turning from liquid to solid neither the liquid water nor the solid water will be colder than 32 degrees. It will stay at 32 degrees until all the water is frozen no matter how cold the air temperature gets. For it to drop below 32 degrees the liquid water has to give up a lot of energy to form solid water. Until all this energy is disipated it will stay at 32 degrees.
Ah ha! I found a flaw in your reasoning. Once all the water freezes won’t the ice drop below 32 degrees causing the water in the cell to freeze thus expanding and bursting the cell wall causing the plant to die? Using ice can’t save the plant. This is so bogus.
Well that would be true if all we did was spray the plants with water before we went to bed. But we use the sprinklers to continually add water — and more importantly energy — all night long. By adding liquid water we maintain a minimum temperature of 32 degree — just warm enough to prevent the liquid in the cells from freezing.
Wait, wait, wait. I thought the water in the plant would freeze as 32 degrees. What gives?
As I mentioned before, as the liquid water we are spraying on the plant starts to freeze it gives off a whole bunch of energy. Some of this energy is absorbed by the plant. This adds to the warmth of the plant and is in part responsible for saving the plant from freezing.
Ok. I think I get it now. You use water as an energy source to keep the plants warm during cold weather. Even though the water freezes into ice it never drops below 32 degree because you are constantly adding more liquid water. And, as the water freezes it releases energy, some of which is absorbed by the plant. That’s pretty cool!
Cool yes! In fact freezing! At some point this approach no longer works. If the weather is too cold the amount of energy released is insufficient to maintain the temperature of the plant above the cell’s water’s freezing point. Another issue is the amount of solid ice that forms on the plant could exceed the plant’s ability to support the ice’s weight. The plant then comes crashing down to earth and kills the plant.
I can see that. It would be hard to make this work in January.
Here at Fresh Earth Farms we use the sprinklers in the fall to save the pepper plants during the one or two cold nights we have in September or early October. We found if we can get the plants through these couple nights we can have peppers for another 3-6 weeks depending on future temperatures. Of course if we see a forecast that has 3-4 nights of below freezing temperatures we figure it isn’t worth trying to save them and let them go for that season. This usually happens sometime in October.
Thanks for the great explanation Mr. Farmer Physicist. What are you planning to discuss next week?
Tune in next week when I prove the existence of the Higgs boson using nothing but kohlrabi and tomatillos!