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I live in USDA Zone 5a, which means we see -25 degF in winter.  That means my greenhouse can drop down to about zero.  There's no way I'm ever going to keep it from freezing.  On a sunny day, my greenhouse hits 70.

I'd like to grow (not just keep alive) spinach in winter.  The best way to conserve heat in the plant zone seems to be to trap humidity with a plastic low tunnel.  Doing this, moisture expelled from the grow beds and will then condense on the cold surfaces of the low tunnel, plant leaves, etc... and release heat.

The question I have is what the effects of 100% RH will be on my plants.  I know that when they attempt to transpire and fail, they take moisture into their cells which can cause fungal problems.  I would think that this is a less serious problem at low temperatures, though I'm not 100% sure.  Are there other problems what would result from 100% RH at low temperatures?

I can ventilate the low tunnels during the day when growth will happen.  But at night I'd like to trap humidity so ventilation is counter-productive.  I've thought of putting my aerator under the low tunnel, which will cause a small amount of ventilation and some heating, helping a little bit.

The roots will largely be kept warm, because they're flooded with 50 degF water on the regular.

Thanks!

Picture is from the Agribon low tunnels from last year.  This year will be plastic and air sealed.

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Hi Jeremiah...fungal diseases are always an issues to deal with cool temps and high humidity...and since the water temps are going to be warmer than the air temp, you can be certain of an RH of near 100% (as you've already alluded to).

Probably the best thing you can do is choose a variety that has some resistance to some of the common spinach fungal diseases, and try introducing a silicate based buffer into your buffering regimen. (Like potassium silicate). The silicon really helps strengthen plant epidermal cell wall tissue, making the plants less susceptible to fungal pathogens. It can also be used as a foliar fungicide in much the same way potassium bicarbonate is used...that is, to raise the plant leaf surface pH, making the leaf surface much less hospitable for low pH loving fungi.

The potassium silicate sold as a fertilizer/amendment is the same as the potassium silicate sold as a fungicide, the only difference being the label. It is much more expensive for a company to be able to label something a fungicide than it is for them to label something a soil amendment/fertilizer...even when they are the same substance. (I have a bunch if you want to try it out...PM me if you want).

Beyond the above two things (resistant varieties, and KSi) all that would be left to you (that I can think of) involve ventilation and heating the air in a manner that would help to dry it out (wood heat) as opposed to heat that would add more moisture (propane)...

Good luck

Wow - thanks Vlad!  I appreciate it.  I'll see about getting some of those fungicide/buffering agents. My water pH tends to run high anyhow.  That might be helping me it seems.

So... fungus in your opinion is the major problem.  You don't see any special issues with cellular destruction from freezing in a high humidity / zero transpiration environment?

Or some kind of oxygen depletion?  I've heard it said that "plants need to transpire to breathe."  I'm not sure what that means though.

I'm curious why you say that propane is drying while wood heating is not.  Don't they both involve the same combustion process (i.e. CxHx + O2 -> CO2 and H2O)?

A tiny HRV would be awesome, if I could find one.  Or something like a heat pipe or thermosiphon.

Wood heating is drying, propane adds a significant amount of water vapour in the air...
Spinach can take some slightly below freezing temps, especially if their are some salts in your system. It uses the salts present as anti freeze. So don't get freaked out by the guttation you see...many noobs will think their insect eggs or other funky stuff.
Yeah they do need to transpire, so you prob don't want to seal everything up and not ventilate...in which case I believe you'll notice fungal problems before you detect any 'breathing' problems..

Ok cool.  I'll work on the chemistry to keep the fungal problems down to a minimum.

Then I'll ventilate just enough to keep RH below 90%  Once I'm below freezing, shut off because who cares if I have 100% RH below freezing, right?

Wish I could justify buying a couple of these.

Is it worth ventilating at night?  I know transpiration is much faster in the day.  Does it matter if they can transpire at night when it's cold out?

I had a conversation with a horticulturist the other day and his primary concern about my system is the constantly-wet roots while at cold temperatures, and the implications for osmotic pressure.  He suggested that my roots typically store energy in sugars and salts, and this will be difficult when constantly wet.  

Any thoughts?

Just this one...



Jeremiah Robinson said:

I had a conversation with a horticulturist the other day and his primary concern about my system is the constantly-wet roots while at cold temperatures, and the implications for osmotic pressure.  He suggested that my roots typically store energy in sugars and salts, and this will be difficult when constantly wet.  

Any thoughts?

Nice :)  A beautiful thought!

How cold does your air temperature get?

Sometimes it's a struggle just to keep the air temperature above freezing...but it fluctuates with outside temps.

Gotcha.  

Keeping mine above freezing where I live would be very expensive.  As it is, it gets down near 0 degF (-17C) in the greenhouse on occasion.  I can improve that a bit but I'm not willing to go to the expense of improving it a lot.  So I'm left trying to figure out how to keep my plants alive at very low temperatures.

I've had some success, but I'm trying to figure out the mechanisms by which my plants die over time and which ones I can remediate, beyond just simply getting too cold.

Yeah, it gets even a bit colder at times outside, so I built a gassifying stove (for about 50 bucks out of some old army barrels. I have lots of woods on my property, and a chainsaw and an axe...so it didn't really cost me much)...you have to work with what you have, and do what makes sense in your set and setting.

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