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Heating a greenhouse on a budget??? Ideas?

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D-Grower:
Hello all! Just getting settled at my new zone 8 location. Have erected two greenhouses but not of the greatest resistance to the 20s° F nights here right now. Many of my prized plants have gotten cold burnt already but most my important ones still live and could perhaps make it through at this point. However the 20s are here to stay for another week or so at night...

Right now I've strung up my best plants with old incandescent Christmas lights and also covered them with sheets even inside the greenhouses. Hopefully that will help get me through the week...

However outside what I've done with my little buget at the moment is there any tricks up your all's sleves? I don't have huge barrels for the water heat retention and radiation at night thing. Probably cost way too much to put a space heater in thin plastic greenhouses and could be even more a fire hazard than what I've currently done...

One greenhouse is 20'×10'×10' and the other 10'×10'×10'. Did double plastic them too on the top at least and the long sides on the biggest one.

Any suggestions?

Thanks! DG

Isaac-1:
My suggestion would be a propane heater, perhaps one of those $100 patio style radiant heater that hold a 20 pound propane tank in their base.  Or even one of these https://smile.amazon.com/Mr-Heater-Single-Outdoor-Propane/dp/B004W7SIUG  On low they will run for about 16-20 hours on a tank of propane, a 20 pound tank typically cost around $17 at one those propane tank exchange cabinets that can be found in front of all sorts of stores (hardware, grocery, dollar, etc.)

KarenRei:
Once upon a time I lived in Iowa and built a greenhouse. I had a choice between propane and electricity. I chose propane.

I chose poorly.  Go electric.  Propane is an annoying chore for a greenhouse. Even though I went with a 30 pound tank to reduce the frequency of trips, and kept the greenhouse super-cold (wasn't growing true tropicals back then, just "garden plants"), the periodic replacements were a drain on my life.  Also note my caveat further down in this post about propane heat.  Now, you're certainly in a warmer climate than I was in Iowa! But you're also surely not wanting to keep your greenhouse super-cold either.

Now, when I say "go electric", am I saying "add electric heaters"?  NO!  Add electric lights!  Your plants will lack light in the winter (even if you do what I did and put reflective insulation on the north walls); mine sure hated it. Ideally go LED if your budget supports it; even with LED, most energy that goes in is wasted as heat, but you get a *lot* more light out than you do with incandescent for a given wattage.

Do black-painted water barrels help?  A little.  But not as much as you'd hope.  Also note that the water inside will steadily get ucky, and if you add chlorine, it'll shorten the barrels' lifespan.  Better for heat storage would be - if your budget allows for it - running tubing underground and hooking up a blower (or a water system, so long as you're sure it won't freeze).  I had an idea for a "low budget" version at one point involving an auger and doing pairs of 45° holes that meet at the bottom, then fitting PVC tubing through them forming a zigzag network. But I never implemented it.

Some other things:

Do yourself a favour and run water to your greenhouse; hauling buckets of water is tiresome work.  And don't do what I did and manually dig a trench with a garden hose in it; get a proper trenching tool so that it'll be deep enough to not freeze, and have a plumber connect a water line to the *inside* of your house.  Or if you don't want to do that, then find an electric heating solution for where the hose is exposed outdoors on its way into the ground. Insulation alone isn't enough to keep that from freezing.  And on the greenhouse end, make sure that the hose comes up *inside* the greenhouse, not outside as I foolishly did in my first iteration.

You can set up an automatic watering system to save you from having to go outside in the cold, but be careful.  You have to keep your electronics from getting wet first off.  But secondly, don't make my ultimately fatal mistake.  I had a painted-steel framed kit greenhouse (which I heavily adapted over time).  My watering system *dramatically* shortened its lifespan, and ultimately rusted it away after just several years.  The thing actually collapsed on me while I was inside. Now, my heavy insulation that I added on the north wall sure didn't help things any (lots of extra weight), but it was fine until the steel rusted; by that point, a windstorm would have taken it down regardless even if the insulation weight wasn't there.

If you don't have them, install vent openers and/or fans.  Leaving the door open all the time in the summer may be fine, and always closed in the winter, but in spring and fall it's a twice-daily task of opening and shutting them manually.  And if you screw up?  Hello scorched plants.  Or more accurately, goodbye scorched plants.  I did implement an improvised solution that did actually work (solar panels hooked up to fans on a small vent in the roof - it vented when the sun shone, and stopped at night), but a proper opener would be better.

Oh, remember that caveat about propane heat that I mentioned above? Here it is: the heater Isaac linked is NOT AN INDOOR HEATER.  At least as far as I can tell from the ad.  It could kill you if you use it in the enclosed space of a greenhouse.  And FYI, your plants can be poisoned by carbon monoxide as well.  You need an *indoor* propane heater.  There are two types, indoor-rated unvented heaters, and vented heaters.  The former - even though its rated for indoor use - will still worsen your air quality (buildup of ethylene is the biggest one for plants).  And if the greenhouse is too well sealed, it'll keep hitting its oxygen cutoff.  I used an indoor-rated unvented heater because I thought the extra CO2 would be worth it, but it wasn't. I'd recommend an outdoor, vented heater if you go propane (or natural gas).

Concerning indoor air quality: you may want to seal every gap, but greenhouses need to breathe.  Even without a propane heater, an overly sealed greenhouse will suffer from ethylene buildup (plants give it off on their own), and ethylene is toxic to plants at much lower levels than carbon monoxide is to humans.  One of my DIY projects which I *was* happy with was my DIY heat exchanging ventilation system, which brought in fresh air while heating it up with outgoing air.

Ultimately, though, after my greenhouse rusted, I didn't rebuild.  Since I no longer had a greenhouse, I began starting my seeds indoors.  And they kept taking up increasing amounts of space.  And needing more and more light.  And then it got to the point of, "hey, I have all this light here, I can grow plants that I never would have been able to otherwise, and just never move them outside".  And so bit by bit began my bad habit of obsessively culturing exotic tropicals indoors  ;)

KarenRei:
Oh, and as for the "double plastic" thing: sounds like it's a film greenhouse.  I had that too.  Hope you realize that you need to change that film regularly.  If it's a good (but conventional) greenhouse film, say, PVC, it should be once every two years, maybe three as a stretch.  If it's something not rated for outdoor use, change it annually, if even that late.  It'll fog and get brittle.  Even PE designed as greenhouse film is generally replaced annually.  And if it's not a proper greenhouse film, you're probably losing more light than you realize, even when it's not visibly fogging.  And light matters a huge amount. 

Again, I really can't recommend doing a "greenhouse on a budget".  Build a proper greenhouse, or don't build one.  I know that sounds harsh, but seriously, picture how you'd feel if after a few years you decided that the thing you built was turning out to be a giant PITA and/or was killing your plants, and all that money and labour you poured into it was for naught.  You don't want that to happen, do you?

If you want a film greenhouse, there's now some ETFE ones out there on the market (expect to pay a lot more for ETFE film than PVC or PE), and ETFE actually does last for decades (UV doesn't degrade it). On the small scale, however, polycarbonate panels are probably the best choice.  You'll have to replace them after 10 years or so, mind you, so keep that in mind.  But you never have to worry about, say, hail shattering your roof, and don't have all that weight on the frame.  And as for the frame, again note my experience... be *very* wary of rust.  I would *never* use painted steel, ever again.  If you already have painted steel, if you can be bothered to tear it down, find a local place that does galvanizing and send it off (note: galvanizing will add a little thickness to the metal, so make sure it'll fit back together!).  There's also zinc-bearing paints, and while they're better than regular paint, they're not as good as proper galvanization.  A galvanized steel greenhouse would be fine, although I'd still prefer alumium. But be careful with alumium - you should use plastic spacers wherever it contacts any other metal, and not anchor it directly in the ground or into concrete. Aluminum resists corrosion superbly in the air and in fresh water, but is very susceptible to galvanic and chemical corrosion.  If you're designing the greenhouse yourself - or if the one you're importing is kinda cheapo - don't underestimate how bad snow and wind loads can get.  And of course insulate the north side, but that goes without saying.

Triloba Tracker:
And this is why i have no desire to have a greenhouse :)

Couple people I know are wanting to get into greenhouses, and I'm saying to them "WHY?!?!"  :o

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