Small Greenhouses & Temperature Control

Why You Might Want A Small Greenhouse?

Fortunately, greenhouses aren’t a requirement to grow in almost all cold climates, but they can be extremely helpful to the cold climate grower.

The greenhouse provides the grower two major advantages in cold climates:

The most useful purpose of the greenhouse is allowing you to put your garden starts outside well before it’s normally safe to do so for frost reasons.  A typical greenhouse will raise your frost zone by practically one whole zone, which usually translates to about two weeks sooner.  When you can heat that greenhouse, it can be a safe environment for much of your garden starts a month (or even more!) before your typical last frost.

The second most useful purpose of a greenhouse is to provide warm weather crops a more ideal environment for their growth.  This is also valuable in cold climates, but not nearly as critical as freeing up your indoor grow space in the early spring.

This post will focus on the most important use.  Getting at least some of your garden starts outside well before it would normally be safe to do so.

Inventing A Small Greenhouse Solution

There are a lot of reasons that someone might not have a greenhouse.  You might be a renter or apartment dweller.  You might not have the space.  Or, you might not have the money or time or skill to build or buy one.

We were in the category of A)  Didn’t have a greenhouse, B) Couldn’t build one and C)  We needed something because we wanted to grow a lot of plants.

So, we kind of pioneered something that isn’t super well documented out there.  The strategy has become an integral part of our cold climate growing process.

The important thing in this post?  A greenhouse doesn’t have to be a permanent structure.  It can also be a temporary structure that can be put up and taken down each season.  Nor does it have to be a greenhouse even!  It could be a temporary hoop house or cold frame, made from PVC and poly tunnel plastic.

Building The Small Temperature Controlled Greenhouse For Garden Starts:

Our plan essentially turned out to as follows:

  • Cheap, small greenhouse
  • Temperature controller
  • Small, inexpensive space heater
  • 6 inch personal clip-on fan

gardmangreenhouseWe were able to pick up a couple inexpensive Gardman Greenhouses on Amazon fairly inexpensively.  They usually go for around $40, but we were able to find them late season discounted down to $25.

The unit features 4 shelves for vertical space savings and a zip up front access that allows watering of the plants and venting.

It’s a “cheap” greenhouse, but we expect with a few zip ties and anchors, that it should be reasonably decent for at least a season or two.  (See our updates at the end of the post.)  We clearly plan to break it down for the winter, it’s just not robust enough to handle an Interior Alaska winter!

The Reality Of Practically All Greenhouses:

The problem with any greenhouse, hoop house or cold frame is always going to be temperature.

Absent external heat, the temperature within the greenhouse can get down to ambient air temperatures quite easily.  If that temperature is below freezing, your plants will be below freezing.  Only the hardiest of cold weather crops can sustain a good freezing.

By design, a greenhouse can also trap a lot of heat.  A LOT of heat.  It’s not uncommon for a greenhouse or cold frame to get 120+ degrees Fahrenheit on a nice 60 degree day.  It’s always necessary to ventilate them, either manually or through some automated fashion.

For us, we don’t always have the leisurely time to ensure our greenhouse is either enclosed or vented properly, based on the conditions.  It was strongly desirable for us to automate the climate conditions, both heat and cooling.

I also didn’t want to dump a huge amount of money into this temporary greenhouse project.  The whole point was for it to be relatively cheap.

Setting Up Temperature Control Of The Small Greenhouse:

We really wanted a way to ensure that our plants won’t get killed off by frost at night.  I simply don’t trust a $25 greenhouse to maintain heat well enough to prevent catastrophe from freezing temps.

The idea was to heat the greenhouse with a small space heater to prevent frost from setting in under any circumstances.  But, I wanted to save electricity and only turn on the heater when needed, really just to maintain an absolute minimum temperature of my choosing.

Fortunately, I have a fair bit of experience and equipment relating to temperature control for my home brewery.  I didn’t have to reinvent the wheel too much for this application.

Enter the Inkbird ITC-308 temperature controller!  This brilliant little device offers relatively inexpensive high and low temperature control.  This temperature controller features a temperature probe that continually measures the temperature.  When the greehouse gets too cold, it turns on a heater.  When it gets too hot, it turns on a fan to help vent the hot air outside.

Figuring Out The Small Greenhouse Heating & Cooling:

We’re not trying to heat a large room here, this greenhouse is at most 12 cubic square feet.  I only need to heat that space to 40 degrees or so, or 10 to 15 degrees above the expected outdoor low temperatures in late April and May.  These requirements could get some of my plants outside 4 to 6 weeks before last frost, which is really great.  The problem was, I had no idea how to figure out what I needed to know.

How many BTU’s are needed to heat this space to the stated requirements?

I taught myself a bit about the BTU requirements of heating greenhouses.  At least enough for this application.  When I first ventured into this, it felt like I was trying to learn some great feat of Tesla engineering.  It turns out there’s basically calculators out there on the internet you can use to figure it out or check your work with relative accuracy.


I was thrilled when I figured out that, with my requirements, I could get by with a reasonably low wattage space heater.  The Lasko 100 “MyHeat” 200 watt space heater seemed to fit the bill here and also fit the “cheap” requirement, coming in a measly $15 each.  The power usage was right up my alley too, in a worst case it’d cost about $10 a month to run for eight hours a day.  Likely much, much less than that.

For cooling, we chose a small 6 inch personal fan to assist the venting and that seemed to do well.  We just have to point the fan towards the greenhouse zipper and crack the zipper door on days we expect the greenhouse might get hot.  We haven’t had problems with 90+ degrees in the greenhouse, so the 6 inch fan is sufficient.

Configuring The Temperature Controller For Optimal Temps:

The temperature controller allows a few important settings that we’ll need:

  • Temperature set value
  • Heating Differential
  • Cooling Differential

The temperature set value is fairly clear, it’s the desired temperature.  The differentials are essentially the values from the temperature set value before you engage either heating or cooling.

What we want is for the temperature controller to turn on the heater at 40 degrees Fahrenheit and turn on the fan at 75 degrees Fahrenheit.  Here’s how we did that:

  • Temperature set value:  45 degrees Fahrenheit
  • Cooling Differential:  30 degrees Fahrenheit
  • Heating Differential:  5 degrees Fahrenheit

The heater will kick on at 40 degrees, which reflects five degrees less than the temperature set point.  It will heat the space to 45 degrees and then turn the heater off.  The heater is on the bottom shelf and since heat rises, it warms the entire greenhouse.  We also found that placing a blanket on top of the greenhouse helped to trap heat from the heater.

For cooling, the fan will turn on at the expected 75 degrees, or 30 degrees warmer than the temperature set value.  All we have to do is zip open the door of the greenhouse on days where we expect the fan will come on.  That’s most sunny days and definitely days over 50 degrees.

The “bad” part of these inexpensive temperature controllers is they work on the whole differential and a single temperature set value concept.  Given the above settings, the fan will TRY to cool the greenhouse down to 45 degrees Fahrenheit.  On a warmer day, that’s impossible, so the fan will continue to spin until the evening when temperatures cool down again.  The only I am aware of to prevent this behavior is to use two separate temperature controllers for heating and cooling.

Your particular settings might be different from ours.  There’s no perfect way to set them up and a number of settings would work.

Scaling This Up To A Real Greenhouse:

There’s a couple things that will change if you were to scale this concept to a larger greenhouse.  The same concepts apply as a small greenhouse.  You can read about how we temperature controlled our large greenhouse here.

First, the heating of the greenhouse.  The small heater specified above will do almost nothing in a larger greenhouse.  Essentially, you have to figure out the BTU required for your space and have a heater rated for that space.  It can get really expensive to do this, either through electricity or a fuel based system.  For a lot of greenhouses, a 110V electric heater just won’t be enough.

When it comes to cooling, you would also use a different fan or even multiple fans.  For most moderate sized applications, I would select at least one standard 16 inch inline fan.  You can calculate the needed CFM (cubic feet per minute) for your greenhouse with an online calculator.

This fan would be installed on a wall, somewhere in the greenhouse.  I would also install a vent on the opposite side of the greenhouse to allow air to flow through the greenhouse.  This calculator can tell you how big the vent needs to be based on the CFM of the fan.  Both would feature louvers that would close automatically when the fan wasn’t running.

It might be a better solution to use something like the smaller greenhouse we talk about to raise your garden starts.  Then, use the larger greenhouse to grow a large supply of warmer climate vegetables over the summer.  That’s how we roll here as it saves immensely on heating costs.

Better yet, you could put a smaller greenhouse inside your larger greenhouse.  This would effectively give you two layers of greenhouse protection.  Each layer of greenhouse protection gives you almost an entire zone of growing advantage.  That would result in very little heating needs, even 4-6 weeks before your typical last frost.  Remember, both greenhouses would need to be vented, so that would mean two sets of these temperature control systems.

Additional Notes On Small Greenhouse Temperature Control

There are a few obvious safety precautions with this setup.  The heater’s output should be kept relatively free of cords and other obstructions.  It’s not a very powerful heater, but you should also make sure it’s not pointed right at the plastic exterior of the greenhouse or other flammables.  Also, water and electronics don’t mix, so be careful when watering.  Better yet, move all the plants out to water them.

We are also monitoring peak low and high temperatures via a wireless thermometer.  It is nice to be able to tell when we’re starting to push the limits of the heater or if we need to adjust the fan a bit.

In our experience with this solution, when you push 4-6 weeks out, you need some additional heat.  We install two of the 300 watt heaters to use through April and this allows us to heat against temperatures in the teens.

Final Thoughts On Small Greenhouse Temperature Control

We are super pleased with this combination and have used it for several years now.  Our migration of some of our transplants worked out exactly as planned.

This inexpensive solution has worked according to our expectations.  We’ve been able to heat against evenings even in the low twenties with no troubles.  Sometimes that heater kicks on a lot, but it’s not super expensive to run.  At only 300 watts, it’s not racking up a major electric bill and that’s piece of mind for me.

For basically the ~$100 budget I had planned to put together a DIY cold frame out of poly and PVC, I was able to put together TWO thermostatically controlled small greenhouse setup that will have more than enough room to handle our moderately sized transplant operation.  This was a huge win for us in our garden.

Longevity Updates:

Longevity update:  We’ve now used this set up for two years and it has worked quite well.  The greenhouses have held up reasonably well, despite being stored outside in sub-zero temperatures most of the winter.  It is a huge benefit to have somewhere other than the indoor grow room to put plants, especially as we get into May when grow space is at a premium.  We prefer to move our cool weather plants out into this, while keeping our warm weather plants inside for maximum protection.  This has allowed us to put off getting into a production greenhouse, although this is still in our plans.

Longevity Update 2:  Well, it couldn’t last forever for $25.  In the third year, the plastic tore away from the zipper seam on one of the two greenhouses.  Amazon does sell replacement covers for the 4-tier and 5-tier greenhouses.  These have been inexpensive enough and we try to pick them up when they go on discount, typically outside gardening season.

6 comments… add one
  • L. Bloom Mar 25, 2017 @ 21:40

    Random question….but I live here in Fairbanks and I am starting my first garden this winter/beginning of spring. I started in my garage a couple weeks ago. I was looking on Amazon for this exact greenhouse a couple weeks ago but all the sellers said they wont ship up here. Do you know which seller you used by chance? Or where I can get something similar that will ship? Walmart doesn’t have in stock and wont ship here either apparently.

    • Jeff Mar 26, 2017 @ 10:02

      Howdy neighbor! Ours was the Gardman brand, linked here. I just checked and it looks like it does ship here, for free, and seller is Amazon itself for a price of $42.94 right now. I hope that helps! Good luck with your garden and thanks for reading!

  • Kendall DuVall Feb 26, 2019 @ 11:35

    This has been very helpful. I have had a greenhouse approx 8 x 5′ and haven’t really been able to use it because I didn’t think I would be able to control the temp. So is it basically the same idea to control the temp from getting over 70/80 deg? What would you suggest

    • Jeff Feb 26, 2019 @ 13:19

      Exactly. If you use a dual-zone temperature controller, you can hook the “heat” side up to a heater and the “cool” side up to a vent or a fan. Usually you’d want to only heat up to 40, or maybe 50 degrees, basically enough to protect from freezing. Your vent (or cooling) would kick on at 70 or 80 degrees, or whatever maximum temperature you want. Your fan may run a lot if ambient temperature is above that setting, but that won’t hurt a thing. You can find greenhouse fans on Amazon for fairly cheap, the kind that have closing vents will reduce undesired heat loss at night. Good luck!

  • David Rosen Sep 7, 2020 @ 17:21

    Super useful article. Do you have any photos of your small and large greenhouse installations? Gracias.

    • Jeff Sep 8, 2020 @ 17:13

      We did capture some this year. We are making some major changes to our site here and one of the tasks is to go through all the current major posts and edit the content more thoroughly, along with posting additional pictures we’ve taken since this was written! Maybe check back in a month or two. Thanks for reading!

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