You Should Consider These Five Aspects of Greenhouse Kits

I just couldn’t resist buying several greenhouse kits the other day. They were on a discount table at our local auto store (I love that store) for $25/each. It got me thinking about how to use them and what I’d like in a small greenhouse. So here’s my thinking.

As a heads up on these kinds of kits, they’re not going to be as good as a real backyard greenhouse and serious gardeners may want to think about which will be more useful to them.

They were only a few dollars and I could use them until the plastic disintegrated with no problem. (This turned out to be about 3 years) I thought I could then recover them with my own plastic. (Turned out to be easier said than done to get a good fit) And if not, I could use them for other things. (Yes, I do) More later on this but generally, the frame became a cold frame when laid on its side and the shelves were repurposed into useful coffee tables for the dock.

Construction ease

Some kits can be a bit on the complicated side; the unit I purchased came with a single sheet of instructions (exploded view) and was a bit of a challenge to figure out but not one that was overly difficult if you sorted all the parts beforehand.

First bit of advice – do sort all the parts and fittings beforehand so you don’t have to figure out if you have enough of those 38cm parts versus the 42cm parts. 🙂

This kit was screw-free. No fittings to tighten or mess up – nothing to drill. It was simply fit the pipe lengths into the plastic corners and push until tight.

Construction durability

Greenhouse kits are going to come in various levels of durability. Let’s be honest here, the unit shown in the pictures isn’t going to be all that durable. It’s light steel tubing with plastic joints, no foundation, no attachments to the ground and a plastic covering that’s not replaceable. One wind storm (if it were out in the open) and this one is well-wrapped around a tree.

But it suits my purpose perfectly. It’s out of the wind, well-sheltered by the house and garage and I’ll put a cement block or two on the bottom pipes in the spring when I have it covered.

If you want a more permanent greenhouse, I’d suggest you look at one with foundations and how you’re going to secure it from the wind.


The unit pictured has a heavy plastic covering that we’ll use first thing in the spring as a frost protector – remove it after all danger of frost and store it carefully for the summer. Because it’s in the shade, it will not be exposed to UV rays that degrade it and I expect to get at least 5 years (as I said above, I got three) from it before I have to make my own covering.

Expose this plastic to the sunlight for one summer and that’s the life expectancy.

UV-treated plastics will last 4-5 years when exposed during the summer.
If you want something more permanent, you’re going to have to look at fibreglass, poly-carbonates of one form or other or glass.

Intended Use of Greenhouse Kits

We use ours as a seed starting frost protection system. That’s it. We’ll toss the seeds in there in the spring and free up some windowsill space. The racks give us all the space we need for medium cold-hardy plants. Perennial seeds can be right outside and tender vegetable crops can either be started in this unit later in the season or given premium space on the windowsills or (hopefully) the indoor growing area under lights (planned for the basement).

If you’re looking for more than a seed starting or frost protection system, then you’re going to have to evaluate some of the other issues I’ve written about – with heating, ventilation etc.


This is an excellent system if you don’t own your own house and tend to move regularly. The system pulls apart (remember no screws to rust) and stores in a box about 3 feet long by 2-inches high and 8-inches wide.

If you are looking for greenhouse kits with portability, look for something like this compared to a heavier system that requires foundations etc.

Update: June 2021.

Those small kits lasted a few years and when the plastic cracked it turned into a major hassle to recover them. I gave up. I now grow my plants using lights in the basement and harden them off before planting.

Five Reasons You Need Greenhouse Fans

We use greenhouse fans to do two things in the greenhouse.

  • The first is to reduce the air temperature so the plants will grow and
  • the second is to move fresh air into the greenhouse to reduce the relative humidity of the air surrounding the plants.

Both are important functions of greenhouse ventilation. No, let me restate this – both are critical to success in the home greenhouse and these are mostly ignored by enthusiastic beginners. (Don’t be one of those people who build first and figure out the details second) 🙂

Relative Humidity

Let’s look at something many home greenhouse growers don’t take into account – the relative humidity around the leaf surfaces.

When relative humidity increases past 80% on many plants (tomatoes or other soft leaf plants in particular) then diseases start and run rampant. Over 90% (typical of many home greenhouses) and you are almost guaranteed to have fungal problems. Under 70% relative humidity and these problems almost disappear.

Two considerations are important here. Number one problem with this of course is that it costs money to heat the air you’re blowing out of the greenhouse. Secondly, you have to heat the incoming air before it hits the leaf surfaces and chills them.

Winter greenhouse operation then becomes a balancing act of ventilation and heating to keep the plants healthy and growing well. We have to ventilate to drop the humidity but we want to conserve energy. This is something for which there are no guidelines on the home scale of things.

You can install humidistats to turn on the greenhouse fans when the humidity gets too high or you can run by guess and by golly. But keeping it below 80% humidity is going to be your goal.
Heating the incoming air can partially be done by running greenhouse fans to blow air around within the greenhouse. Good air circulation and air mixing is important.

In a small greenhouse, a house fan with 18-24 inch blades will mix the air if it is suspended at roof level and blown down the greenhouse. In larger home greenhouses – say over 20-feet long – I’d use an old furnace fan unit connected to commercially available plastic ventilation tube. The tube carries the air down the greenhouse and holes in the plastic force air downwards, mixing the warmer upper air with the colder incoming air.

Determining Your cubic footage

The math to determine your greenhouse fan need is to take the square footage of one end wall and multiply it by the length of the greenhouse.

This will give you the cubic feet of your greenhouse.

Figuring this Out: Rule of Thumb

  • Winter air changes should be 2-3 total air changes an hour.
  • So if your greenhouse is 15 feet long, 10 feet wide and 8 feet tall, your total cubic area is 15x10x8 or 1200 cubic feet.
  • You will need to move 2400 to 3600 cubic feet of air per hour and your fan sizing should be able to handle this.

Greenhouse Fan Sizing Measurement

Fans are measured in cubic feet per minute capacity. So moving 3600 cubic feet per hour requires a fan of 60 cubic feet per minute. (3600 cubic feet divided by 60 minutes) But check out the summer ventilation rates to understand why you’re going to be using two greenhouse fans.

If you need a full set of instructions on greenhouse management and growing advice, click here

Reducing Air Temperatures

Again, we need to reduce air temperatures for the plants to grow properly. Once leaf temperatures start to climb into the 80’s F, then the plant reduces its growth. It is too hot. We use greenhouse fans to ventilate and blow this warm air out of the home greenhouse.

A single visit to a greenhouse during the summer will quickly convince you of the power of solar energy and the resulting heat (if not ventilated) can quickly cook plants and soil.

In the winter, we need to move 3 air changes an hour. In the summer we need to move one air change a minute.

So the greenhouse example above of 3600 cubic feet of space means that our fan now has to be rated for 3600 cubic feet per minute.

You can see greenhouse fans and vents here

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