How to build a small polytunnel for seedlings and other small plants

I recently made a small polytunnel slash cold frame to assist me and also my and young seedlings by offering me an easier way to germinate seeds and to also give my seedlings a more comfortable start to their life cycle.

Knowing how to care for the seedling is knowledge for success in the diverse and vast gardening world. And so, such knowledge should be held invaluably!

The seedlings benefits amazingly from polytunnels. By planting seeds within these mock greenhouses your seedlings and young plants are protected from the harsh elements and also certain insects as well. Although to a lesser degree, but not forgetting how vulnerable they'd be in the outside.

Some people may wonder, what is a polytunnel? Well, firstly, a polytunnel is often but not always a large structure constructed out of various materials and is then covered in a polyethylene sheet. They're usually made out of easily accessible materials which is why they're favoured so much.

Polytunnels are often hoop shaped and set up as cheap greenhouses. They generally have the same function although  at a fraction of the cost. They can be formed into other shapes other than the usual semi-circular model although this is the most popular as it allows rain to run off smoothly.

The main benefits of polytunnels are to protect plants from harsh winds, heavy rain and hale, frost, cold and to also create a growing environment by trapping in heat and moisture within the polytunnel. Plants that enjoy tropical climates gain the most advantage here. It would be impossible to replicate said environments without structures such as greenhouses or polytunnels.

Now you know what a polytunnel is. Let's move on to how to construct my miniature version for starting of the seedlings and for protecting other young plants.

So how do you make a greenhouse or polytunnel? I hear you scream in extreme anxiousness. Well read how I did it below. I've included illustrative pictures so anybody can have a go!


Tools and necessities required:

1x saw
1x manual drill and 1x power drill
2x 90" length by 2"x2" pieces of timber
2x 90" length by 1"x1" pieces of timber
1x At least 30 feet of 1" width polyethylene piping
1x Screwdriver plus a pack of 4" and 3" screws
1x piece of polyethylene or any opaque piece of polythene, transparent plastic sheeting or tarp (At least 5x5 metres)

I've designed my polytunnel around housing 4 of these little pot holders. They are approximately 12" by 22" so my 90" piece of timber was easily cut in half to fit the length of the polytunnel. You can obviously change the measurements to fit your desired build.





As I new the sides of my pot holders were 12". I just simply doubled that by 2 and added an extra 4 inches to make room for the front and back sides of the polytunnel.

I then cut one length, aligned it up to my remaining piece of timber. Marked it off and cut the final side.

We have children running around in the garden alot so it's best to sand rough edges down to avoid nasty splinters!

As you can see the 4 pot holders will fit nicely with the correct measurements.

The next step is to measure the entire front side so we know where to drill our holes to mount the hoops. I already knew the front side would be 45"+2"+2"=49" but it doesn't hurt to measure again.

As I wanted 3 hoops I simply divided 49" by 3 which is 16.33. 

Measured that amount along the entire front side and marked it off at each occurrence (Note: I later changed the amount of hoops to 4. But I kept the same measurement of 16.33 and it worked out fine.)

To save time and to get an even measure on both the front and back lengths just simply use a straight edge and draw a line. I used a saw which is fine as long as you place end of the handle exactly straight along the side.


Next you will want to mark off at 1" across your front length of 2"x2" piece of wood.

Repeat that with both lengths until you have something that resembles the picture below. 

You will also want to do the same with both sides of your polytunnel. On mine I marked a cross at 1" across both ways.

Now simply drill all of your holes. I chose a depth of 1.5" although 1" is fine. The drill bit I used was 1" as my piping I had for the hoops was exactly 1". You may have to alter the measurements to fit your needs once again.

The pieces of piping used for the hoops simply slide in like so.

Now it's time to get your hoops ready. Mine were all arched and bent so I had to straighten them out somewhat before I could get a correct measurement. You will want to tinker around with it a little bit to get your desired height.

I started with a piece around 8 foot but it was far to tall so I just sawed off a few inches, tried it again, and repeated that same procedure until I found the height I wanted. I finally chose a length of 66".

 Next step is to mark and cut all of your other hoops with your chosen length.

Once that's done. Drill your screw holes through the sides and screw in your 4" screws also through the piping like so. I chose to do this to give the end hoops more support.

Repeat with the other end. Slot in all of your other hoops and as you can see it's starting to take shape! I didn't bother screwing in the other hoops as the 1.5" holes I drilled for them would hold very well.

Now it's time to apply your sheeting.

The best thing to do is to fold one side under and unravel the remainder of the sheeting on the other side. Like so.

Next, use a straight piece of wood and align it parallel to your polytunnel. (Doesn't have to be exactly straight as there will be some excess sheeting anyway).

Cut along the piece of wood as straight as possible.

Now you need to crimp the front side of your polytunnel with clamps whilst you screw in your piece of 1"x1". As the picture describes below.

Cut off the excess sheeting off of that one side. And repeat the above procedure with the back side.

Now it's time to sort out the sides. I just simply folded up the remaining sheeting and screwed it down and I also screwed in a small piece of wood in the centre of the side and did the exact same with the final side.

And that's it! One beautifully crafted polytunnel constructed! You can now sit back and marvel at your wondrous creation!

This polytunnel cost me nothing to build as I already had everything I needed laying around. Although to build one like this it shouldn't cost to much.

The completed structure should last for approximately 5 years. Give or take a year or two. During really hot episodes you may want to raise it slightly to allow some hot air to escape and to give your plants some extra co2.