Polytunnel Temperatures


Polytunnel with Doors Open

I’ve been enjoying the terrific weather this week, it’s been better than summer! OK, not quite as warm as summer but bright warm sunshine and blue skies beat the monsoon season we often get.

One sunny morning I went up to the polytunnel at 10am and it was like walking into a sauna – 37 degrees! It had only been in direct sun for an hour and the night temperature had fallen to just 3 degrees. The next morning I had the doors open at 8am ready for the direct sunshine to appear over the hill and hit the tunnel.

Come the summer, assuming we have one, I can see I’ll have to have the doors open at night but at the moment the night time temperatures have been very low and I’m closing up around 7pm when things start to cool down.

I’ve been keeping an eye on the temperatures in the greenhouse and polytunnel with maximum / minimum thermometers. My feeling was that the larger volume of the tunnel would give more stable temperatures at night but it seems that the tunnel drops lower at night. Having said that, I still need to check the thermometers are reading the same. They weren’t the cheapest but price isn’t always a good guide. If one is reading lower than the other at the same temperature then it would explain why it isn’t performing as I expected.

The soil temperature in the tunnel seems very stable, just under 15 degrees. This is around 5 degrees more than soil outside. This is ideal for the tender plants like sweetcorn and tomatoes as it will warm the air at low level even when it’s cold higher up.

Polytunnel Performance

Up in the tunnel most things are doing well except for very poor germination of the carrots and the potatoes that have a large amount of top have no real tubers yet. I spoke with Dr David Shaw from the Savari trust as he’s an expert on potatoes who reckoned it could be light levels slowing down the tuber production.

Feeding Plants & Soil

I gave the tomatoes, peppers and sweetcorn a high-nitrogen liquid feed (Miracle Gro) to get them away. Thing is with a liquid feed is that it goes in quickly and acts fast. The tomatoes and peppers will go onto a liquid tomato feed when the fruit starts to form but the sweetcorn will be getting pelleted chicken manure and weekly Miracle Gro.

The soil in the polytunnel is thin, poor pasture that was turned over and then covered with about 10cm of a mixture of spent and new commercial multi-purpose compost. It’s going to take a few years before the organic matter and fertility in there is up to much.

I’ll be adding compost and forking to open up the soil below as I go. Hopefully it will start to build up a worm population – there are very few worms in the soil here – and once they arrive we should see the underlying quality improve dramatically.

Posted in Allotment Garden Diary
13 comments on “Polytunnel Temperatures
  1. Kathryn Ost says:

    Agree with most of your comments especially the open/close routine for the tunnel. Haven’t tried with carrots yet this year (rarely seem to get good germination/crops) but my early potatoes, like yours with lots of top, are producing. Got 1lb 6oz off the one tuber I lifted earlier this week so not too unhappy with that. They look nice too.

  2. John Harrison says:

    I realised I was expecting a bit much from my polytunnel spuds – it’s only been 11 weeks since I planted! That’s a nice early-season result, Kathryn 🙂

  3. Drew Adamson says:

    I have two 2m x 4m tunnels. In one I have a max/min thermometer which last week registered 61 degrees C.

  4. Ivan Mendez says:

    Could you advise me on how to obtain a temperature of approximately 40degrees celcious in a polytunnel.

    • John Harrison says:

      Ivan – assuming you’re in the UK. You’re going to need some serious heating. Be better to use a purpose built double glazed greenhouse or conservatory rather than a polytunnel to get a consistent 40ºC

      • Ivan Mendez says:

        Hello JOHN,
        MANY THANKS FOR YOUR RESPONSE. I AM ACTUALLY conducting an experiment to ascertain the maximum temperature one can achieve in normal polytunnels without the need for heaters, etc.
        It has been suggested that using Bubble Wrap, an Internal Mini Tunnel, or a Twin wall POLYCARBONATE sheeting would suffice.
        Please let me know what you advise.

        • John Harrison says:

          The maximum temperature in a tunnel will come down to the ambient external temperature, wind chill and the amount of solar energy hitting the tunnel. Most often the problem people have is that it is too hot in the tunnel.
          Once the soil in the tunnel (thermal mass) has warmed, consistent warmth will depend on insulation. Internal tunnels, fleece etc all serve to retain heat, not to add it.

  5. Terence Belton says:

    Hi this is my first year at Poly tunnel growing, I was wondering about the optimal growing temperature. My current temp 10 Degrees C at night 30° Degrees C morning.
    It is situated in my garden so I have some protection.
    I tried spring greens, they have been fantastic, no blight and huge plants after feeding them with Compost Tea, Same with my Lettuce bed.
    I sometimes water the walking area, (I have interlocking rubber mats that are available all over now). When I open the tunnel on a sunny morning, my glasses steam up with the moisture in the air so I suppose that is good atmosphere for the plants. Of course I have the normal Tomatoes of various strains, Cucumber, Peppers and I am trying a couple of Melon plants that will share the trellis along one wall with my cucumbers. Thinking about peas that I have never tried before, all very exciting. I could post some Photo’s but don’t want to bore everyone.

    • John Harrison says:

      In an ideal world you’d keep the temperature down to around 25ºC for tomatoes but it is difficult keeping a polytunnel from overheating.
      I’d love to see your photos – you can email them to me (see the link below for contact)

  6. Mike Cunningham says:

    just wondering where you bought your poly tunnel?
    I am in search of a good company to use who also supply Crop bars and correct doors with mesh or aeration windows of some kind.
    Mike C

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