Winter night time temperatures in a Polytunnel

Hi, We've just got our first polytunnel up and running and all is going well.

What I'm wondering is is how cold is it likely to get in the winter. i.e. At night if it's -5C outside what temp am I likely to get inside the tunnel?

Also if I built a big fleece cloche inside the tunnel, what if anything would that do for the min temperature?

The tunnel is 14 x 25 ft, single sheet polythene. I'm based in Rush North Co Dublin, Ireland about a 1km inland. (Zone 8b)

Reason I ask is that in addition to wanting to grow veg I also want to use the tunnel to overwinter my pelagoniums and a huge Yucca I have that has suffered really badly the past 2 winters. I've also bought a Boungainvillea and a lemon tree, I'm wondering weather to plant them in the ground in the tunnel or to put them in pots so I can bring them indoors if the tunnel gets too cold.

Any advise on worst case scenario winter night time temperatures & what to do to mitigate them would be much appreciated.

 

I don't think that Polythene gives much gain over outdoor temperatures (glass would give you a bit more), but it will keep the wind out, which has an associated wind chill for plants outside, of course, and it will keep the roots dry - cold & wet roots are often the cause of things that are not totally hardy failing in a cold winter.

You could put an inner cloche-tunnel inside the big tunnel (I would use plastic, rather than fleece), and some night-light-candles, or some other form of background heating (a small gas or paraffin greenhouse heater, for example) on nights that were particularly cold.

 

I keep meaning to check the internal/external temperature differences with my small polytunnel, but need another max/min thermometer, plus another soil thermometer, as I think there could be heat released overnight from the soil within the polytunnel that's warmed during the day (a bit like a storage heater).

 

Yes, I would expect that the temperature falls slower in a tunnel than outside - and, at this time of the year, by the time its getting close to the minimum outside daybreak will have happened and temperatures will rise - which is great for young plants being brought-on.

But I reckon a cold snap like we had just before Christmas, -10C day and night for a couple of days, the Polythene won't give you any gain.

We had -10C minimum air temperature here, and my Max/Min in the glasshouse was -6C - but there is probably some "error" in the two thermometers, so not sure how accurate the +4C delta in the glasshouse was, in reality.

 

I don't have a polytunnel, but I have a summerhouse and a greenhouse where I overwinter plants. I would agree with the others. The minimum inside is often about 3C warmer that the minimum outside. Because of the delaying effect of the shelter. However If it was well below freezing day and night for some time I wouldn't expect the inside low to be any better than the outside.

However, your location means that you will be a bit warmer in winter than many of us. I don't know about the other plants, but I have no doubt that you could overwinter Pelargoniums in a polytunnel. They will take a bit more frost than many people imaging. I have had some survive outside overwinter in North Yorkshire, albeit close to the house.

If they are zonals let them dry out completely and they will go fully dormant. However Regals don't go fully dormant, they will continue to grow very slowly even at 0C - so you can't let them totally dry out.

 

Sorry Esthalon, I don't have a polytunnel but I have had a greenhouse.
I don't know what the night time temps in your polytunnel will be but reckon it will be 2 or 3 degrees warmer than the outside temp - without the wind and the rain. I think you have to work out what's possible over winter according to your own winter weather.

 

Is there any sort of minimum volume enclosure required for burning a night-light candle in to prevent any adverse conditions inside?

It's just that i was wondering about putting them inside some home made 'cloches' placed over some early plantings of marrows & pumpkins, and if lit at the right time i wouldn't have to worry about getting up too early before the sun came up and things got overheated inside. About one cubic metre is the plan.

And another question, has anyone tried the inverted terracotta pot(s) over a candle method (radiated heat rather than convection plus the soot sticks to the underside of the pot)?

 

YouTube - polytunnels's Channel

Don't take it as Gospel though ... "Pink Fir Apple" is not an Early potato, for example ... and Early Potatoes don't take 14 weeks either ...

See from 1m20s onwards. I should have said "Tee-lights" instead of nightlights - you can get them to burn for longer periods.

Yes, I think the terracota pot is a good idea to spread & radiate the heat.

You'll need Max/Min thermometer in the Tunnel and Mini-tunnel to know what sort of frost protection you are actually getting. A thermostatically controller heater would be better - Electricity if you can get that to the tunnel, otherwise Bottle Gas. Anything else is going to be costing you money-for-fuel on mild nights and run the risk of not-enough-heat on cold nights - plus a fair amount of Faffing. Paraffin is OK, but needs setting the wick right - which means returning about 20 minutes after lighting to trim the wick, and Paraffin produces a lot more condensation than Gas.

Personally I would go for an Eden (now known as "Lifestyle" but eBay ones may well still be Eden ) gas greenhouse heater second hand off eBay. Something like this:
Lifestyle Gas Greenhouse Heater 1.9kW

 

Thanks all for your advice. I now have a much better idea what to expect.

I now plan to take no chances :D.

I'm going to buy one of these Bosmere Electric Greenhouse Heater

And using the frame of an old 4 man dome tent, use bubble wrap to build a "tunnel" (dome) within the tunnel. I'll proably use H Fleece too for good measure.

Thank you all so much for your posts and sharing your experience.

 

Hmmm I tried to reply to this thread, but my post never appeared...trying again.

Thanks all for your insights and advice, I now have a much better idea what to expect...i.e. the worst!

I'm now plannign on getting one of these Bosmere Electric Greenhouse Heater

And came up with an idea to use an old frame from a 4 man dome tent with bubbel wrap to create a "tunnel" (dome) within the tunnel. I may even use H. Fleece too for good measure!

Thanks all so much for your help.

 

I've been trying to post a reply but no luck. 3rd time lucky, I hope!

I just wanted to say thanks to all for your insight and advice. I will now prepare for the worst!

I plan to get an electric greenhouse heter that will kick in if it goes below 5C, I can get one online here for €29.99 (Not going to try adding link as maybe that is why I couldn't reply?)

Also going to use frame of old dome tent with bubble wrap to create a "tunnel" (dome) in the tunnel. And I may use fleece too for good measure!

Thanks again all for your helpful replies.