The Accidental Smallholder Forum
Growing => Vegetables => Topic started by: martcol on April 30, 2019, 08:17:28 am
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Probably a bit of a how long is a piece of string question, but worth a try.....
It's our first year with the polytunnel. 60ft. West Wales. The temperature this weekend is scheduled to get down to 1 or two degrees outside. I have tender plants, tomatoes, courgettes and so on in the polytunnel (still in pots) and they are doing well.
Any opinion on how much warmer it will be in the polytunnel? does it tend to give say 3 degrees of protection, or 5 degrees? Rough guesses will do. At the moment, I think I'll bring all the tender plants into the house, until the colder period is over, but I'd appreciate opinions.
Thanks in advance.
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One of those interesting speculations!
I doubt that there's any definitive outdoor/polytunnel temp differential. The reality has to be more to do with soil temperature. By that I mean what has been happening for the several days prior to the speculated event.
Meteorological temps are air temperatures and we are concerned with temps very close to the ground for our plants so if there have been several sunny days and the radiant heat has transferred into your polytunnel soil then that will radiate it back out at night and keep the temp above the soil perhaps many degrees higher than outside air temp. Obviously the opposite is true.... many dull days with wind whistling through your polytunnel and soil temps in there aren't much higher than outside so no heat sump effect.
One of the theories with geodesic dome greenhouses was to build a central raised pond to act as an additional heat sump to keep night temps higher (add fish for food and fish excreta fertilizer) much in the same way that some vineyards place rocks under the vines for the same effect of storing daytime sunpower. I paved the central walkway of my greenhouse with concrete slabs for that effect as well as keeping it tidier.
Local forecast here gives a low of 3C this weekend. Since we are higher then that forecasts central point I'd guess 2C forecast for this farm but my soil has started to warm up so a frost won't happen. My seedlings that are hardening off are in trays outside on paving against a wall so also should be fine.
(unless i lose the lot!)
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That is a good point. I have a probe thermometer in the soil and it was showing 16C at about 08:00 this morning. It's been high 20s in the air for a week, and into the 30s over the warm easter. I have a weather station, that records indoor and outdoor temps, so I will be able to see the difference at the weekend. Maybe I'll leave one or two plants of each type in the polytunnel and see how they do.
I've seen permaculture vids where they use large rocks to keep the temp up, and people who keep 100l IBC tanks inside for the same reason. It's all been a bit rushed this year, but maybe I'll try that next winter.
Thanks.
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We have experimented with a cubic metre fresh horse manure hot bed in a polytunnel. Used top as a germination place and idea was heat helped tunnel air temp.
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That's another thing I wanted to try. We have access to lots of manure, fresh and rotted. There is a slim possibility I could sort that by the weekend, though it would need to be already warm, and I don't know if it would stay hot in moving it from the farm next door to here. I'll have a think and report back if we get it done.
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Grass doesn't grow until soil temp is over 45F (yeah I learned stuff in imperial units 'cos I'm ancient). So if like me you're having to mow the lawn you can be fairly safe against ground frosts unless several days of low temps happen...
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I have found that in spring, the polytunnel gives one degree C of protection, another degree inside the greenhouse, which is inside the tunnel, and a degree if we use fleece. Later in the year when there is more vegetation in the tunnel there is a bit more protection. The main reason we have a tunnel is for wind protection.
The thing about tender plants is not only that they can be damaged by actual frost, but cold temps below about 12C can cause problems later on. Here we just have to put up with the later problems as we don't heat our tunnel or greenhouse.
I would take the tender plants indoors overnight. Better safe than sorry. We have three cold nights forecast here in S Scotland, so my toms etc have not even been outside yet, so are just staying put until the cold spell is past.
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I agree with Fleecewife. There’s a difference between tolerance and thriving. Tomatoes, if kept frost free, might tolerate the cold but they don’t like it. They develop a mauve tinge when subject to cold and it checks their growth. So I would bring them in. Unless you are devil-may-care like pgkevet.
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Yes, I've seen that blue tinge before. Good points. If I had my old 8x6 greenhouse, I'd not risk it at all. I just wondered whether the polytunnel was different. It would need to give 6 or 7 degrees of protection for me to be happy, I think. I doubted that, but thought it worth asking.
I'm also more careful with seedlings and small plants than I might be with more mature plants. I may still try one or two and see what happens.
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Yup, just a rash devil! Anyhow most of my greenhouse stuff is already in the borders so bringing it in would be an issue - toms 18" high at least. We're not expecting a drop below 5C outsde until Sunday night. I'll watch the forcasts (as I do) if there'sa risk of a prolongd low spell I just might chucka heater out there but I'm frugal with more than keeping it above freezing. It's really only the 100 sweetcorn in 3" pots I'd put back inside.
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I have read that the polytunnel air temperature will often be colder than the outside temperature due to evaporation from the surface.
While I find this often to be the case, damage by frost, which causes a lot of the cell damage and tissue death doesn’t tend to occur under protection of the polytunnel. So, for examle, my potatoes came through in the tunnel in early March and are now producing tubers - they have displayed no sign of damage or being knocked back, despite temperatures inside dipping as low as -5, which would certainly be a problem outside.
Covering plants that are tender with fleece will help to keep the soil’s warmth in and the plants should be fine. In a 60’ tunnel you are unlikely to raise the temp much with hot rocks or water reservoirs as the volume of air is huge although is definitely worthwhile in a small area e’g 6’x8’ greenhouse.
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One way to add a bit more protection is to shut the tunnel early, so condensation forms on the polythene - this will reduce heat loss upwards a bit.
I am wondering if it's better to have the soil wet or dry when a cold snap is expected - probably dry, so less evaporation from the soil. Has anyone found this to be so?
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..mental image of IBC tank in 8x6 greenhouse...
I'd guess dry soil means air pockets as insulation.
Rocks work outdoors under vines so seed trays on or just above rocks ought to help. radiant v convection?
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No, I think you'd need an 8x6 IBC tank to keep an 8x6 greenhouse warm...... ;)
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I tried 20 x 5 litre wine barrels (filled with water) in our 6' x6' greenhouse, the idea being that they warmed during the day and lost their heat overnight keeping the inside warm. Sorry to say they seemed to make no difference at all- the heat loss through the glass was far too much to be offset by the tiny amount of heat given out by the water barrels.
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Good quality garden fleece works wonders! You can out more than one layer loosely over your plants, since I have started to do this every evening during April and May (Scot Borders, so frost possible til end of May) for a few years now I haven't lost a thing. But it does mean yoiu need to pot on a few times and by the end peppers and tomatoes etc will be in quite big pots.
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The forecast for this area has worsened somewhat since I posed the question. It's now giving -1 or -2 for Early Sunday morning, so everything still in pots is coming back into the house, I think. I do have some fleece, and I'll put that over some of the other plants. By Monday it's back up to 8 degrees plus, and looks better next week, so hopefully things will be fine after that. Reckon I can hold things back for a couple of weeks and then they will need to go in. Should be clear of nasty weather by then.
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OK, so, forecast vs actual outdoors, and inside the polytunnel.
Fri Forecast 3C, Outside actual 6C, Polytunnel 8C
Sat 2C 3.9C 5.7C
Sun -1C -1.5C 0.6C
Mon 2C 2.6C 4,3C
So, the forecast was pretty good, and it looks like a fairly steady 2C difference in the polytunnel over the outside. Looking at those as well, I'm glad I brought the tender plants back into the house.
Unsurprisingly, coldest temp was dawn-ish, around 5-6 am.
Daytime temps peaksed at around 25C, btw, around 15C warmer than outside, when the sun was out.
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Good call. We never got down to frost although i did bring the trays of stuff back into the greenhouse and throw a heater in there. I don't think it ever came on (was set as low as poss).
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We use those cheapy upright mini greenhouses inside the polytunnels. Bought them for next to nothing at the end of the season. You can still put fleece on top of the plants at night.the greenhouses are then dismantled and stored away for the summer