Heating a greenhouse, in Spring, so that seedlings are kept warm enough (tender things, like Tomatoes, Cucumbers, Aubergines, Peppers, etc. need a minimum of 10C day and night) can be done, more economically, by putting a smaller structure, like a tunnel or a blow-away, inside the greenhouse, and then putting the seedlings inside it and only heating that, smaller, enclosure.
I’ve done this in the past with a tunnel in my (unheated) conservatory, and then put a Metal Halide growing lamp in the tunnel, turned on at night. The seedlings get the extra light hours and grow more quickly, and the 400W Metal Halide bulb chucks out enough heat to keep the tunnel toasty warm.
Here’s the construction of the tunnel in Spring:
and then the tunnel with growing lamp and seedlings:
Chatting to some gardening friends someone mentioned that they didn’t bother heating their tunnel (actually a 4-shelf blow-away), inside their greenhouse as they found that it provided enough protection on its own. I was a bit sceptical because when I monitored the temperature in my tunnel it fell like a stone the moment the lamp went out in the Morning – the polythene doesn’t provide any significant insulation benefit.
I’ve got an old 4-shelf blow-away, and some logging thermometers (the type which you press a button to start, and then when the experiment has finished they can be plugged into a USB port and the data downloaded), so I constructed the blow-away in the greenhouse:
and I put the logging thermometers:
- Top shelf of the Blow-away
- On a pot standing on the border in the greenhouse (the plastic bag & pot at the bottom left of the Blow-away in the photo)
- On a brick outside the greenhouse (North/Shaded end)
Turns out that there was a flaw in my choice of positions for the thermometers, but maybe that mistake is serendipity!
The blow-away was colder, overnight, than the greenhouse temperature! I suspect the reason for this is that the Blow-away thermometer was on the top shelf (covered with a pot so that it was never in direct sunlight), whereas the “greenhouse” temperature was monitored by a thermometer sat on a plant’s pot, on the ground (also covered by a pot to keep the direct sun off) and that would have been influenced by the ground temperature. I suspect that if I had put the thermometer on a table, at the same height as the shelf in the Blow-away, it would have registered a colder temperature.
Anyway, I think it shows that, without any supplemental heat, the Blow-away is not providing any useful insulation, and indeed the temperature inside both the greenhouse and the Blow-away feel to, as near as makes no difference, the same temperature as the external temperature, and on 3 nights hit 0C – which would not be good for any young seedlings
Other points to consider:
The Blow-away was kept zipped-up all day, so it got hotter than would actually be the case. If such extra heat contributes to storage, overnight, that would have been apparent, but clearly that was not the case.
There were no pots / soil in the Blow-away. Some “mass” would have helped buffer the temperature. It would be interesting to put a container of water (say) in the Blow-away to see if that helped.
A Blow-away might well be kept reasonably warm just using a Tee-light (putting an up-turned clay pot over the Tee-light can help to radiate the heat).
My measurements are to 0.1C but in practice the thermometers probably cannot measure that accurately, so 0.5C here or there may be experimental error, rather than one part of the greenhouse being warmer than another.
Here’s a zoom’d image of just the 21-Feb. As is to be expected there is a useful lag of falling temperature inside the greenhouse compared to outside. The temperature starts to fall at 16:00 and at that time the greenhouse (and Blow-away) are 12C and outside is 5.5C. The greenhouse (as shown in the table below) lags behind the outside temperatures by some 2.5 – 3.5 hours.
The temperature outside falls from 5.5C at 16:00 to 2C by 18:00, and then more slowly to 1C by 21:00, and 0C by midnight. Minimum temperature is -1.1 from 6:30 to 07:50 when it starts to warm up again.
Inside the greenhouse the temperature does not fall to 5.5C until 17:50, two hours behind the outside temperature. Here’s a table of the key temperature points:
|10C||N/A||16:20||Temperature started falling below 10C (in greenhouse)|
|5.5C||16:00||17:50||2 hours lag|
|2C||18:00||21:20||3.5 hours lag|
|1C||21:00||23:40||2.5 hours lag|
|0C||00:10||03:40||3.5 hours lag|
|-1.1C||06:30||07:00||-0.9C MIN in the greenhouse, 1.5 hours lag|
|Warming||07:50||07:20||Greenhouse starts warming, when the sun rises, before external temperature rises|
|0C||08:20||08:00||20 minutes ahead|
|10C||N/A||09:50||10C being the safe temperature for tender seedlings.|