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Heating a lowland greenhouse


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#1 mlong

 
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Posted 08 February 2012 - 11:09 AM

Hi there

I've got a few lowlanders in a terrarium but am starting to think about how to house them when they get bigger (or when I get my hands on a few more!). I was thikning about setting up a small area, perhaps to start off with one of those small PVC greenhouses (roughly 2' x 2' x 5') which will be situated inside a greenhouse already kept above 20 oC.

What wyould people recommend for the heating? I think I've seen similar set ups here in the forum in the past, can you use a normal greenhouse heater like this http://www.greenhous...CFcQTfAodYioFfg or are they unlikely to be able to raise the temperature high enough?

Any tips or advice would be fantastic.

Cheers,

Mark

#2 manders

 
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Posted 08 February 2012 - 17:31 PM

There are quite a few 'lowland' neps that would survive quite well with a minimum of 20C, and for the majority raising it to the mid 20's would be all that is required so a 2Kw heater would be more than ample. (one of those greenhouse heaters can raise a 10ft x 8ft grrenhouse by 15C above ambient easily), the built in thermostats are rubbish though so I would control it with an external thermostat or you may run the risk of cooking your plants.

#3 mobile

 
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Posted 08 February 2012 - 17:35 PM

What are you currently using to maintain 20°C?

#4 Gareth Davies

 
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Posted 08 February 2012 - 20:38 PM

The only issue in a "mini greenhouse" is that a fan heater is way too powerful... I tried it a couple of times and the problem is that even on the 1kw setting, it chucks out way too much hot dry air in such an enclosed space. The plants hate it, especially smaller ones.
For a small space, such as an enclosed area where I grow my lowland seedlings, a better solution is soil-heating cables. If your greenhouse is already at 20 degrees, setting up soil heating cables as per their instructions, buried in a couple of inches of sand, will provide a much more gently, steady heat. I place gravel trays on top of the heated sand, then put 1cm of gravel and water in the gravel tray, meaning the plants are sitting in a small amount of warm water. This method has worked wonders for me, generating humidity and heat at the same time.

#5 mlong

 
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Posted 09 February 2012 - 09:27 AM

What are you currently using to maintain 20°C?


Ah, good question. I look after the CPs at Treborth Botanic Garden (http://www.treborthbotanicgarden.org/) near Menai Bridge and Bangor. It's part of the University at which I'm a student and anyone's welcome to visit it and its grounds. We've been growing carnivores for a few years now, the lowlanders are currently in a terrarium in one of the heated greenhouses and this is where I'm hoping to be able to build a larger grow area. The heating is provided in the greenhouses by pipes acting as radiators from a boiler. Ideally it would be great to tap into this for additional heating, but I imagine it would be difficult to set up and control.

The only issue in a "mini greenhouse" is that a fan heater is way too powerful... I tried it a couple of times and the problem is that even on the 1kw setting, it chucks out way too much hot dry air in such an enclosed space. The plants hate it, especially smaller ones.
For a small space, such as an enclosed area where I grow my lowland seedlings, a better solution is soil-heating cables. If your greenhouse is already at 20 degrees, setting up soil heating cables as per their instructions, buried in a couple of inches of sand, will provide a much more gently, steady heat. I place gravel trays on top of the heated sand, then put 1cm of gravel and water in the gravel tray, meaning the plants are sitting in a small amount of warm water. This method has worked wonders for me, generating humidity and heat at the same time.


Brilliant, thanks for that, may well have saved me from cooking a few plants! Do the soil-heating cables raise the air temperature that much, or is keeping the growing medium warm usually enough? I see your point with the fan heaters providing too harsh a heat, would something like greenhouse tube heaters be a better option for raising the air temperature?

Thanks for the replies,

Mark

#6 Gareth Davies

 
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Posted 10 February 2012 - 17:57 PM

For me, 300w of heating cables heats an area of 3' x 4' (approx 3' high) to 12 degrees above ambient, using a triplewall polycarbonate enclosure. The plants are sitting in shallow water that's around 30 degrees. (!)

So if you're using a mini-greenhouse, something along those lines might work.

Even with tube heaters, the plants nearest to the heater are going to receive too much dry heat. It might not be an issue if you have large, well-established plants. I was finding that when I used assorted sources of dry heat in small areas, I fried no end of delicate seedlings (things up to 2" across.)

The issue here is trying to heat a small area without exposing your plants to too much direct heat. My experience now tells me to use soil warming cables buried in sand for small areas. I can't wait to graduate out of small enclosures to a whole lowland house now... it's going to be a challenge....

#7 Dicon

 
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Posted 11 February 2012 - 08:57 AM

It sounds to me like the heating is not an issue, I assume that the main glasshouse keeps an absolute minimum of 20c at night and rises with solar input through the day?
in which case, all you are really needing to achieve by way of further enclosure is a higher, more constant humidity.
I keep 20c in my greenhouse 15x8 and the only thing that slows growth is the lower light levels during winter.
If you were to add light to the enclosure this would provide an additional "daytime" lift to temps and provide the much needed winter light.
Heating a small volume is always tricky as Gareth has eluded and the gentler the source the better, tubes below bench level are fine and soil warmers too, these can also be placed at perimiters above ground to heat air directly, but are more efficient if burried in a heat sink as Gareth describes.
Whatever you decide, you must provide good air movement.

Edited by Dicon, 11 February 2012 - 08:59 AM.


#8 mlong

 
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Posted 12 February 2012 - 20:18 PM

It sounds to me like the heating is not an issue, I assume that the main glasshouse keeps an absolute minimum of 20c at night and rises with solar input through the day?
in which case, all you are really needing to achieve by way of further enclosure is a higher, more constant humidity.
I keep 20c in my greenhouse 15x8 and the only thing that slows growth is the lower light levels during winter.
If you were to add light to the enclosure this would provide an additional "daytime" lift to temps and provide the much needed winter light.
Heating a small volume is always tricky as Gareth has eluded and the gentler the source the better, tubes below bench level are fine and soil warmers too, these can also be placed at perimiters above ground to heat air directly, but are more efficient if burried in a heat sink as Gareth describes.
Whatever you decide, you must provide good air movement.


Cheers guys.

I'm hoping to set something up in the next month or so and think I'll give the soil heater cables a go as Gareth suggested. If it works well I'll post some pics in the summer.

Thanks again,

Mark