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I'm just kinda throwing this out the hope that it might be of some use to someone, somewhere...

What we're looking at is a 4'x8'x32" high seedling table that I built in the greenhouse. I've velcro-ed bubble foil wrap around the table so that I can open and close and get in there with relative ease. The plan is to use the space under the table to start seeds. Inside the space are 2 sets of 36Watt T-8's (that give off very little heat) and a 250Watt MH bulb suspended in the center which gives off significant heat. The MH bulb is on a timer and only comes on from 12:15am to 7:30 am (when electricity here is 4 times cheaper than during the day). Night time temps here have been at or below freezing for many weeks/months now, so the ambient air temps in the greenhouse run about the same(1 or 2 degrees above whatever it is outside at night).

Except that under the table night time high temps are generally between 24 and 30 degrees Celsius. Night time lows even without the MH running are still 10 to 16 degrees Celsius which is way better than I thought they'd be. Of course ventilation interferes with the temps, but you need ventilation. I left one of the pepper plants in there for 4 days with no ventilation just to see what would happen (I knew it couldn't be wasn't) and lost a couple of the smaller fruits. But I only ventilate during the day time when temps are higher, with no ill effects thus far I understand that this may not be the case with tender, damping off prone seedlings. There are enough little gaps between the velcro strips so that some air escapes even when the set up is 'closed'. I've been using this for about a month now, and have been/am comparing the temperatures under her a few times a day to the outside and ambient greenhouse air temps. Usually around 10:am then 3:00pm and 2:30am respectively, and am pretty satisfied. 

The pepper plants are some leftovers from a vermiponics and organic hydro "experiments" and are doing well for a sub-tropical type. There were more of them (doing well too), but they've been sacrificed to various other "experiments".These are/have been fertilized using a humonia, wood ash, water mixture.

 I know none of this is very scientific, and this whole deal was based just on 'guess-timation' and some previous experience with HID lights and bubble wrap, but it's working out wonderfully thus far. I may try a much larger 'greenhouse within a greenhouse' next winter when hopefully AP will be in full swing, and use the four sets of 400Watt MH's I have in that "seedling and post seedling area. I doubt that the info I extrapolate here is 100% scale-able, but my current thinking is that it might get you close enough for hand grenades . (Volume of space/air by Wattage of lights = a given air temp, it just wouldn't seem to work that way with all the different variables). Anyways...

I would not suggest using any HID's above 250Watts (400, 600, 1000Watts would all seem out of the question) at this distance (at or under 2 feet) from your plants, particularly young seedling as you will in all likelihood damage (i.e kill) them. (Pictured here is one of the T-8's, there is nothing directly underneath the MH yet).

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Comment by Vlad Jovanovic on July 11, 2012 at 1:57pm

Yeah, eliminating the need for supplemental lighting seems like a good one :) The winters here are a dark and long affair.

Mirrors huh? Sounds interesting...Tom Parsons has constructed out of maylar (and then supplemental lighting) an indoor set up hoping to achieve something of the likes in Cananda. It looks psychedelic enough, maybe see what his thoughts/experiences are...I know he is growing lots of toms and peppers. I probably wont be as bold...I'll be happy to keep my pipes from freezing and be ecstatic if my carp and bacteria and some leafy greens aren't totally devastated by the end of winter...

Thanks Bob...Mirrors........

Comment by Bob Campbell on July 11, 2012 at 11:52am

Vlad, since you are thinking about doing a green house within a green house I thought I would share my experiment.

I have built an indoor system inside my 2000 sq ft metal shop building.  I insulated the walls & ceiling with R19 the floor with R7.5 and each tank is insulated again with R11.   I use a 1500W inline heater to keep the water at 70F during the winter and an evaporative cooler as seen in the picture above during the 100F days of summer.  Temperatures are quite stable and energy demands are low. 

The 100 sq ft grow room has 40 sq ft of single pane glazing.  This vertical glass does not admit enough light and I'm thinking about expanding the front of the grow room out so that I can take advantage of more light, but this might throw the energy demands higher.  The other idea I have and this will be done first is to place a mirror such that the additional light is reflected into the room.  It's a little bizarre but if it works it will be cheaper, easier, and it will not be detrimental to the insulation of the room.

My goal is to reduce or eliminate the need for supplemental lighting.

Comment by Vlad Jovanovic on December 12, 2011 at 7:40am

Thanks Sylvia. It seemed better than having to walk back and forth between 2 different buildings, as I originally had originally planned (so as not to take up valuable/expensive greenhouse space). But this seems to work too, and any heat that does escape, stays in the GH. I hope to test it out with a batch of seedlings in net pots, hydroton and styrene rafts this week.

Comment by Sylvia Bernstein on December 12, 2011 at 7:08am

Love your creative use of space, Vlad!

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