building DIY large coldframe (heated, ventilated)

9 years ago

I'm thinking of building a larger sized coldframe for seed starting.

Basic goals:

Size/location - Maybe 4' deep, 6' long, 4' high in the back, with a sloped roof (maybe a foot high in front?). Only the roof need be transparent. It would be south-facing with about 6 hrs sun.

Purpose - starting mostly vegetable seedlings (peppers, tomatoes, etc), possibly some tree seedlings. Would use only from early March - mid May at the most.

I was thinking about a wood frame and strucure, and possibly using twinwall polycarb for the roof to add a bit more insulation and light diffusion.

To start tomatoes and peppers in March, I have to have some sort of heat, most likely, as temperatures CAN get as cold as the teens (we hit 19 degrees on March 25 as recently as two years ago), but typically our freezes aren't as severe at that time. For something that size, I'd need a heater (electric, as we have no gas) capable of raising temps about 30 degrees worst-case scenario (most likely far less than that, typically).

Would be against the house or garage on the south side, and open to the ground, which should produce a BIT of radiant heating at night.

Since I have this thing called a 'job' - haha - I can't "babysit" it and would need some sort of automatic ventilation to avoid overheating. Maybe a vent opener, but perhaps a fan? April can be dicey here - as cold as the twenties or as hot as the nineties, so I need to be prepared to ensure my plants neither freeze nor fry.

So basically, a large coldframe with some greenhouse-like characteristics.

Any thoughts and suggestions?

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Comments (4)

hairmetal4ever
Original Author
9 years ago
Would, say, 500 watt soil heating cables be sufficient for my needs?
UpperBayGardener (zone 7)
9 years ago
last modified: 9 years ago
I actually found this post looking for your posts, because I'd wondered what happened to you!

I am in the same boat in a way but for different reasons. I want a lean-to kit for certain smaller, subtropical, dry climate plants that should be able to survive with minimal winter heating, like Aloes, Cotyledon orbiculata, and a Puya I've had for years. The difference is I want to be able to break it down every spring and rebuild it every fall. I do need it to be taller than what you want, but most mini greenhouse lean-to kits go up to 7' on the steeper side. I would mostly use a heating cable in the soil as the heat source, with a tiny electric or propane 'milk room heater' to keep the temp above 32F during cold winter nights. Not even that actually. All of the plants are zone 9 hardy so if the coldest nights dipped to say 25F, even that would be ok. Maintaining a 25F temp differential (above a hypothetical all time min. of 0F) is a lot easier than a 55F differential, if I were trying to grow tropical orchids in there or whatnot.

Still shopping around and have a couple leads, but it's a bit surprising nobody offers kits that are designed to quickly breakdown. I think I might be able to adapt one of the existing kits somehow.
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Kevin Reilly
9 years ago
daviddrt, keep us posted on your project. I'm actually moving to Atlanta area (Zone 7a) later this year and plan on constructing a lean-to "greenhouse" type structure. I live in 9b now, and yes subtropcials in general can take a few degrees below freezing for a few hours. My goal would be to keep something at 32 minimum knowing that there is a little leeway if that goal is not met. I want to be able to grow the things I do here in ground (citrus, guava, passionfruit) in ground in Georgia. Like you mention, heating requirements would be pretty minimal with a nice passive solar setup. But a permanent fixture in the humid south would be hard to keep cool in the summer. I plan on building something that would have removable sides for the hot months, and the roof would stay on to protect from rain (I would catch rainwater in IBC's and water via drip).

The best thing I can think of is solawrap for the side walls. It has an R-value of 1.7, lets over 80% of light in, diffuses the light, and can be rolled up.

https://youtu.be/WtQBwmZt1TI

For the roof, a twinwall poly carb (or maybe solexx which has R-2 value and 100% light diffuse).

Then using passive solar ideas to warm the structure in winter, thermal mass (water, stone, or PCM materials), possibly a climate battery, possibly having the greenhouse 2-4 ft below grade for temp regulation (depending on water table)
hairmetal4ever
Original Author
9 years ago
I’ve been around, David. Since it’s the dormant season (as mild as it’s been…), I’ve been working on the house itself a bit, and I also have a wife, kids and job!

I’ll post more as spring sets in. This season for me will probably be more focused on edibles, and just maintaining the ornamentals/trees and slowly working on the landscape.

I don’t need anything quite that large as you do, I’ve given up (for the time being) on a full-on greenhouse for plants that aren’t really hardy, instead just want a “season extender” and something to start veggies (and probably tree seeds and grafts) in a bit early, as I stated above. My temps in March have been known to get to the teens, (like in 2014 and 2015, IIRC), being a bit more “inland” than you are…although this year, who knows?

Let me know if you find a good kit.