As much as I’d love a full-on greenhouse, it’s not really in the budget or timeline.
However, a straight up coldframe might not quite meet the needs, either, without a heat source.
I am mainly looking for something I can use for seed starting and hardening off - I’d use it probably from January (to start onions) through early May (hardening off tomatoes and peppers).
However, our weather is both hot enough and cold enough at times, and my schedule busy enough, that I’d like SOME type of automatic or semi-automatic climate-control system.
This includes both ventilation and heating.
Something sunken into the ground a foot or so, tall enough to handle plants of maybe 2’ tall at most, maybe of say, 4’ X 8’ or so.
Has anyone done this? A coldframe with perhaps a thermostat-controlled ventilation system and heating system?
Would a ‘radiant’ heat source like buried soil heating cables be sufficient to keep it warm enough to start onions in winter in zone 7? If not, any suggestions?
For cooling - sunny days could get pretty hot in there even if it’s cold outside - so some type of crude automatic ventilation may be in order. I’m not some retiree, I work all day, so I can’t always be there when I need to ventilate - it could feasibly be too cold when I leave for work, but still get too hot by the time I get home in a closed space.
Hot frames have been around for a long while- I’ve never tried it but it should be doable. (Composting manure used to be the heat source in some.) Of course you could hook a heat source to a thermostat, and there are openers that will react to heat and open a vent.
See if this link gives you any ideas, (and then maybe somebody here who has actually done it can help):
Dig a trench 2 feet wide by 2 feet deep and put 6 inches of partially rotted horse manure in the bottom followed by 6 inches of good topsoil. Build a wood frame around the trench 6 to 8 inches high and cover it with a sheet of transparent plastic arranged so that water can’t puddle on the plastic, in other words, rig a couple of piece of wood so it forms a dome. When you want to access the trench, lift the wood frame cover and do what you need to do.
If you need more details, I have a 1910 book that tells all about it. The only weakness with this method is that very heavy rainfall can fill the trench which is bad for seedlings.
Depends on how handy you are, a total climate controlled system can be designed and implemented with very little cost.
Low cost Raspberry Pi board has wifi ready, you can attach a camera watching you plants grow through you cell phone while you are at work.
By attaching different relays you can control the water ; attaching temperature sensor you can adjust the ventilation according to the temperature… of course, it also can turn on/off heating element or fan during cold/hot days. It’s just involves a bit learning and trying, but it is fun and can combine your home security system and garden control system all in one
Electronics in the cold/HOT wet/WET environment of a cold frame/hot frame are not going to last long. It can be done for sure, but I’d look into simple fans vs. trying to get something to open/close a piece of the structure. It works great on a windy day, but passive ventilation can be ineffective when it’s calm outside.
Having said that, I’ve seen small passive cold frames grow wonderful greens well out of season. You can’t grow lemons in them or whatever but they would allow you to harden off starts and get onions going early also.
My basic idea so far is to build it sunken a bit into the ground, against the south side of the house. It won’t be true ‘full sun’ in that there will be shade from structures at times, but on a sunny winter day, it will still get about 5 hours of sun, which is enough for seed starting, and increasing to about 6 hrs by mid-March.
I could get more sun going farther out in the yard, but then I have the challenge of getting power out there, and trudging through snow and mud to care for the plants on nasty winter days.
One thing I’m not sure of - whether I design it so that I place soil and or media (or amended soil)) inside to grow plants directly in, or just plan to grow in containers inside the coldframe.
I use a gigavent .heat operated , nonelectric, opener ,on louvers in my high tunnel . Works good.
While these type of vent openers say they can move “x” pounds ,i think they are best used to open a vertical hinged vent ( like a small door") or louvers , with a spring to close. These work by heating up a cylinder that expands, that cylinder needs to be up high,where the heat is.
For heat I would think, putting foil backed foam insulation on bottom , heat cable / heat pad on it ,buried in several (3-4) inches of sand, potting soil on top.,
Would work good.?
I tried the compost for heat method once ,
I believe it generated ethylene ,and stunted my plants.
Has anyone had luck with the compost method ?