Plant growth has slowed considerably in the green house. I'm pretty sure it's due to the cold temps, because I had this problem trying to grow out in the metal garden shed over the winter. When I moved the grow into the house, in the bathroom off the master bedroom, the growth took off like a bat out of hell. But I can't afford to grow in the house. If I can't grow with daylight, then it just ain't gonna happen. I'm hunting for cheap ways to heat inside on cool, overcast days and colder nights. Electric space heater is out of the question. Would a patio or camping type propane heater be good to maintain 70*ish? Would a smudge pot hurt anything? Got anything else?
Get some black 33 gallon trash cans, fill with water, put in sun all day and hopefully those will keep them warm enough throughout the night. Repeat the next day. Problem with those propane heaters is they get really hot and theres no way to control them i.e. thermostat and you could overheat the plants. Also they sell soil warming cables. I would look into those.
Bring them in at night at place out in a sheltered area during the day, away from views open to the sun
I just mentioned this to someone else - one of the best ways to maintain temperature stability in a greenhouse (i'm at altitude and we experience 60 degree daily temperature swings in summer) is to fill a couple of 50 gallon drums with water and leave them in the greenhouse. They will absorb heat during the day, and they will release that heat during the night, stabilizing at least somewhat your temperature, and keeping the inside of your green house warm for a little while longer. You can also earth berm your greenhouse - either with a dozer, quite literally, or buy building planting boxes filled with dirt to put up against the greenhouse. Earth bearming is another way of keeping temperatures stable, and you can use the planting boxes outside for sunflowers or other low-maint. plants. I'm pretty new to this type of growing, but my greenhouse operates feb - nov, and we experience temperatures here as low as -50.