Mounted a surplus flat-plate collector at a steep angle (for winter sun) and attached it to a loop of PEX tubing filled with 50% glycol to prevent freezing.
The small PV panel drives a DC pump that circulates the glycol solution through a 400 gal. water tank under the floor— the heatsink.
With good sun, water temps in the heatsink can get above 120°F. An AC pump circulates the hot water through a maze of PEX tubing in the sand floor.
I also mounted four black tanks on a south-facing wall, as passive heatsinks. They're also the source for watering plants, so there's no conventional plumbing.
Took apart a quartz radiant patio heater and mounted the elements on the ceiling, on a thermostat, to warm the foliage directly.
For severe cold, I have a small propane radiant heater attached to an outside tank. It gets used only a few times each winter. I've lost a few plants to cold air leaks at the corner intake vents, but in 12 years it's never frozen inside.
