I have a collection of citrus trees and guavas and some tropical patio plants, most of them have gotten too big to move around easily. This house came with an old half sized basketball court - home owner poured asphalt (not level or square). So I tore down the goal posts and built a 20 by 30 hoop house on top of it. I heat it with a large wood burning stove. I keep super delicate things on some cheap plastic shelves that are draped with extra plastic sheeting and use an electric space heater under there. It is set at 40 degrees. I have only heard it come on when the nights are bitter cold. I also use a standard kerosene space heater on super cold nights or when I really need my sleep. Tending the fire in the wood stove means getting up every couple of hours and walking out to the greenhouse and throwing more logs on the fire. I can get 15 to 20 degrees heat gain with the wood fire and almost that much again if I use the kerosene at the same time.

So on a 20 degree night, with some work I can keep it about 55 degrees. It wouldn't bother me to have everything cool down to 40 since most of the plants would then go dormant and not be so sensitive to the Ethylene gas given off by the kerosene heater or the wood smoke given off by the wood stove. But citrus shut down under 50 and I am trying to keep them actively growing all winter and blooming/fruiting on a schedule.

A lot of "tropical" plants can handle it down to 28 degrees but you'll know in the summer that they went through a tough winter. It will show in their leaves and how well they bloom and fruit.

I basically claim to be trying to keep everyone above freezing and have found that easy to do here in zone 7b. If I can keep it above 50 at night then I will have more fruit the following year (they fruit in the winter.

It's 20F right now. The GH is sitting around the 60F mark. I won't be keeping it going much longer as I have super hot peppers in there and just wish to ripen the final crop before closing the season.

I heat with wood in the spring to grow bedding plants, mostly tomatoes. Thankfully, the season is only a couple months, because tending the fire is way too much work if you like to sleep. I only have to keep it above freezing. I have learned to seal off one end of the greenhouse to heat when my plants will fit in a smaller space.

I have a 10x12 Harbor Freight greenhouse, stuffed full of mostly cacti and succulents. I keep plants in it year round, but in the winter all my non-hardy potted succulents are piled in there too. Some large plants under my benches are sitting on inexpensive lazy susans I buy at thrift stores; this allows me to rotate them through the winter, so the lower light under the benches doesn't ruin their form.

Many of my cacti could go lower, but I keep the minimum temp at around 43F to baby the fussier succulents. I use two inexpensive electric 1500w heaters, plugged into thermostats that let me set when they kick on. Remote thermometers let me keep an eye on the greenhouse temps from inside the house.

I also keep a very small fluorescent light plugged in out there, so I can tell at a glance at night that the greenhouse power is OK. (Yeah, I worry about my "kids!")

Two really good ideas: 'rotating' your stock and the night light....

I use a string of LED Christmas lights.

Cheers

Trianglejohn and Cole_Robbie both have my admiration for heating with wood stoves. I am positive my plants would be certain mush if they had to rely on me waking up and adding wood. Bravo...

apg4, good idea about the LED Christmas lights. That would be less bulky, and I could run it along the shelf over my potting bench.