Does your heater have a fan? Or do you have fans to circulate the air? Radiant heaters heat the air immediately around them. Chickens can move closer to the heat source but plants can't.
I only try to keep my greenhouse above freezing as citrus and the few other plants that I winter in the GH can handle those temperatures. The orchids move to the sunroom.
You can use a heat lamp over top of especially sensitive plants - I use a couple over the cymbidiums.
