“We have a greenhouse made out of old windows. What is the lowest temperature suggested for winter? This morning, the thermometer read 39° F – is this too low? Most of the plants inside are tropical.” -Dianne
For most tropical houseplants, the minimum recommended temperature is around 55° F, although it really depends on the plant. Some plants respond to colder temperatures by slowing growth or going dormant, which means they may recover when temperatures warm. They will likely not survive temperatures below freezing.
For overwintering tender or sub-tropical landscape plants, temperature tolerance varies widely depending on the plant. Check the plant’s hardiness zone to determine the minimum recommended temperature for that plant.
It would also be helpful to determine if the plant is “root-hardy” at colder temperatures, which means the top of the plant will die back while the roots or bulbs survive. Such plants may be stored in a dormant state and replanted in the spring.
Even if your greenhouse is unheated, it can be a great way to get a head start on spring plantings, or to overwinter plants that otherwise might be suited to a warmer hardiness zone.