Zone pushing in an unheated greenhouse

[NC_Palms]

A relative of mine is giving me permission to use her unheated greenhouse to grow basically whatever I want. And as you probably suspect, I am planning on growing a wide variety of palms and other plants that may not be able to survive in zone 8a without protection. The greenhouse is medium sized and you can comfortably walk inside of it. I've noticed that the inside of this greenhouse can stay about ten degrees warmer than the actual temperature outside during the daytime. But how much warmer should I suspect the inside of this greenhouse to get when it's like 15ºF outside in the middle of the night? 

[Chester B]

I suspect the answer is “depends”. Siting, amount of insulation and thermal mass inside the greenhouse are all factors. Get a temperature sensor that logs data to your phone to truly know what’s going on. Set it up ASAP and track the rest of this winter to get real world data. 

[DAVEinMB]

It may stay a degree or two warmer but without supplemental heat I would think you're basically just getting wind protection. If you have room I'd try to get a hold of one or 2 plastic 55 gallon drums, fill them with water, and put them inside the greenhouse somewhere. They will absorb solar energy during the day and slowly release it through the night hours. 

[RJ]

28 minutes ago, NC_Palms said:

A relative of mine is giving me permission to use her unheated greenhouse to grow basically whatever I want. And as you probably suspect, I am planning on growing a wide variety of palms and other plants that may not be able to survive in zone 8a without protection. The greenhouse is medium sized and you can comfortably walk inside of it. I've noticed that the inside of this greenhouse can stay about ten degrees warmer than the actual temperature outside during the daytime. But how much warmer should I suspect the inside of this greenhouse to get when it's like 15ºF outside in the middle of the night? 

It'll drop pretty close to the outside temperature. Duration will be the key. If you fit them use some big plastic (55g??) barrels as shelf legs, this will help hold heat in during the night and release in the cold spells. 

[Allen]

Since the palms will be in pots they are still susceptible to cold, especially in nursery pots.  The greenhouse will keep the cold air from high up funneling down onto your palm at night and may buy you a little bit of cold protection, otherwise since they are in pots they won't be any hardier than those in the ground.  Greenhouses are good for keeping frost and moisture off sensitive plants.  

[oasis371]

I purchased a remote thermometer from HD for under $20 and I LOVE IT!  It records the daily maximum and minimum temperatures for a 24 hour and 48 hour time period and also the extended all time maximum and minimum temperature recorded, and I can see this from in my house (the garage is detached).  You need real temperature data as your conditions will depend on a number of factors.  My garage/greenhouse is in Zone 7A/B NJ. The top panels of the garage where replace with replaced with a plexiglass. The garage faces directly south and gets unobstructed sun all day long, there's a loft ceiling and some heavy duty plastic around the walls and ceiling.  However, there are two small heaters an electric radiator and an infrared one. This keeps temps around 20 degrees warner than the outside. Last year, the minimum got down to 3 and the minimum garage temp bottomed out at 27 F.  and I saw no damage. Too cold for my real tropicals but mild enough for Butias (Pindo palms), Phoenix canariensis, Queen palms (Syagrus), some Chamaedorea (some that are surprisingly cold tolerant btw), Cycads (including C. revoluta and Dioon), Norfolk Island Pines, and a large Citrus collection. 

Begin by getting a remote thermometer and good luck!

[Manalto]

In the old days they used composting under cold frames to generate heat which was trapped by the glass. This helped to moderate the temperature at night. I'm not suggesting you bury manure under the floor of your greenhouse but, along with the passive solar recommended by Dave, maybe you could come up with some way to more efficiently trap geothermal heat. Covering the north side of the greenhouse with a heavy tarp may help with heat loss.

Edited January 20, 2020 by Manalto