green house question - much colder then outside temperature - Ask Extension
So I got a greenhouse (small high tunnel) 10 X 25'. I put in two raised beds on the sides and left the center open for grow bags and containers.
...
Knowledgebase
green house question - much colder then outside temperature #895446
Asked March 25, 2025, 6:47 PM EDT
So I got a greenhouse (small high tunnel) 10 X 25'. I put in two raised beds on the sides and left the center open for grow bags and containers.
I put in two different thermometers and they are within two degrees of each other.
The green house temperature is at the least 2.5 degrees colder then the outside temp with the sides open.
5 degrees colder when just the small side windows are open
up to 10 degrees colder when everything is zipped up.
I understand the cooling effect now with water condensing on the sides and then turning to ice.
However now knowing this I was wondering how a greenhouse can extend your season if this is the case?
I have noticed plants seem to be growing better in the greenhouse but not sure if that is the soil, the higher temperatures during the day or something else completely.
Howard County Maryland
Expert Response
Hi- the average ambient temps will be higher inside your high tunnel, but as you've seen, there can be times when the outside temps are higher. This mostly happens on cold, clear, windless nights. Your high tunnel is actually trapping cold air under the plastic cover. This is more likely to happen in relatively small high tunnels like yours because the smaller air volume heats up and cools down more quickly than in large high tunnels. Air temps inside the high tunnel would also be lower if it's in a low spot in the landscape.
The more rapid growth is due to higher soil and air temps and higher humidity levels. Covering plants with row cover will increase temps around plants a bit. A brick walkway or barrel of water would also help absorb heat during the day and release the heat at night.
Resource:
https://fcfreepresspa.com/high-tunnels-for-homeowners-season-extension-and-winter-gardening/
Jon
The more rapid growth is due to higher soil and air temps and higher humidity levels. Covering plants with row cover will increase temps around plants a bit. A brick walkway or barrel of water would also help absorb heat during the day and release the heat at night.
Resource:
https://fcfreepresspa.com/high-tunnels-for-homeowners-season-extension-and-winter-gardening/
Jon
