Gardening in Alaska : How hot is your greenhouse?

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How hot is your greenhouse?

Posted by talkdirt

Posted: June 9, 2009 - 11:18 am

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Photo by Fran Durner.I have a question from a reader about what is the best temperature to set a greenhouse with thermostatically controlled ventilation. He grows tomatoes and peppers and basil and recently the electricity went out at his place and the greenhouse was way up over 100 degrees. He's not sure how much heat the plants can take. I don't have a greenhouse so I thought I'd throw this question out and gather the info.

Mid-70s is ideal for tomatoes; 76 I think is the magic temperature but you could probably Google it to make sure. Basil also flourishes at the same temp...I don't know what to think about peppers - I don't have a lot of luck w/ them. - Elizabeth Bluemink

The optimum soil temp for tomatoes is 78. Over 85 the tomato pollen gets "sticky" and you end up with cat-face tomatoes. No big deal, they just aren't tomato pretty. There shouldn't be any problem this early in the season. Just open windows as much as possible and don't worry. - Rosemary Kimball

Well, once it gets over 95 degrees you have a real problem. Around 80 degrees is ideal for tomatoes during the day....nothing lower than 55 at night. - Jeff Lowenfels

Generally, they say to keep the greenhouse between 60 and 80 degrees - to get the best fruit production. - Mel Monsen

I have a greenhouse and grow the same plants. Tomatoes grow best between 60 and 75 degrees. Over 80 they will suffer and not produce as well but I think that would be over an extended time. A one day event would slow them down a bit but I don't think it would as harmful as several days of extreme heat. Keeping them watered would be helpful in such heat. - Mary Shier

I have a thermostat on my ventilation system and I turn it to come on whenever the temperature gets 60°. Sometimes when the greenhouse is full, I turn it down to 55° to get the fan to come on more often to keep the air moving with all the new seedlings. Then, a week before I get ready to take the annuals out of the greenhouse to be hardened off, I turn the thermostat down to 50°. I do these things also with the help of a heater in the greenhouse, adjusting the gauge on the heater as well. - Nickel LaFleur

The thermometer for my fan that blows hot air out of the greenhouse is mounted on the rafters above the plants. It gets hottest up there soonest and is often hotter than where the plants are. I have that thermostat set at about 90 degrees. It's my understanding that tomatoes, etc. shut down about 90ish. The hundred degrees is a bit to warm for them. They seem to like mid range temps. from about 60's to high 80's. - Kathy Wartinbee

I haven't done any scientific study on the question, but find my tomatoes and basil (don't grow peppers) do best when the temps are above 60 and below 90. Good air flow is essential. - Brenda Adams

My sunroom exhaust fan’s thermostat is set at 75f and it has tomatoes, cucumbers, angel trumpets and some cape primrose. As to the upper max for the plants he is growing, a lot would depend on the hydration level, the length of time they were in the 100+ degree heat. Tomatoes, peppers and basil are heat lovers and can withstand a good bit of heat when grown outdoors. - Mari Wood

I do not even touch the thermostat, winter or summer and I keep it at the lowest setting. When we installed our greenhouse we were advised to purchase a cooling system, a sun visor and a very good fan. The cooling system and the visor have never been used but we keep the fan running all the time. Yes, it gets warm in there at times but it is perfect for those plants (except for the constant watering that goes with the territory.) I have to mention that my greenhouse is equipped with automatic vents that open up when the temperature is too warm. They work very well. - Amelia Walsh

I keep my Greenhouse at 80 deg. partly to help save electricity. On sunny days the exhaust fan is on continuously and cloudy days if it was set cooler would do the same. Tomatoes and peppers do fine for me at the 80 deg. setting as do my fruit trees. When a greenhouse gets over 100 plants will not do well and tend to shut down if not wilt without adequate moisture. Misting during that time would be beneficial to keep the plants cool. Tomatoes love cool nights and warm days. Our climate provides us the cool nights but the sunny days can overheat any greenhouse. - Kevin Irvin

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How hot is your greenhouse?