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Thursday, January 5, 2012

Hobby Greenhouses a Special Challenge in the Hot Desert

Q. My helpful husband gave me a 6 x 6 foot greenhouse for Christmas and I'm wondering what in the world I can do with it here in Las Vegas. I am thinking that I can get a head start on growing seedlings and things like that, but during the summer months, it will probably be a storage area since it's so hot here.  I have it located on concrete on the southwest side of my house, and unfortunately there is no other place for it. Also, maybe I can extend the season into winter months? Any information would be great.

A. That was very thoughtful of your husband. As you have probably already figured out, greenhouses can be tough to manage in our climate. But it can be done. You have one thing going for you. You have a greenhouse. What you don't have going for you is that size and its location.

            Small greenhouses are difficult to manage in this climate because they heat up so rapidly. By modifying your greenhouse you may only have to shut it down for two or three months during the summer.

Generally speaking in hot climates with lots of sunshine, tall greenhouses are better than short greenhouses. Anything you can do to make the greenhouse taller is a good thing. The heat accumulates toward the top of the greenhouse. As the heat builds it puts heat on the crops growing lower. The taller the greenhouse, the easier it is to keep the heat off of the plants.

That Southwest exposure is also tough in the summer but would be nice in the winter.

            During the summer the sun is nearly overhead and in the winter it is at a low enough angle that it mostly comes in through the side walls. You might want to consider orienting your greenhouse so that the door is on the East or North side.

            Next, I would put some shade cloth on the roof or paint the roof with white latex paint to reduce your solar load. There is some bamboo fencing that is quite reasonable that you might consider placing over the roof to create some shade during the summer months.

            These are all options. I would probably look at the bamboo fencing first since you can kind of adjust it by putting more or less of it on the roof. These should be done on the outside of the roof, not the inside.

            If it's possible, you might consider putting a couple of vents in the roof that you can open or that will automatically open for you if there is too much heat in the greenhouse. This will allow some of the heat that accumulates at the top of the house to exit and will help reduce the heat load somewhat. It will not cure the problem but it will reduce the problem.

            You might also consider a small swamp cooler that draws air from the outside. Put it on the cooler north or east side if possible. It is also important that the greenhouse is not sitting in a rock landscape.

            If it is surrounded by rock, it will compound your problem. About the most you will be able to drop the temperature from that little swamp cooler is going to be about a 20° difference from the outside air temperature.

            Another way to drop the temperature is to replace one wall, or build into the wall, a pad system similar to commercial greenhouses. This uses recirculating water just like a swamp cooler but the pad is built right into one of the side walls.

            On the opposing wall is a fan that pulls air through the wet pad cooling the inside of the house. If the pad is 4 to 6 inches thick, it could cool the house significantly provided you reduce sunlight entering through the roof.


  1. Covering the roof with shade cloth will cut the heat and direct sunlight in the summer.
    A greenhouse with a solid north wall and a solid roof that angles up toward the south will prevent direct blistering sunlight during the hot months and allow full sun during the winter months. Indirect sunlight is fine in the summer and will keep the greenhouse cooler. This is a design I saw in Sunset magazine some 25 years ago. My father has one here in the northwest up near Lone Mountain. Ir actuall works really well for a hot desert climate.

  2. I just put up a greenhouse in Tucson. It seems the only way to keep critters from beating me to the vegetables. We have an automatic venting system and louvers, but it is still pretty hot inside (October) and yet the indoor temperature was 57.8 this morning...so no heat is being trapped inside. I can't garden outdoors because of the critter issue, so this is going to be a real challenge in the summer! Even with the shade cloth up, I suspect it will be terribly hot.

  3. You are right, it will be hot in that house in our desert climates. University of Arizona greenhouse people have told me to always use the tallest greenhouse you possibly can in the desert. This helps to get the heat off of the crops since heat rises and accumulates at the top of the house. The second thing to do is make sure there is some shade during the summer months. In cooler climates they would use whitewash on the house to reduce the amount of heat gain from the sun shining into the house. That's fine in cooler climates but we really need to use more shade cloth in our climate. The best houses I have seen so far for our climate are what they called the Chinese style. I will try to post a couple of pictures about them.