Very Cold Temperatures in the Greenhouse

It can get very cold in a greenhouse. It can get very hot in a greenhouse.  The trick to growing anything in a greenhouse is finding a way to moderate the temperature fluctuation, and finding a way to do so that fits within your economic comfort level.  This morning we woke up to a frigid -15’F temperature.  For reference, 1 year ago today it was 40’F here.  This entire week has been exceptionally cold, but sunny.  I use a 15 foot fiberglass greenhouse about 10 months of the year and this is about the time of year that I get out there and start to prep for spring seed starting and early spring edibles like leaf lettuce and radishes.   I like to start lettuces early because they do pretty well even if frozen during the night.  But, it has to be warm enough for the seeds to germinate and grow.

Often I am asked if you have to heat a greenhouse.  The short answer is yes.  There are many factors that go into determining how much heat is necessary and that makes up the long answer which is a subject for another day.  I like to remind people that most people don’t heat all of their house to the same temperature but instead heat the rooms according to the amount of use they get and the comfort of those using the room.  The same goes for the greenhouse.  If you don’t need the entire greenhouse space to do what you want to do in it – for instance just starting a crop of leaf lettuce for an early spring harvest – why go to the expense of heating the entire space?  Depending on the type of greenhouse you have, take a look at your space and figure out how you can subdivide or partition it off into a usable space and then how can you safely heat it enough to grow what you want to grow.

One of the easiest ways to partition it off is by using flexible PVC pipe and covering it with plastic making a tent.  The plastic will allow for sunlight to penetrate while adding a layer of insulation to hold in the heat.   Now consider how to add a heat source.  I have a ceramic heater with a blower and thermostat that I use overnight when starting my major seedling crops in a large area of the greenhouse.   For the small area that I will be using for my crop of lettuce, I will be growing in a raised bed about 8 feet square.  My heat sources is going to be an incandescent light bulb covered by a clay flower pot.  The pot will help to radiate the heat more evenly.  I used a light bulb screwed into a ceramic base and wired with a plug.  This I plugged into a grounded outlet.  I have my raised bed heating now and will monitor the warmth of the top of the soil and get those lettuce seeds sowed in the next couple of days.