Temperature Control for Winter Cool Crops

    When asked what greenhouse owners plant over the winter months, the more common responses are that some grow a variety of summer veggies which require high heating and lighting but honestly, who wouldn’t eat ripe tomatoes and cucumbers all year round?  Some don’t grow anything at all due to high fuel and energy costs but interestingly enough, there is a growing group of vegetable growers, private and commercial, that prefer to grow “cool crops.”  For temperature controlled environments such as greenhouses, these cold resilient vegetables are ideal for winter growing not only because they thrive at cold temperatures but also because of the minimal amount of heat energy needed to successfully grow them. Less energy required equals less costs to warm a space that inherently dissipates heat.
    All root vegetables (kohlrabi, turnips, beets, carrots, radishes, potatoes), hardy greens (kale, chard, spinach, mustard greens), anything in the cabbage family (cauliflower, broccoli, brussel sprouts), onions, and various herbs (parsley, cilantro) prefer cold temperatures.  They can grow outdoors in places where average low temperatures are around freezing but they flourish in greenhouses that maintain temperatures right above freezing.  
   Setting your thermostat inside your greenhouse at 35 degrees Fahrenheit will create an efficient temperature to grow cool vegetables as well as keeping your water supply from freezing.  Depending on the weather conditions of where you live and the construction of greenhouse you have, maintaining temperatures right above freezing, may easily be accomplished for the majority of places in the US through passive solar heating.  By simply storing heat from the daytime in some type of thermal mass inside the greenhouse, it can then be released at night when temperatures drop.
    Some different types of thermal mass greenhouse owners use are water, rocks, bricks, concrete, dirt, mulch, manure, straw bales and various types of phase change materials.  Some are better than others.  
   The performance of phase change materials are better solely because it stabilizes temperature while releasing its heat.  A good example is savENRG PCM at 32 degrees F.  This PCM is made from salt hydrates and is non-toxic and non-flammable.   It will consistently release a temperature of 32 degrees F and will require lesser quantities than other forms of thermal mass, providing more growing space inside the greenhouse.
   Regardless of what thermal mass is used and how you heat your growing spaces, keeping temperatures right above or around freezing during the cold season is reasonable and can be achieved with minimal energy costs.  At the same time, planting cool crops provides families and local communities with healthy and seasonal vegetables throughout the winter months.