The type of frost that our Greenhouse Buckets are most effective at protecting your plants from is radiation frost. A second common type of frost is advection frost which typically occurs with the passing of a cold weather front and air temperatures drop below freezing. For information regarding on how our Greenhouse Buckets will provide protection against advection frost, please see the Section, “Freeze Protection”.
Radiation frost almost always occurs on clear nights with calm winds and the ambient air temperature near the freezing point, but it can occur with air temperatures as high as 37°F (2.8°C). Radiation frost is a result of radiant energy (heat) being lost into the open sky causing objects to become colder than the surrounding air. Under clear night time skies more heat is radiated away from horizontal objects than they receive and thus their temperature will cool below the surrounding air temperature.
A common example of radiation frost is when on a calm clear night, with the air temperature above freezing, frost forms on the horizontal (top) surfaces of your car but not on its sides. The reason for this is that the horizontal surfaces of your car are facing skyward toward cold outer space and the sides of your car are facing warmer surfaces here on Earth.
How does our Greenhouse Bucket protect your plants against radiation frost? Since your plant is under our miniature greenhouse it is no longer facing cold outer space, but instead is facing the relatively warm top surface of the Greenhouse Bucket. Thus it does not loose heat, i.e. radiant energy, as rapidly as it would have if it were uncovered. An example of this is demonstrated by a car parked under a carport. As long as the ambient air temperature is above freezing the car will not experience any form of frost on its horizontal surfaces. That is because the top of the car is facing a relatively warmer carport roof and not cold outer space.
Without some form of an internal heat source all greenhouses don’t provide much, if any, protection against sub-freezing temperatures. Also because greenhouses aren’t insulated it would take a significant amount of supplemental heat to affect the greenhouse’s interior temperature. Since our Greenhouse Buckets and almost all other greenhouses do not have insulation or a supplementary heat source it is best to not start using them until it is unlikely that air temperatures will fall below freezing.
That being said, from personal experience our Greenhouse Buckets can protect plants from subfreezing temperatures given the right circumstances. If, for example, there is sufficient solar radiation (sunlight) in the daytime to heat the soil beneath our Greenhouse Buckets, then during the night the soil will re-radiate heat back and slightly moderate the air temperature inside our Greenhouse Bucket. If outside air temperatures are hovering at or slightly below freezing, then this heat gain may be just enough to protect the plants. It is important to remember the longer and/or lower the temperature is below freezing the less effective our Greenhouse Buckets or any other unheated greenhouse will be at protecting plants.