As I understand it, the USDA zone formulation, based on average yearly
minimum temperatures, implies that temperatures for any given year may have
a 50% or even greater chance of falling below the specified range for a
particular zone. That is, if say at a particular location, the minimum
yearly recorded temperatures for 6 years were -15, -12, -11, -9, -8, -5, the
average yearly minimum would be -10, and the location would qualify as zone
6a (-10to -5) despite 50% of the low temperatures being below -10. Yet, it's
common practice to choose plants that are hardy only to the lowest
temperature in the zone description,and so these plants may be at risk many
winter seasons, though perhaps for a small number of total days over many
years.
Given this, I would think it sensible to invest time, effort and expense
consistent with that alotted to other types of plant care protecting at risk
plants during those few days of extreme cold. A good system for this purpose
needs not only to be effective but to be quick and easy to install and later
remove in the bitter cold. Also, I would think that target plants could be
no larger than shrubs or dwarf trees. I invision something simple like huge
plastic bags covering individual plants to prevent heat loss by convection,
with a heat source such as a light bulb within the created chamber. Possibly
this would be enough to supply that extra 5 degrees that will save the
plant.
Appreciate any suggestions or other ideas for systems.