plant hardiness zone is a way to describe a geological area where the
average low temperature in winter will fall within a certain range.
Plants have a general range of preferred temperature in which they will
grow. In terms of low temperature the plant may be damaged or die from
being exposed to the low range of it's preferred temperature zone.
zone maps are a general way to identify the low temps in your area. You
should speak to your local ag extension agent to find exact low temp
ranges known to occur for your area.
a plant or tree will be able to survive an extreme low temp with some
help. Mulch will protect roots and plastic, foam or fabric can help
protect the trunk and/or upper part of a plant.
wind chill and drying effects produced by winter winds is what does the
most damage. A small amount of preparation on your part can help save
your plants and trees.
In larger orchard
operation the growers will use water/ice to protect their orchard as
ice can act as a temporary shield to extreme low temps. Often the
extreme lows will only last for short periods. This is what makes the
water/ice work as a temporary shield. This is not practical for a home
orchard so you should take care to plant trees that are known to
survive in your area.