Aspects of dormancy in the temperate Southeast.  

First, understand that virtually all our native plants, and many from other temperate countries, REQUIRE a cold winter dormancy in order to leaf out and grow the following year.  Hence, wild ferns and wild flowers do NOT make good houseplants because they would not get this dormancy required to renew growth.  Some species require a LOT of cold to break dormancy (apples, lilacs, other northern shrubs); while at the other end of the spectrum, some require little cold period (e.g. winter honeysuckle, witchhazels, alder, forsythia – many of which are  exotics from milder climates)

How plants go dormant:

            1. Plants detect shorter days in late summer (very important).  This then triggers--

a.   Chlorophyll breaks down, leaves die and turn colors, then drop off - this is a strategy to reduce water loss all winter

b.   Plants builds up colloids in cell sap - colloids thicken cell sap like anti-freeze, to keep from freezing (this is much of what determines hardiness, which plants can tolerate what degree of cold).

  c.  Buds on twigs are actually new growth for next spring hidden down inside of overlapping layers of bud scales with packing material to protect from cold.  

1. There can be separate leaf buds and flower buds, and sometimes the flower buds are less hardy than the leaf buds and are killed by a cold period.  15 degrees F is a magic number. Below this roots and flowers buds may be killed on certain unprotected plants (e.g. the roots of plants in containers on top of ground that would other wise be hardy if planted in soil.)  Examples of plants with separate flower & leaf buds: Camellia, azalea, rhododendron, honeysuckle, witch hazel, banana shrub, pussy willow, alder - many winter-flowering types.)

2. Cooler temperatures, even freezes, stop growth of some plants and they "go dormant" on the spot.  Some subtropicals do this (Mimosa, Tulip-tree, tropicals).  

2.  Roots can grow as long as temperature is above the magic number of 40 degrees F.  Thus, many plants grow roots long after their tops are dormant. Tis is why Fall is such a good time to plant and transplant.  Start planting around Nov. 1 when dormancy comes.  Plants may still be establishing new roots into Dec-Jan.  They can even be fertilized after Nov 1 because tops are dormant and roots can take up fertilizer.  DO NOT fertilize or prune heavily in late summer-- certainly not after Sept. 1, or new growth may not harden before frosts in October.  

               3.  Likewise, a sudden cold snap can even injure plants going dormant naturally.  If,  for example, the temperature went from 60 degrees to 20 degrees in one day in late October, some plants COULD be injured.  Similar sudden drops any time can be disastrous-- even in the dead of winter if it goes from a high to a low very quickly.  

4.  Usually introduced exotics are hurt more often because they come from a milder climate and they tend to need less cold and start to grow earlier.  Our native are almost never hurt by cold spells or temperature changes or extremes.  The minus 6 degrees F we had here in Charlotte in 1985 did not hurt any native hollies, azaleas, rhododendrons, etc.  Burford, Nellie Stevens hollies however were killed to the ground. (as were camellias, sasanquas, banana shrubs, etc.)  

                 5.  How to PROTECT from winter damage:

                   a. Mulch to keep roots from freezing and thawing, and to prevent some root damage from severe freezes.  Mulch helps to keep the surface ground cold so some plants don't try to come up too early and be frost damaged (e.g. Bletilla orchid).

                   b. Wrap trunks with paper and evergreen shrubs with burlap to prevent drying from severe winter winds.  When ground is below 40 degrees, roots can't take up water (when frozen, it's impossible).  Therefore, the leaves could lose water on a warm sunny day or in drying winds, and if roots can't replenish water, the leaves will dry out and turn brown.  This is winter burn.

                  c. Protect trunks, stems and leaves from bright winter sun.  If direct sun shines on frozen leaves & stems, they could quickly warm up and crack if they are frozen solid.  This is what kills stems in winter.  Leaves can be killed too.  Try to shade susceptible species from winter sun, especially winter Daphne.  I lost a Gelsemium from winter sun striking exposed stems when they frozen to -6.  The whole plant was killed.  Protect by planting on the north side of something: a wall, evergreens, house, rock etc.  Even bare twigs of other plants can break the winter sun. Wrap with burlap during extremely cold periods.

                  d. In Summary: winter SUN and WIND and FROZEN SOIL can be injurious to plants, especially evergreens, but deciduous as well.

                  e. Cold air drainage can be a factor. If cold air accumulates in a hollow or against a building, it can injure plants growing there at certain times.  Allow cold air to drain downhill, and use evergreens as windbreaks.  

6.  Also consider the term HARDY.  It is best used to refer to the ability of a plant to withstand cold temperatures.  This is the basis for the important hardiness zones, which is the first thing you need to know about a plant when determining suitability.  These zones are based on average minimum winter temperature  (winters can have warmer or colder days than average) as it is the lowest temperature that causes the damage and determines what can grow in a given region over time (the average can be misleading, it is actually the absolute lowest temp that should define zones if you want a plant to be long lived).  HARDY should not be used to describe an easy to grow, adaptable plant.  The term there is "hearty," but that is confusion.  Consult a Hardiness zone map to see what zone you are in.  Realize that you can probably grow plants two zones colder than you, and maybe one zone warmer – except for the extremes of heat and cold.