Damage to rhododendron and azalea leaves and buds can occur at low temperatures.  Exposure to cold can cause dry, brown areas on leaves and brown-colored buds.  Sometimes new plant growth may not have had sufficient time to harden off before cold weather sets in and may be killed.  Give the plant plenty of time to send out new growth as temperatures warm before pruning off the damage.  Warm temperatures followed by a quick freeze, such as early freeze in the Fall or a late freeze in the Spring can be more damaging than a gradual drop in temperature.

USDA hardiness zones (and similar maps in other countries) can be consulted to determine annual minimum temperatures at your location.  Where your garden is situated in a hardiness zone and the garden's specific details are important, as local microclimates may be different than the general hardiness zone you live in.  Cold hardiness estimates provided by the ARS for selected rhododendron and azalea species and hybrids can be used to find plants suitable for your garden.  The ratings give an indication of minimum temperatures that a well-established plant can be expected to survive without damage.  Certain rhododendrons and deciduous azaleas can survive temperatures as low as -35°F (-37°C).

It is normal for some rhododendrons to exhibit leaf droop and curl at around 32°F (0°C).; the lower the temperature the tighter the curl.  Leaf movement occurs rapidly and it is reversible as temperature warms.

Rhododendrons and evergreen azaleas lose some of their leaves each year.  Typically leaves are retained for one to three years dependent on the variety.  Leaves may turn yellow, red, or purple before they fall off.  For some rhododendrons and azaleas the retained leaves turn red or bronze-colored in the cold months.  In some azaleas the only leaves remaining are those that surround the flower buds at the tips of the branches.  The degree of leaf coloration or loss is determined by a plant's genetics.  In cold areas, certain evergreen azaleas may lose more leaves than they would in milder areas. 

Drying winds and frozen ground deprives plants of their natural moisture intake.  A good soaking in the late fall before freezing and a good mulch will greatly help a plant's survival.  In cold climates, rhododendrons and evergreen azaleas can benefit from an application of an anti-desiccant, such as Wilt-Pruf.  Read and follow carefully the manufacturer's instructions.  Spray in late Fall when temperatures are near 40°F.

Even with recommended varieties, plant performance will be improved with reasonable protection from drying winds.  In some windy areas gardeners protect rhododendron plants by building a windbreak around them or screening them with burlap or other protective material during the worst part of the cold season.  Rhododendron or azalea especially valuable to you can be protected with a mesh enclosure filled with oak leaves, or with a teepee-like structure constructed using three or four evergreen branches with the points forced into the ground and other ends tied together, or by snow fencing alone or with a polyethylene plastic sheeting attached to it.  Protect the plants just before freeze occurs and remove the protection after all the frost is out of the ground.

Among the major types of rhododendrons (big leaf and small leaf) and azaleas (evergreen and deciduous) there are differences in their tolerance to various weather conditions.  Generally large-leaf rhododendrons are less tolerant of sun and wind than small-leaf rhododendrons and evergreen and deciduous azaleas.  Planting locations with early morning or late afternoon sun or dappled sun throughout the day from an overhead canopy or a shade structure, protection from high winds and proper watering can minimize leaf sunburn and wind damage problems.  Generally, the east and north sides of the house are better locations than the west and south.  Some varieties will not tolerate full sun, developing quite yellowish leaves under such conditions.  There are others that become a better shaped plant if grown in a location with lots of light.  Deciduous azalea species are very heat and humidity tolerant, and are widely grown by gardeners in the mid Atlantic and southeastern regions of the U.S.

 There are many exceptions to the above so an awareness of what specific rhododendron or azalea you want to grow and and attention to where you want to plant them are important if you want to be successful.  With hundreds of different rhododendrons and azaleas to choose from you are sure to find plants suitable for most climatic conditions.

For further information on protection of rhododendrons and azaleas consult the following Journal ARS articles:

  Winter Desiccation Injury of Rhododendron by John R. Havis
  Flower Bud Hardiness Of Rhododendron Taxa by Harold Pellett, Susan Moe and Wayne Mezitt
  Cold Hardiness Ranking of Rhododendrons By Means of Flower Bud Damage by Russell Gilkey
  Notes on Winter Hardening Rhododendrons
  Causes and Significance of Winter Leaf Movements in Rhododendrons by Erik Tallak Nilsen
  Rhododendrons and Hot Weather by George W. Ring
  Growing Rhododendrons In The Gulf South by John T. Thornton
  Rhododendron Species Hardy in Southern New England by J. Powell Huie
  Azaleas Suitable for Different Areas in Connecticut by By J. W. Oliver
  Growing Azaleas In New England by Charles W. Findlay, Jr.