Some rhododendrons and azaleas can tolerate severe winter conditions while others cannot. The main problem is desiccation of the foliage when the ground is frozen and sun and wind attack the leaves. There are four solutions:

1: cold-resistant varieties of plants,
2: winter windbreaks
3: winter sun shades
4: chemical antitranspirants

In catalog listings, the cold resistance of a hybrid rhododendron or azalea is usually indicated by a code that indicates the lowest temperature the flower buds can tolerate during the winter and still open perfectly in the spring. The US Department of Agriculture introduced Plant [Cold] Hardiness Zones in 1960 to classify the cold hardiness required of plants in certain areas. Prior to USDA Zone system, there was an international cold hardiness rating system for rhododendrons which ranged from H-1 to H-5. Plants bearing the code designation H-1 do well down to -25, H-2 to -15°, H-3 to -5°, H-4 to 5°, H-5 to 15°, H-6 to 25° and H-7 to 32°. Most varieties grown in the USA range between H-1 and H-4 in hardiness. Many catalogues use to the USDA cold hardiness zones. Zone 4 corresponds to H-1, Zone 5 to H-2, Zone 6 to H-3, Zone 7 to H-4, Zone 8 to H-5, etc. Other factors are important. If your plants are subject to desiccating sun and wind in the winter or warm spells in winter that may compromise a plants dormancy which, then you should probably use plants that are one zone hardier. For example, I live in Zone 5, H-2, but try to use plants that are good for Zone 4, H-1. Plants with a H-2 rating need winter protection.

There is a third cold hardiness zone system developed by Sunset Magazine for 13 states in the Western US. It is completely different and tries to divide the West into 24 plant zones. If you are using a Sunset Magazine garden book then you will have to become familiar with this system. That is about the only place it is used. Apparently, even though the climate zones were established by the University of California Cooperative Extension, the Sunset Magazine climate zone map is copy written so it was never intended to be used outside Sunset Magazines own publications.

Rhododendrons and azaleas may be harmed in winter by drying winds and bright sun; protect their shallow roots with a mulch of oak leaves or pine needles and their foliage with a loose blanket of evergreen boughs or specially built screens. Such screens must provide shade without capturing heat. A burlap screen will protect a plant while a black or clear plastic bag will cook a plant. Keep the mulch away from the trunk of the plant. This avoids bark split, fungus and rodent damage.

Chemical antitranspirants effectively cover the stomata, the pores through which the leaves loose moisture. However they must be designed to naturally degrade so they don't interfere with the normal operation of the stomata during the growing season. This usually means the antitranspirants need to be applied at least twice during the winter, but not too close to the growing season.

To insure that a plant has the ability to make it through the winter, it must be dormant. Dormancy is a normal process in which the plant goes into a rest state during the winter. Dormancy is caused by a number of things including short days, low temperatures and drought. Several things can break or prevent dormancy.

* Too much nitrogen fertilizer after mid summer will keep a plant in a growth state when it should be going into dormancy.
* Warm weather spells during the winter can break dormancy in a plant that has gone dormant.
* A warm fall followed by a sudden winter, can expose a plant to the cold before it has gone dormant.
* Too much light such as being planted under a spot light that is kept on all night.

A study in New England determined that a plants hardiness rating varies by about 10 degrees from winter to winter. A decrease in hardiness is usually caused by warm spells followed quickly by very cold spells. The hardiness rating that is reported is usually the lowest temperature a plant has been known to survive without damage. In the real world, the actual capability may be 10 degrees warmer than that some years.

Click here to look up your hardiness zone.

Rhodyman, thank you. I am not really sure what zone they are rated for, but I am hoping that I buried the tag with the plant, which I try to do when thinking (which is not all the time). I am thinking of going ahead and protecting from the wind, although I don't remember really bad wind last winter, which was our first winter here. I will also mulch well. Do rodents like to eat their roots?

Yes, voles and other rodents can wreak havoc on rhododendron and azalea roots. The recommendation to keep the mulch away from the main stem/trunk of the plants is to help prevent this. If you are the victim of this kind of damage, you will see a perfectly healthy plant die. Sometimes when you touch it, there are no roots left holding the plant in the ground.

If you have evergreen azaleas and have a prevailing dry wind in the winter, I would protect the azaleas for a minimum of three years from winter wind and sun. It takes them about that long to develop full winter hardiness.

Larger more mature plants can tolerate more dessication in the winter. If the ground doesn't freeze or you are in a wetter area or an area with snow cover, this is less of a problem. I don't imagine there are many areas in western Oregon or Washington where winter wind is a major problem except perhaps in the Columbia Gorge, but that area is usually moist in the winter.