JARS 49n4 - Tips for Beginners: Understanding Rhododendron Cold Hardiness

Tips for Beginners: Understanding Rhododendron Cold Hardiness
Ted Van Veen
Portland, Oregon

Rhododendron varieties have long been rated or given a range of temperatures for hardiness. The H-1 rated plant is hardy to -25°F, H-2 is hardy to -15°F, H-3 is hardy to -5°F. These ratings are relative guidelines; sometimes hardy plants die and less hardy plants survive. Many interrelated factors contribute to the successful hardiness of a rhododendron. The ARS recently formulated a new hardiness rating system that takes into account these many interrelated factors.
The following is a listing of some common hardiness factors:
1.  AGE. It takes about five years for a rhododendron to reach full maturity for hardiness. Young plants need extra protection.
2.  TRANSPLANTING. Mature plants that have been moved will need added protection until established.
3.  VARIETY. Some varieties are slower to harden in the fall. Damage may occur when sub-freezing weather comes suddenly, without the benefit of light frosts.
4.  GARDEN LOCATION. Each garden has micro-climates. Temperature may vary within a garden as much as five degrees. When the hardiness of a plant is marginal choose the best garden location, using the following criteria:

Choose areas of good air drainage; keep away from frost pockets.
High, filtered shade or northern exposures protect the plants from rapid temperature changes.
Mulch plants after the ground has cooled.

Avoid the following locations when the rhododendron hardiness is marginal:
Windy locations: wind will draw moisture from the plant.
Eastern exposures: bright morning sun warms the frozen foliage too quickly.
Sunny western exposures: sun-warmed foliage will cool too quickly.

5.  CULTURE. The hardiness is related to the general health of the plant. Poor summer care will reduce the hardiness of a rhododendron. To ensure good health take the following precautions:

Water adequately during the growing season. Less water in late summer helps bring on dormancy. Water the ground well before it becomes frozen.
Fertilize with a good rhododendron food. Avoid high nitrogen in midsummer; plants may grow too much and not harden off.
Pest control is a must. Healthy plants survive better.

6.  SOIL. Good soil structure, with proper drainage for healthy root systems, will increase rhododendron winter hardiness.

Ted Van Veen, a member of the Portland Chapter, is owner of Van Veen Nursery.