JARS v63n2 - Tips for Beginners: Wind Chill Effect on Rhododendrons

Tips for Beginners: Wind Chill Effect on Rhododendrons
Alan Campbell
Shawnigan Lake
British Columbia, Canada

Reprinted from the March 2007 Cowichan Valley Rhododendron Society Newsletter

An interesting discussion took place in 2007 on an Internet chat group that concerns itself with the growing of rhododendrons. This rhodo forum has some 400 registered users with a wide range of knowledge and experience.

First, it was stated by someone that the Webster Dictionary defines wind chill as "a still-air temperature that would have the same cooling effect on exposed human skin as a given combination of temperature and wind speed - also called chill factor, wind chill factor, and wind chill index." As plants do not feel, having no nerves to transmit impulses, to use the term wind chill in relation to plants is thus meaningless.

However, a posting from Bruce Clyburn of District 12 of the ARS in Nova Scotia revealed a different point of view: "I, on the other hand, drift from the literal definition and interpret it as the combined effects of temperature and wind. Wind can increase the evaporation of soil moisture, thus speeding drying and making water harder for the plant to obtain. It also speeds evaporation of moisture from the plant's leaves and bark. The faster the wind, the faster the moisture is lost."

The next posting was by Steve Henning (moniker "rhodyman") of Pennsylvania in District 8. His comments give in-depth reasons how and why our gardens should be prepared for winter: "This is correct; the biology (of plants) is entirely different. In humans, the body temperature should be near 98.6°F, and the wind chill is the effect of the environment to combat our body's effort to keep that temperature. Also, our extremities are susceptible to frostbite, but our core is producing heat and trying to combat that. So the colder and the windier it is, the harder it is on the person.

In a plant everything is different. The plant has no body temperature, but must maintain cell integrity, which it loses when the cells freeze (frostbite) or they are desiccated. The temperature at which cells freeze varies, depending on the variety and hardening of the plant. There are at least four different scenarios:

1. When the ground is thawed and the air temperature is above freezing also, sap can flow and replenish the moisture lost from the leaves until the ground is desiccated. Here the effect is the drying root zone. The severity depends on the moisture available. It causes drought damage, wind burn, sun burn and possibly death.

2. When the ground is frozen, but the air is warmer, the plant is very vulnerable to desiccation. The leaves are not curled up and the biological processes in the stem portion of the plant can occur. So a variety that will maintain dormancy under such conditions will do better; those that don't will suffer more. They will suffer even more depending upon the dryness of the air.

3. When the ground is frozen and the air is below freezing, the plant is better off until the temperature gets down to where some of the cells are freezing and being destroyed. So this depends how hardened off the plant is and on the variety also. The wind is not much of a factor here. It will make the desiccation worse, but most hardy varieties can cope. Varieties that can't cope will suffer. Here moisture in the air has much less of an effect.

4. When the ground is thawed but the plant is frozen, the plant should be okay unless the temperature gets low enough to compromise cell structure. The wind is not much of a factor here either.

So for plants it is complex and variety dependant. It is also dependant on how hardened-off the plant is. Hardening off means that cell tissue has adapted to cold weather and has maximum resistance to cold, wind and sun. This usually means the plant cells "bulk up" on chemicals that inhibit freezing and drop their moisture level to increase the concentration of chemicals in solution. So the cold weather tolerance of a plant can vary considerably, depending on how well it has hardened off. Too much nitrogen fertilizer or freakish weather with warm spells can compromise a plant's ability to harden off. So can bacterial infections.

A human being burns food for energy and is a warm bodied creature with a thermostat, a large thermal mass and some clothing for insulation. A plant is a small cold bodied entity with no thermal heat source, very little stored heat because of very small thermal mass, and no insulation unless it has snow cover, leaves or some other protection. The difference between the cell temperature and the ambient temperature is negligible. The only protection consists of the dissolved chemicals in the cell that act like antifreeze and suppress the freezing point. The composition of these chemicals is what distinguishes a hardy plant from a tender plant or a hardened off plant from one that is not."