Pilchuck Chapter ARS

Most folks who have ever read a gardening book or plant catalog, are familiar with the plant hardiness zone maps. With this mass of information in hand, gardeners traced the zone lines around the country to find out which plants they could possible grow. Let's see here...Raleigh NC is in the same zone as Reno Nevada and Dallas Texas...could this be right? You are beginning to see a few of the problems with the map.

Each of the USDA 11 climatic zones are based on the average minimum winter temperatures. In zone 7, which includes Raleigh, our average winter minimum temperatures should be from 0-10 degrees F. What the map doesn't tell us, is how many times the temperatures dropped that low, and how long these low temperatures lasted. There are a number of plants that can survive 0 degrees F for a couple of hours, that could not survive these temperatures for a longer period, or more than once during a winter.

Another factor not taken into account is winter acclimation. A plant growing in our gardens in mid summer can be easily killed by temperatures in the 20's. The same plant, if properly acclimated, can withstand temperatures of -20 degrees F.

Plants have developed several ways of shutting down their growth and preparing themselves for the winter season. Many plants shut down based on light. A good example is hostas. When the days begin to get shorter, a point is reached, usually in early October when the shorter day lengths trigger the plant to begin to go dormant. No matter how much heat is applied after this trigger has been pulled, the plants cannot be kept from going dormant. There are a number of plants that are triggered by day length, which can be a tremendous advantage, if fall temperatures are abnormally warm.

If we have an abnormally warm fall, many plants that rely on cool temperatures to trigger dormancy can be killed when the temperatures suddenly drop. .

Another factor in hardiness is precipitation. I trade plants with friends in New Mexico, which are in the same hardiness zone. In New Mexico, the plants receive less than ten inches of rain per year, while we receive more than 6 times that amount. We have discovered that there are a number of plants that can survive our cold temperatures, but cannot tolerate winter moisture.

There are a few factors, that we as gardeners can control. Some plants tend to continue growing, despite cold temperatures and reduced day length. For example, eucalyptus, which are used to growing on the other side of the world, have to be slowed down by withholding fertilizer and water...in other words the growing conditions must be used to slow down the plant.