Heat, as well as cold, affects plant prospects

This is my 31st summer in the Rogue Valley. There's one thing I can say to all the gardeners in our region with certainty: it will soon get hot. Most likely, it will get very hot. We will probably have seven to 10 days when the temperature will exceed 100 degrees Fahrenheit and many more days when the temperature will rise above 90. In fact, the thermometer will raise above 90 for approximately 55 days.

Why then have the hardiness ratings of plants that we purchase in nurseries only been concerned with the lowest temperatures that a plant will endure? Don't plants suffer and perish due to extremes of heat as well as cold?

Of course they do. I think most gardeners are well aware of the injuries endured by plants like rhododendrons when they are planted in too sunny a location. Most of us are becoming used to seeing tree trunks that have been scorched by the reflected heat from pavement in street tree plantings where improper selections have been made. If there were only some way of codifying plants as to their heat tolerance, surely we would be better able to choose plants that are suitable for the summer extremes we experience in Southern Oregon. Well take heart; much of that work has already been done for us.

The Arnold Arboretum introduced the concept of hardiness zones in 1938 using 40 years of weather information from U.S. Weather Bureau maps. The U.S. Department of Agriculture published its own version of a hardiness map in 1960 using more weather station readings to derive its figures. When it was revised in 1990, it tripled the sources of data and became the standard reference on cold hardiness. Other organizations, such as the National Arbor Day Foundation, have developed their own maps, but have not disrupted the USDA's claim to prominence. For my money, they need to take a look at Sunset Magazine's Garden Climate Map that covers the entire U.S., but that's a story for another time.

In 1998, the American Horticultural Society brought out a Heat-Zone Map that was divided into 12 zones that represent a range of summer heat. These ranges are defined by average annual days above 86 degrees Fahrenheit. That temperature is the point at which plants can experience damage to cellular proteins.

Zone 1 averages less than one day where the temperature exceeds that point and Zone 12 is subtropical, with an average of more than 210 days per year above 86. The Rogue Valley is classified in Zone 7, with 60 to 90 days each year surpassing the 86 level. Purely by coincidence, that is also our Sunset Gardening Climate Zone.

How can the gardener use all this information to grow a more successful landscape? By cross-referencing the USDA map with the AHS one and knowing individual plant zone ratings, one can select plants based on year-round performance instead of information for only half the year. Certainly, if a plant can survive our coldest likely winters but be severely stressed by our hot summer temperatures, it would not be a good choice for our yard. Conversely, if a plant thrives in our summer heat but is damaged or killed by subfreezing temperatures, it is not a candidate to live in my garden.

Keep in mind that maximum summer temperatures are not the only factor to consider when determining whether a plant will make satisfactory growth in our heat zone. The American Horticultural Society is exactly what the name implies: a society that deals with the horticultural aspects of plants and growing. It assumes that you will provide water when, and if, it is necessary for the growth of your plant. It does not take into account drying winds, soil type and the amount of sunlight a plant receives.

Some would argue that the next zone map that needs to be developed is a Humidity Zone Map. Certainly we in the Rogue Valley understand how humidity (or the lack of it) affects the growth of so many choice Oregon natives like salal and sword ferns in our dry climate. Dogwoods, Japanese maples and rhodies would all love a boost in atmospheric moisture to thrive here. Until that time, if we arm ourselves with the currently available information, we'll have a good head start on a great garden.

Stan Mapolski, aka The Rogue Gardener, can be heard from 9-11 a.m. Sunday mornings on KMED 1440 AM and seen in periodic gardening segments for KTVL Channel 10 News. Reach him at stanmapolski@yahoo.com.

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