"Different researchers and scientists have devised various schemes based chiefly on the weather data which is routinely collected at hundreds of weather stations across the country. They all wanted to be able to predict whether or not a plant would survive in a certain area and they knew that a major factor is the ability of a plant to survive low temperatures. So by mapping weather records across the country they hoped that it would be easier to answer this question. A number of systems have been devised, some of which have disappeared while others are still in use.
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