Each plant performs best within a range of pH and nutrient levels. The plant hardiness zone map is based on a limited amount of data, and the zones represent five degree increments of low temperatures. This averaging and smoothing of data give only a general picture of temperatures. It does not include local variations called microclimates. A low, wet location, for example, may be a lot colder than a higher, dry location.
Cities tend to be warmer than nearby countryside.
Large bodies of water make temperatures less extreme. High mountain areas tend to be colder than lower elevations. One guideline states that a 1,000-foot rise in elevation means a 5 degree F drop in temperature. There are two areas of Maine for which there are differences between site data and the map. The first is Houlton.