NGA Articles: A Gardener's Guide to Zone Maps

A Gardener's Guide to Zone Maps (cont)

By: Joseph F. Williamson with NG

Rutgers and Floradapt Plant Hardiness Maps

These maps are based on winter-cold isotherms. The Rutgers Plant Hardiness Map, Zones of the United States and Canada (Cook College, Rutgers, Martin Hall, P.O. Box 231, New Brunswick, NJ 08903; $7), has 26 zones based on absolute recorded minimum temperatures. To its credit, the Rutgers map divides the United States into 13 climates east of the 100th meridian, and 13 different ones west of it. Despite the usefulness of this map to gardeners, it's rarely used. Likewise the Floradapt Map (by John Sabuco and the White Oak Group, Inc., 320 202nd St., Chicago Heights, IL 60411). It is a 10-zone, USDA-like map of 10° F increments overlaid with lines and symbols that refine zone descriptions.

The AHS Plant Heat-zone Map

The significance of winter's lowest temperatures decreases as we shift from places where winter freezes may kill many plants to areas where freezes merely mean frost on lawns and windshields. Obviously, winter lows above 20° F, and especially lows in the high 20s, are much less damaging than lower temperatures are. But on the other hand, areas with mild winter temperatures often have soaring summer temperatures. Gardeners have discovered that summer high temperatures can limit plant survival just as surely as winter low temperatures can.

That's why the American Horticultural Society (AHS) published a map (also created by Cathey) that takes heat into account. Called The Heat Map, this 12-zone isotherm map indicates the average number of days each year when given regions experience temperatures of 86° F or higher. According to the AHS, that's the temperature at which many common plants begin to suffer physiological damage. The zones range from 1 (one day or less at 86° F or warmer) through 12 (210 days or more per year at 86° F or warmer). Color posters of The Heat Map cost $15 and are available by calling the AHS at (800) 777-7931, extension 45.

Sunset National Garden Book Map

This map has more in common with the Koppen or Ecoregion Map than with Humboldt's. Long the standard among gardeners who live in the 13 western states, the map now extends to the Eastern Seaboard.

Dividing the United States into 45 garden zones, the Sunset map first and foremost recognizes the difference between garden climates in the East and West. Sunset zones 1 through 24 are west of the 100th meridian, and zones 25 through 45 are east of it. This zone-mapping system acknowledges the West's complex gardening regions and recognizes that in many cases neither minimum nor maximum temperatures determine a plant's survival.

Like Koppen's zones, Sunset's are based on regions where particular plants grow, not on regions that share a feature such as temperature or heat; instead of matching a plant to an established zone, the zone is created to match the plants.

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