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Understanding Your Climate Zone

The USDA Plant Hardiness Zone Map was created by the USDA to help growers and gardeners understand which plants are most likely to thrive in a specific location.The map is based on the average annual minimum winter temperature, divided into 10-degree F zones. Hardiness zones are based on the average annual extreme minimum temperature during a 30-year period in the past, not the lowest temperature that has ever occurred in the past or might occur in the future. Gardeners should keep that in mind when selecting plants, especially if they choose to "push" their hardiness zone by growing plants not rated for their zone. This map was updated in 2012. To use the map (above), enter your zip code or view the map by state, region or nationally.

The next map to understand is the AHS Plant Heat Zone Map. This map is like the Hardiness map except that it’s reporting heat rather than minimum temperatures. By using the map to find the zone in which you live, you will be able to determine what plants will "winter over" in your garden and survive for many years. Use the AHS Plant Heat Zone Map in the same way that you do the Hardiness Map. Start by finding your town or city on the map. The 12 zones of the map indicate the average number of days each year that a given region experiences "heat days"-temperatures over 86 degrees (30 degrees Celsius). That is the point at which plants begin suffering physiological damage from heat. The zones range from Zone 1 (less than one heat day) to Zone 12 (more than 210 heat days).


Thousands of garden plants have now been coded for heat tolerance, with more to come in the near future. You will see the heat zone designations joining hardiness zone designations in garden centers, references books, and catalogs. On each plant, there will be four numbers. For example, a tulip may be 3-8, 8-1. If you live in USDA Zone 7 and AHS Zone 7, you will know that you can leave tulips outdoors in your garden year-round.

The last map to discuss is the Sunset Climate Zone Map. This map takes into account length of growing season, timing and amount of rainfall, winter lows, summer highs, wind, and humidity. This map basically combines the Hardiness Map and the Heat Map into one. The Sunset Climate Zone Map considers temperature (where plants will thrive year round vs. just winter or summer), latitude, elevation, ocean influence, continental air influence, and microclimates. To use the map, search for your location zone. For example, if you select Northeast, you’ll find 9 zones within that region. Determine your specific zone by looking at the map. If you live in New Hampshire, you’ll find Zone 38 is what you want to focus on.


Growing season: May to early Oct. Summers feature reliable rainfall and lack oppressive humidity of lower-elevation, more southerly areas. Winter lows dip to -10 degrees to -20 degrees F/-23 degrees to -29 degrees C, with periodic colder temperatures due to influxes of arctic air.

As you can see, these three maps can help significantly when planning your spring and fall harvests. Don’t forget to check the maps regularly as you experiment with new and different crops. And remember, if you want to garden year round, Rimol Greenhouses has just what you’re looking for!