Zone Data

Zone data is very useful, but certainly not foolproof. It provides a framework from which gardeners can base planting decisions. More importantly, however, we need to know our own gardens and their microclimates. 

My gardens are well protected by stockade fencing.  They benefit from heat radiated by nearby patios and enjoy mostly southern exposure. In addition, I protect tender plants with fleece blankets for light frosts and overwinter perennials with considerable mulch. I also utilize a hoophouse which adds four weeks of frost-free protection. I estimate that my first damaging frost comes later than most gardens in the area. But even with the best of planning and preemptive measures, it is easy to misjudge when to plant and harvest spring and fall crops. Even a seasoned gardener loses plants over winter that were supposedly hardy to their zone.

Some resources are as follows:

• A primary reference is the USDA Plant Hardiness Map found at http://planthardiness.ars.usda.gov/PHZMWeb/. The map was updated in both 1990 and 2012 and reflects recent data sets.  A new feature allows you to easily determine zones by entering a zip code. The zonal boundaries in central Massachusetts shifted in 2012 from Zone 5b to 6a. The minimum temperature ranges for Zones 5 and 6 are:

                             Zone 5a      −20 °F to −15 °F
                             Zone 5b      −15 °F to −10 °F

                             Zone 6a      −10 °F to −5 °F

                             Zone 6b      −5 °F to −0 °F

           See:  http://planthardiness.ars.usda.gov/phzmweb/AboutWhatsNew.aspx

• According to the Old Farmer's Almanac (http://www.almanac.com/content/frost-chart-united-states/MA/Westborough) we similarly fall into Zone 6a. Using Medway, MA as our climate station, the chart shows a 50% probability of a last spring frost on May 10 and a 50% probability of a first fall frost on September 25. This range allows for a growing season of 138 frost-free days.  

• Recently found an excellent website, Weatherspark.com, that provides an immense data base for your area.  It may be found at https://weatherspark.com/. 

• The website http://veggieharvest.com/calendars/zone-6.html gives "whole zone" data for the various zones.  For instance, Zone 6 has frost free and first frost dates of May 15 and October 15, respectively.  I'm interpolating that each half zone equates to approximately two weeks. I usually follow this range and consider our growing season to have 145 frost-free days. 

• According to Sunset Climate Zone maps, http://www.sunset.com/garden/climate-zones/sunset-climate-zones-northeast we fall into Zone 38.  Here is a description of that zone: Zone 38 - New England Interior and Lowland Maine. Growing season, May to early October. Summers feature reliable rainfall and lack oppressive humidity of lower-elevation, more southerly areas.  Winter lows dip to -10 degrees to -20 degrees F, with periodic colder temperatures due to influxes of arctic air. 

• Another resource is a heat zone map found at http://www.ahs.org/gardening-resources/gardening-maps/heat-zone-map.