You’ve Seen the Pretty Colors, on the USDA Plant Hardiness Zone Map So What Do They Mean?
The USDA and Harvard’s Arnold Arboretum established a set of guidelines, a Zone Map, to help gardeners figure out how well a particular plant would survive the winter cold in their particular area. The first zone map was created in 1960, followed by a revision in 1990; both used historical weather patterns, and the Hardiness Zone Map was created dividing the US into 13 Zones. Looking at these earlier maps, you can see that each hardiness zone differs by 10F. The ‘Gold Standard’, the current map, was updated in 2012 using sophisticated methods and equipment; the new version added 2 new zones (12 and 13) and further divided into 5 degree Fahrenheit zones using “A” and “B”. This version includes a “find your zone by zip code” feature – a pop up box will provide the zone, giving gardeners the exact coldest average temperature for the zip code, and the latitude and longitude.
Why Is Your Hardiness Zone Important?
Remember, plants are different! Some can survive in cold climates, and some cannot. Almost all plants have been assigned a hardiness zone, and the Zone Map is a guide to determine what plants you can grow in your garden based on the average of lowest temperatures in your area.
Before you purchase a plant, KNOW YOUR ZONE! This map is one of the essential tools for gardeners; you need to know whether a particular plant will be able to thrive and survive in your area.
Weather Is Not The Same Thing As Climate!
The Zone Map is only concerned with the average of the lowest temperatures. When we talk about climate, or climate change, we are talking about high and lows in a particular area. Your garden, your plants, are affected by weather. They really don’t live long enough to ‘experience climate’ ~ trees are affected by climate, herbs are affected by weather!
Not The Whole Story!
The Zone Map is essential tool for gardeners, but it is not an ‘insurance policy’. Too much rain, too little rain, humidity, wind and unusual weather patterns that cannot be predicted ~ all factors that can affect your growing. Remember, temperatures CAN and WILL dip under what is stated on the map. It’s what gives us topics of conversations in the spring: ‘can you believe we got down to ….. last winter’!
Don’t ignore the many other factors affecting your gardening: soil conditions, too much or too little water, pests and disease. A Zone Map helps you get started on the right foot, but the rest is up to you!
And of course, we have microclimates; areas where the climate differs from ‘the norm’ due to topographical differences ~ a hilltop, a valley, a slope, a body of water. In your own garden, you may have wind-sheltered pockets on the south side of your brick house where you can grow a plant that is not supposed to grow in your zone. That brick wall is absorbing heat basically making that area considerably warmer than the specified zone for your zip code. In order to really understand these variations, you need to get to know your own growing areas.