Madman Finds His USDA Zone

We are in a new zone – a new USDA Plant Hardiness Zone, that is. Our zone here in Farmington has gone from 6a to 6b. Actually, about half of the United States has shifted to a warmer plant zone. This is great news, right? Well, before we start trading our potato crops for watermelon, let’s talk about the zone maps themselves.

Finding The Zone

Most gardeners know that the USDA has divided our country in 13 different zones based on temperature. Each zone represents a span of 10° Fahrenheit. And each of these zones is split in half, as in “a” and “b”, 5° F. each. The hardiness map uses colors to easily differentiate between zones.

It’s important to know that these zone maps are based on the average annual minimum temperature. As we all know, temperature is variable from year to year – some winters are colder than others. As of mid-November, Farmington is in Zone 6b, -5° F. to 0° F., but remember this is an average figure – it is not a guarantee that it won’t get colder on a given night.

Too Many Dead Plants

The madman is sure that hardiness zones came about because too many garden enthusiasts were introducing too many plants only to watch them freeze. They were simply in the wrong place. The first attempts at guidance came from the Arnold Arboretum in Boston. In 1927, taxonomist Alfred Rehder published a Temperature Zone Map to accompany his Manual of Cultivated Trees and Shrubs Hardy in North America. The map broke the country into eight zones based on the lowest mean temperature of the coldest month. In 1938, Arboretum horticulturalist Donald Wyman revised the map for his publication Hedges, Screens and Windbreaks. This map went through several updates, but eventually fell out of use.

USDA Gets Into The Act

In 1960, the USDA introduced its own hardiness zone map based on data from thousands of weather stations throughout the United States. A major revision occurred in 1990. Based on data from 1974-1986, this USDA Hardiness Zone map added the states of Hawaii and Alaska. Also at this time, each zone was divided in half (“a” and “b”). Another update occurred in 2012 (while we were studying for our Advanced Master Gardener Certification). This revision used a wider set of data and moved a lot of up a half zone. Farmington went from Zone 5b to Zone 6a. For the first time, the map became internet friendly, adding a zip code search option.

More Data

So, as you would expect, November’s new release uses even more data. And, it also incorporates advances in Climate Science. Thanks to complex algorithms, this map better reflects local microclimates. For example, a large city with lots of concrete and blacktop may be warmer than the surrounding areas. Or, the top of a mountain may be colder than the surrounding lower elevations. You get the idea.

But, in reality, as a gardener you really care about your individual hardiness zone. Let’s go further and talk about what it is and what it isn’t. As we said before, this map represents your lowest average annual temperature, not your lowest possible temperature. Since the beginning, these maps have been intended as a guide for trees and shrubs. It’s fortunate that they sort of work for other things you put in your garden. In fact, labels on plants and seeds almost always note hardiness zones.

Heat Zone

Note the use of the word hardiness. This implies cold tolerance, not heat tolerance. No mention is made of high temperatures in this zone map. But there is a map for that. The American Horticultural Society (AHS) has published a Plant Heat Zone Map. Here’s an interesting tidbit: Farmington, CT and Minot, ND are in the same Plant Heat Zone, But the madman says try spending a winter up there. The heat zones are delineated by the average number of days above 86° in a year. Zone 1 has less than 1 per year; Zone 12 has more than 210. These zone maps talk about highs and lows, but neither mentions frost dates, a bigger concern for the madman.

The madman says the data is ready for an update, but it does offer some guidelines. If you use the USDA Hardiness Zone Map and the AHS Heat Zone Map, you’ll find the perfect plant, right? Not quite. As we always say, nothing in nature is simple. It’s a start, but only that. If gardening and landscaping were that easy, we’d all be good at it. But plant survival depends on a number of other factors. The madman feels that the most important of these are water, soil, light and wind.

Other Factors

As we have said a zillion times, plants need water to grow and flourish; but the amount of water needed varies with the plant. A plant that has evolved in a wet environment will not do well in dry conditions, and vice versa. That’s why you shouldn’t plant a Cactus in your water garden.

Plants need a place to stay, and soil provides that place. Soil also, provides nutrients for growth. For thousands of years, plants have survived in their native soils which have just the right mix of nutrients. Moving the plant to a different soil in the same zone may have a negative effect on the plant. It all boils down to the fact that you have to know your plant and know your soil no matter what zone you are in.

Some plants like it sunny, some like it shady. Native plants evolved either in sunny fields, along the edge of forest or under the canopy of trees. The light received by each varied with its particular location. You can grow a sun loving sunflower in almost any zone you want – unless that zone happens to be in the middle of a dark forest.

The last factor we want to mention that affects plants and their hardiness is wind. And, here the big issue is too much. A windy environment can put stress on a plant’s structure. It can also cause dehydration and cooling. In fact, too much wind can put you in a lower zone.

Right Plant, Right Place

All of this takes us back to the concept of right plant, right place. Use the zone map for general guidance, but don’t ignore the intricacies of your location. Mother Nature has always delighted in creating hundreds of microclimates.

The madman is happy with a higher zone, but he started to think about what it may mean for native plants and wildlife, his head almost exploded. What does this mean for migrations, reproduction and food sources needed to sustain our environment. Those are things we need to start thinking about, and the madman has to do a lot more thinking.

Guess it’s time for hot cocoa.