Vol. I, No. 2: 2. Gardening, Indoors & Out: Hardiness Zones

There are lots of maps out there about hardiness zones. But few of them provide the kind of detail of the map put out by the National Arboretum. {You can click on the map inset in the table below for a full, high quality image of North America.}

The hardiness zones are based upon "average annual minimum temperatures," which the USDA defines as "the lowest temps recorded for each of the years 1974 to1986 in the United States and Canada and 1971 to 1984 in Mexico."

USDA Hardiness Zones and Corresponding Common Trees
3 -40 to -30 F -40 to -34.5 C	Berberis thunbetgii (Japanese bayberry) Elaeagnus angustifolia (Russian olive) Junipercus communes (Common juniper) Lonicera tatarica (Tatarian honeysuckle) Malus baocata (Siberian crabapple) Thuia occidentalis (American arborvitae)
4 -30 to -20 F -43.5 to -28.9 C	Acer saccharum (Sugar maple) Hydrangea paniculata (Panicle hydrangea) Juniperus chinensis (Chinese juniper) Ligustrum amurense (Amur River privet) Parthenocissus quinquefolia (VA creeper) Spiraea x vanhouttei (Vanhouffe spirea)
Source: USDA, National Arboretum. Publication No. 1475. January 1990.
Click here, or on map above for high quality image of North American map

Here in Vermont, as you can see from the map inset below, there are at least five different zones. That's because, except for zones 1 and 10, the USDA actually breaks the usual numeric zones down further, into sub-groups of a and b. In that light, Vermont hardiness zones range from 5a down in the southern counties, through 4b for much of the Champlain Valley, 4a for most of the central mountains over to the Connecticut River, and 3b, with a small pocket of 3a, throughout most of the Northeast Kingdom.

By looking at the key to the map on the right, we can see that the average minimum temperature in the southern counties bottoms out at -15 to -20F, while at the northern end of the state, minimum temps reach -35F, and in at least one small pocket around the Canadian border, 40 below.

In the Champlain Valley -- mostly rated a zone 4b -- the minimum temp is listed at -25F. However, anyone who lives in any of the 'cold pockets of the Valley knows it can get colder than that.

Certain features make for climatic islands. Cities, for example, become heat sinks and generally tend to stay warmer than the surrounding countryside. Low-lying valleys, especially smaller 'bowls', also have a tendency to both heat and cool more, and so have wider temperature variations than more exposed areas. Finally, and most obviously, elevation also influences temperature considerably.

Each of these elements can contribute to significant fluctuations in temperature. And it isn't limited to the most obvious or dramatic instances only. For example, on the west side of our place, which is in a stand of hemlock and beech, the winter morning temp has a tendency to be at least 5-7 degrees warmer than on the east side, which slopes away from the house into a small hollow.

So, while the USDA has built much of this info into the map for broader regions -- the foothills and Green Mountains are a colder zone than the Valley -- the map is not sufficiently detailed to provide micro-climate information.

Nonetheless, there's nothing frustrating in quite the same way as laboring over those catalogs and landing on a plant you know would just accent your border plantings perfectly, only to be defeated by the fact that this isn't Maryland of Jersey, or even The Berkshires.

For more information on hardiness zones, as well as a host of other useful gardening information, check out this month's Links to Farm & Garden On-Line, which includes the link to the USDA Gardening site.

If you would like to submit a feature for our Farm & Garden section, don't hesitate to let us know. Simply e-mail us at farmgarden@downstreetmagazine.com. The e-mail should contain your name, address, and a phone number where we can reach you. You may also send a copy of your proposed article. The text can either be included in the body of the e-mail, or you can send it as an attachment in just about any word processing format. If your piece is accepted, we will pay a small honorarium for your interest & your time. [See Freelancers Wanted for more details.]