June 12, 2007 at 12:00AM
by Leslie Land
Maps that divide the country into roughly 15 cold-hardiness zones
are the tools you love to hate. The more experience you have, the less faith you put in those numbers and yet some belief is essential; no way to predict unknown-plant survival without SOME guidelines, be they ever so crude. I've been reading reports (both scientific and anecdotal) on this ever-vexing subject for about 35 years now. And for the last 15 of those years I've been tending two gardens, one 400 miles Northeast of the other, both of them in old-fashioned USDA
Zone 5 -- or the more up-to-date Arbor Day Foundation
Zone 6. Insights gained are below. This is just to keep you amused; it has nothing to do with climate zones except that being a daylily (Hemerocallis spp), it'll grow almost anywhere).
Rules that Help: a) Allow for 1 zone on each side of the one you're allotted. If the map says you are in zone 5 you can probably grow a Zone 6 plant, but on the other hand your winter may kill something that has Zone 5 as its permitted lower limit. b) Do the same thing with plant labels; they tend to be either overcautious or overhopeful, or (on those that offer a range of zones) both. c). Learn your garden -- if it's big enough to call a garden, it's probably big enough to have warm and cool spots, windy and sheltered ones, and quite possibly different soils within 10 feet of one another. Hardiness Maps that Attempt to Improve Things: By refining the description of the plants: The American Horticultural Society has created a HEAT zone map
, which makes a great deal of sense; most plants have death points at both ends of the thermometer. This map made its debut in 1960, but like the metric system it hasn't made much headway. It IS used by the society's magazine, where spicebush, for instance, is referred to as Lindera benzoin, Zones 4-9, 8-1. The first set of numbers are the cold zones, by the way, should you happen to run into a pair like this on a plant label somewhere, and if you do please write and tell me about it. By refining the description of the zones: Sunset publishing offers -- and works from -- a national map
that includes not only heat and cold but also length of growing season, humidity, rainfall and rainfall patterns. It is silent on the subject of soils, probably just as well given that there are 45 zones as it is.
Writer Leslie Land blogs about gardening, food and design at Leslieland.com.