We humans, and many other animals, have the option of living where it's warm or cold, traveling north or south depending upon our preference and the time of year. Plants don't have this option.
Plants' preferences are dictated by the climates to which they are adapted. That's where they'll grow the healthiest. For example, there was a paper birch tree planted next to my parents' home in Sarasota, Fla. The neighbors brought it with them from the northeastern United States. It lived for 10 years, never growing beyond seven feet in height. It was the saddest sapling on the block, while tropicals, such as oranges and palms, thrived.
Cut to Canada, a northern part of Quebec, the Gaspe Peninsula. My wife and I visited Metis in Mont-Joli a couple of weeks ago. This is one of the many native habitats for paper birch trees. Their brilliant white bark is evident throughout the woods. Here they grow to 70 feet, with trunks that are more than two feet in diameter. Of course, an orange or palm tree wouldn't survive one winter in Quebec.
To rate plants by where they're hardiest, a map and numbering system was developed for North America through the National Arboretum. It's called the "USDA Plant Hardiness Zone Map," and it's well known to just about anyone who has browsed through a gardening encyclopedia or mail-order catalogue. The zones range from 1 to 11, coldest to warmest, and are based on the lowest temperatures recorded in the United States from 1974 to 1986.
Hardiness is determined by how well plants survive cold temperatures. Paper birch, for example, can survive to 50 degrees below zero, making it a Zone 2 plant. Royal palm, on the other hand, can't live where it gets colder than 30 degrees, which is a Zone 10.
The Agriculture Department's hardiness zone map shows Washington, Northern Virginia and Southern Maryland as Zone 7. Montgomery, Frederick, Howard and northwestern Prince George's counties are shown as Zone 6. So the plants to buy for your garden in this area are those rated for Zones 6 and 7.
This system is excellent as far as it goes. But since the birch didn't thrive in Sarasota, even though it doesn't get colder than 50 degrees below zero there, obviously something was missing: a way to measure the plant's heat tolerance.
To address this shortcoming, horticulturists have been showing the range of temperatures a tree will grow within by giving a zone "spread." Paper birch is listed as hardy from Zone 2 to Zone 7. But that's only partially accurate. It's prone to many disease and insect problems in Zone 6 and Zone 7 and is best planted only in Zone 5 or colder. This is because of heat and humidity, not cold.
H. Marc Cathey, who authored the most recent version of the hardiness zone map when he was director of the National Arboretum in 1990, introduced a complementary map in 1997, the "American Horticultural Society Plant Heat-Zone Map."
Mary Ann Patterson, the society's director of marketing and public programs, said many authors and publishers now are listing the heat codes and the map. The first books to do so are "Great Plant Guide" (DK Publishing, $16.95) and Cathey's "Heat-Zone Gardening" (Time-Life Books, $24.95). The first CD-ROM program using the heat zones recently was released. It's the Burpee 3-D Garden Designer Heat Zone Edition, published by Macmillan Digital Publishing.
The heat zones are based on the number of days that the temperature tops 86 degrees. Above that temperature, plants begin to show heat stress. On this map, the Washington area is rated as having 60 to 90 heat-stress days per year, which is easy to believe after this week's heat wave.
The fewer sweltering days, the lower the zone rating goes; the more hot days, the higher the number. These zones run from 1 to 12. The data were taken for a 20-year period, from 1974 to 1994, and supplied by 4,745 weather stations. The map puts the Washington area in heat Zone 7--the same zone number as the cold-hardiness map.
Paper birch has a heat-zone rating of 1 to 7. So, it's at the edge of its ability to withstand excessive warmth in this area, yet Washington is also barely cold enough for paper birch to survive. This double hardiness zone rating is what gives us enough information to know, definitively, if a plant will thrive where we are planting it.
Cathey is promoting this double-zone labeling for all nursery stock. About half a dozen large plant catalogue companies are now using it, with more on the way.
The best chance of having success with plants in this area is to plant those with a U.S. Department of Agriculture (USDA) cold-hardiness rating of 5 to 8 or greater, and a separate American Horticultural Society (AHS) heat-zone range of 4 to 8. That would place our zones of 6 to 7 right in the center of these plants' hardiness ranges.
Following are some plants that should be perfectly matched to this region by zones:
Japanese anemone hybrids: USDA hardiness Zones 4 to 8; AHS heat Zones 9 to 3
Astilbe: 4 to 8; 8 to 2
Hosta: 3 to 8; 8 to 1
Lavender: 5 to 8; 9 to 3
Peony: 3 to 8; 8 to 1
Rodgersia: 5 to 8; 8 to 5
Blue star juniper: 5 to 8; 8 to 5
Blue or Meserve holly: 5 to 9; 9 to 5
Oakleaf hydrangea: 5 to 9; 9 to 2
Double-file viburnum: 4 to 8; 8 to 1
Viburnum opulus: 4 to 8; 8 to 1
Incidentally, for those of you who want a birch tree in this region, there is only one that I would recommend getting for Zone 6 or Zone 7. It's a hybrid named heritage river birch, and it is highly resistant to the insects that plague most other birch trees here.
But continue to try some other plants that might not be as hardy to the area. My wife, Sandy, and I are excited to try the meconopsis seeds, also called blue poppies, that were given to us at Metis, where they are being bred. The seeds stay viable for only a few weeks, so we must plant them immediately. After all, it's worth a try.
Lerner is president of Environmental Design in Capitol View Park, Md. His e-mail address is firstname.lastname@example.org
CAPTION: (This graphic was not available) The American Horticultural Society Plant Heat-Zone Map puts the Washington area in Zone 7.