Hardiness Zones – Gardening Planting Calendar
What do those zone numbers mean… and how do they apply to my garden?
For gardeners, “being in the zone” matters—a plant hardiness zone, that is. An interactive online map of Canadian and USA zones allows gardeners to zoom in and discover their zone to Planting their veggies, plants, and flowers. But what do those seventeen numbers starting at zero, through 1a, 1b, and culminating at 8a mean?
The first Hardiness Zones Map and the “New” Map
Canada’s original plant hardiness zone map was created in 1967. However, as we gardeners know, times—and climatic conditions—are changing. Scientists from Natural Resources Canada’s Canadian Forest Service, Agriculture, and Agri-Food Canada, and Natural Resources Canada’s National Atlas of Canada collaborated to create the new Plant Hardiness Zones Map. Published in 2000, it used data captured between 1960 and 1990.
The new data incorporated information on elevation, so exposure effects are factored into the hardiness equation. The hardiness zone map is useful for home gardeners through to nursery owners and large-scale orchardists to refer to these zones before investing in plants.
What is the Relevance of the Numbers?
The nine major plant hardiness zone numbers describe Canadian growing conditions, where zero (0) describes the harshest environment and 8 represents the mildest. A simplistic way to remember the numbers is to think of zero being harshest, and 8 being very mild. When we look at the map of Canada divided into its zones, this approach works. All of Canada’s north including the Arctic region falls under zones 0a or 0b, the color is blue—icy blue.
Usefully, warmer zones go from green to yellow to red, where Fuschia (zone 8a) represents the mildest growing conditions, found on British Columbia’s West Coast. The general rule of thumb is that when you’re buying plants, you buy plants that are tagged for your zone (or lower) because you know the plant will survive in your zone’s specific weather conditions.
Buying Plants for your Zone and What Does ‘a’ and ‘b’ Mean?
Buying Plants for your Zone
My husband Eric and I planted 24 fruit trees in 2011: 22 apples and two pears. When consulting the nursery operator from whom we purchased bare-root whips (meter-high saplings), we identified our location and hardiness zones (4a/4b) with an occasional zone 5 species surviving.
He recommended varieties hardy from zone 3a to 4b. Species include Snow and Wealthy (3), Northern Spy, and Winesap (4). Looking now at the Silver Creek Nursery website, I realize we could have experimented with Rescue, which is hardy to zone 2.
Why bother dipping into zones below the sub-zone in which you live? Because the plant does well in zone 2 or 3, it will likely grow even better in zone 4.
What’s key is that as the numbers increase, so does the mildness of the zone. Not all lower zone plants would do well in milder zones: it’s prudent to be cautious and ask the experts at the nursery proprietors.
What do A and B Mean?
The numbers from 0 to 8 are broken down into sub-zones, such that each number (except 8), has an associated a and b. These descriptors break the numbers into a finer set of sub-zones. Agriculture and Agri-Food Canada’s website informs us that these numbers were derived from plant survival data, as well as many climatic variables ranging from seasonal temperature fluctuations and frost-free periods to summer and winter precipitation, and wind speed.
The subzones come in handy to explain micro-climate conditions. According to the website, “some significant local factors, such as micro-topography, amount of shelter and subtle local variations in snow cover, are too small to be captured on the map. Year-to-year variations in weather and gardening techniques can also have a significant impact on plant survival in any particular location.”
A Micro-Climate Example
Over time, gardeners may recognize the subtleties of micro-variations in their gardens. Let’s look at an example. My farm, Spiritwood, is near Ottawa, but on the northern, more elevated (read: exposed, harsher) Quebec side of the Ottawa River. Whereas Ottawa is in hardiness zone 5a, according to the zone map, my farm, located 50 minutes northwest of the Parliament Buildings, is between zone 4a and 4b.
Interestingly, my growing season is different than that of some friends who live a 20- to 40-minute drive west of me, in harsher conditions. While my tomatoes ripen on the vine; theirs frequently must be gathered to ripen indoors.
Moreover, because of microclimates my husband Eric and I have created since 1989, we can extend our plant hardiness zone to (somewhat unreliably) include species requiring 5a.
Beyond the Zones
Natural Resources Canada’s Canadian Forest Service is now calling on the public to tell them which plants from a comprehensive list are surviving in their zone. These will be added to range maps for individual trees, shrubs, and perennial flowers. Take a look at the website to discover how you could contribute data for this project.
CLIMATES AND Hardiness ZONES
When gardeners gather, discussing the weather is more than idle chitchat. Weather is an important consideration in deciding how you’ll use your yard and what types of features you need in it. And gardening success is vitally linked to selecting plants that suit your climate.
One important piece of information is your USDA hardiness zone, which indicates the average annual minimum temperature in your area. It’s a basic rule of gardening to choose plants that are hardy in your zone, unless you want to experiment, knowing that certain plants you’ve invested in may not survive through the winter.
Plants also have a limit on the amount of heat they can withstand. Many garden jewels such as lilacs, rhododendrons, peonies, and white pine develop heat stress, pest problems, and fungal diseases south of Zone 7.
The cost of cheating too much on a plant’s recommended zone is higher maintenance. Growing a semitropical plant such as an angel’s trumpet (Brugmansia spp.) or Japanese camellia in Zone 7 means that you must be prepared for the occasional years when the plant dies in winter. You will either have to start over or consider growing the plant in a container and plugging it in and out of the garage as the weather changes.
If you want a low-maintenance garden design, stick with plants for your zone. But if you want to expand your plant palette and experiment in the garden, push the limits. Also, keep in mind that new heat- or cold-tolerant cultivars and species come on the market each year. If you see a new offering, check its label. It just might be worth giving a try.
MICROCLIMATES The USDA
bases the zone map on the law of averages, and it works on the theoretical premise of flat, open, uniform landscapes. But few yards are completely flat and open, and that’s a good thing for gardeners. Slopes, depressions, trees, ponds, driveways, and other landscape features affect growing conditions, creating microclimates – small areas that are naturally warmer or cooler than would be expected based on your zone.
A microclimate exists for example in a shaded depression at the base of a slope where cool night air pools or on a paved patio backed by a southwest-facing masonry wall that traps and reflects heat.
As you develop your landscape plan, think about how you can take advantage of the microclimates in your yard. Or if you’re starting with a bare lot, consider how you can create microclimates as you install hardscape or structures. Use a cool, shady depression for growing columbine that wilts in the heat of a sunny, exposed place in your garden. Build a gazebo nearby to provide cool respite for sitting and viewing the columbines on a muggy afternoon.
Further, enhance the setting with a trellis on the downhill side to provide dappled afternoon shade and trap cool night air as it sinks the slope. If you have a sun-drenched patio, use it for experimenting with citrus in containers.
Set out a chiminea, a table, and a few chairs for a cozy gathering spot where the radiant heat of the paving and walls draws you outside on cool evenings in fall and early spring. The more microclimates you create, the more options you have for using and enjoying your landscape.
WINDBREAKS, PRIVACY SCREENS, AND SOUND BARRIERS
Creating a windbreak, a line that is perpendicular to the prevailing winds forms a protected niche for cold-sensitive plants. A windbreak also protects plants in summer from drying winds. Also, a windbreak makes an effective privacy screen and also diminishes sound from a busy street or another noise source. Use a fence, wall, hedge, or berm of mounded earth to make the windbreak.
A combination of these elements makes the most effective screen. For a sound barrier, the most effective control comes from sitting thick evergreens, such as Southern magnolia, rhododendrons, or Burford hollies, near the street or noise source. The living baffle diffuses and scatters the sound waves.
The opposite of windbreaks, wind tunnels improves cross-ventilation by opening up the sides of a garden room and capturing prevailing winds. Using open basket-weave brickwork or courtyard walls and planting in rows parallel to prevailing winds improves air circulation in a yard. Converging the planting rows in a funnel shape forces the wind toward a point, like water through a nozzle.
The area just beyond the narrow opening gets the advantage of the brisk breeze, becoming a cool summer gathering spot as well as an ideal microclimate for plants vulnerable to fungal diseases in stagnant air, such as roses, phlox, and zinnias. Locating a pool, pond, or splashing fountain upwind cools the breezes even more.
Areas beneath decks or the eaves of a house, inside the dripline of a tree with dense foliage, or behind a structure often remain dry even after a soaking rain. The way to handle rain shadows is to select drought-tolerant plants and ensure that your watering system reaches all plants and gives them adequate water.