Plant Hardiness Zones and Your Garden
Most gardeners rely on the Plant Hardiness Zones published by the U.S. Department of Agriculture (USDA) to help guide their plant choices. Zone recommendations are published on seed packets, added to those plastic seedling spikes included with new plants and referred to in articles like those posted here. I think hardiness zones are somewhat misunderstood, though. Here's why:
Many gardeners believe zone listings include more information than they actually do. Those numbers aren't the result of complex algorithms of multiple geographical and weather related characteristics. Zone designations refer to the average anticipated low temperature for a specific geographical location. That's it. Here's how the USDA puts it:
"Hardiness zones are based on the average annual extreme minimum temperature during a 30-year period in the past, not the lowest temperature that has ever occurred in the past or might occur in the future." (USDA Agricultural Research Service)
Zones don't take into account heat, humidity, or microclimate variables like proximity to water or buildings. These last two are particularly significant because they can have a big impact on temperature but are too localized to be evaluated by zone, even with the more specific "a" and "b" zone designations in use today. (More on that in a minute.)
Here's an example: You have a pond, creek or other water source near your home. Wind coming from that direction will be somewhat cooler that the surrounding air because soil, which is dense, holds heat better than water. As hotter air passes over a volume of water, its temperature drops. The reverse is also true: Developed areas with lots of concrete roadways and tall buildings will hold heat better than the surrounding undeveloped terrain.
|Protected areas can be warmer in winter.|
This is even the case on a smaller scale:
In summer, the shady side of your home will be cooler that the sunny side.
Elevated areas like decks and balconies will be cooler.
In winter, unprotected locations will be colder than sheltered areas.
If you live at the base of a hill or in a dale, your microclimate will likely be colder than the surrounding terrain because cold air is heavier than warmer air and will migrate to the lowest level it can reach.
If your home is on a south facing slope, your yard will be somewhat warmer that the folks on the other side of the hill with a northward facing garden.
These microclimates create conditions the savvy gardener can take advantage of -- or prepare for.
Test Your Garden -- It's the Only Way to Be Sure
So, if you think parts of your garden are warmer or colder than the norm, grab a thermometer and do some research. The difference between zones is actually 10 degree Fahrenheit for an average winter low. That number is further broken down by an "a" or "b" designation, an incremental difference of 5 degrees Fahrenheit.
In English, that means you can look at a zone map, discover you're in Zone 5b and be 5 degrees Fahrenheit away from a Zone 6 designation. Natural microclimates in portions of your yard or geographical area could easily adjust for the 5 degrees and open up a whole new world of plants for you. You may find you have a sheltered spot where rosemary will thrive, or a chilly corner with asparagus potential.
|Proximity to water can affect climate.|
Here are some tips on how to take advantage of microclimate variations:
Watch your garden this year. Take a look at where the sun shines in the morning and afternoon. Pay attention to how the wind blows. Review topographical maps. Your findings should factor into your garden planning.
You can Install the Google Earth to take a look at 3D maps for your area. The software/service is free. The installation page is here: Google Earth
You can also visit the U.S. Geological Survey site for access to maps for your area. Start Here: USGS Map Locator & Downloader
Invest in some outdoor weather temperature equipment and start making notes during hot summer days and cold winter nights. Focus on areas you think may be average, protected or particularly vulnerable.
This can be a more reasonable investment than you think. There are reasonably priced wireless weather stations that allow for up to three independent outdoor sensors. A system will include a convenient indoor console with a summary readout. (*Outdoor sensors are typically pretty durable. They run on batteries and have weatherproof cases.) Perform a web search on "wireless garden thermometers" for options. I know that La Crosse Technology (WS-9160U-IT Digital Thermometer) has one, but there are definitely others.
You can also use the old fashioned method, and position an outdoor thermometer where you want it and check the readout during peak hot and cold periods. (This may take some garden love when you're slogging through the snow in the middle of the night, though.)
Once you have a rough grid of your garden's microclimates, you can start investing in perennial plants with a real sense of what you can expect from them. You may be surprised at what you discover.
*Based on product reviews, these batteries can last up to a year.
Cornell University Gardening Resources, "Microclimates" (06/14/2013)