Picking plants whose intrinsic biological and cultural needs fall within the parameters of the soil and climate in which you hope them to thrive is perhaps the most basic gardening principle. The question “Will this grow in my zone?” seems as though it should be one with a yes or no answer. Nothing is ever that easy–not even understanding what your ‘zone’ is and what information that zone number is conveying. There are several references available to gardeners which tell us what zone we garden in. Let’s look at a few of them:
USDA PLANT HARDINESS ZONE MAP
Nine times out of ten the zone number you see on plant tags and labels is the UDSA Plant Hardiness Zone.
A collaboration of climatologists, meteorologists and horticultural experts produced a lovely full color map first published in 1960 and updated in 1990. The USDA zone reference leaped forward to the digital age in 2012 with an additional update. There are no print maps produced of the 2012 update–you can access and download the maps from the USDA website http://www.planthardiness.ars.usda.gov or just type your zip code in the window provided on the site and you will be rewarded with your zone number.
But what does this magic number tell you? The USDAPHZ (yes, they actually refer to it that way now on the website) is a scale which shows the average annual minimum winter temperature, divided into 10-degree F zones, during a 30 year historical period. Each 10-degree slice has an “a” and a “b” part so the USDA Zones and corresponding temperatures start at 1a (coldest winter) and rise in 5 degree F increments to 13b (warmest winter). For example my zip code 93711 is in Zone 9b with average annual minimum winter temps of 25-30 degrees Fahrenheit. Typically a plant label or description will state a range of zones or use the wording ‘hardy to zone__’ indicating the lowest zone number recommended for the plant. For gardeners in cold weather winter parts of the country this zone number is critical in determining whether a plant will “winter over” or be lost to temperatures simply too cold for the plant’s cultural requirements. Bear in mind that past weather records are not a guarantee of future weather. Additionally, all gardens have a variety of microclimates within them–valleys or low spots where cool air collects, warm pockets created by the reflection of the sun off concrete or blacktop, etc. The number is never a substitute for your experience and that of fellow gardeners in your area. It is simply a place to start.
For those of us in areas characterized by milder winters and screaming hot summers the USDAPHZ leaves us in a quandary. If my plant choice is burned to a dry husk or wilted down to a barely recognizable lump by September it really is of no benefit to me to know if it will winter over! The need for a scale to identify heat tolerance easily must have inspired the creation of this next zone map.
AMERICAN HORTICULTURAL SOCIETY HEAT ZONE MAP
Cold is not the only factor which determines whether what you plant will survive and thrive in your garden. Heat was finally given its due with the creation of this 12 zone map. The zone number indicates the average number of days each year that a region experiences “heat days”–temperatures over 86 degrees F. The AHS designates this as the temperature point at which plants start to suffer physiological damage from heat. The zones range from Zone 1 (less than one heat day per year) to Zone 12 (more than 210 heat days). My garden is in AHS Heat Zone 9 which equates to 121-150 days annually exceeding 86 degrees F. When a label or reference lists the recommended zones they are listed from high (hottest) to low (coolest).
The above label is a perfect example that the zones, whether hardiness to cold or heat tolerance, are only a starting point for your selections. The ‘Blue Waterfall’ campanula’s hardiness number tells me that my winters are plenty warm in USDA Zone 9 as this plant can tolerate winters in zones well below mine but I am pushing beyond the recommended AHS Heat Zone by a couple of notches. My colonies of ‘Blue Waterfall’ are sited in early morning sun only and get adequate water and have been very successful. I suspect that if they were planted in anything close to the full sun they can take in cooler zones and they would be toast!
As recently as a few years ago the AHS website had a zip code zone finder tool similar to that of the USDA website. This has disappeared from their site and other sites which formerly had links to it. I suspect that AHS would like you to purchase their map. The Society has also classified more than 2000 plants by AHS Heat Zones and offers that information in a reference manual called AHS Great Plants Guide. Their website, http://www.ahs.org, is packed with lots of gardening basics/resources and you can purchase both the book and the map online.
The excitement of having heat tolerance classifications for plants and a nifty full color map to find our zone is dampened in that relatively few plant growers or garden centers mark their stock with the AHS Heat Zone. AHS is promoting a label format that will offer both cold hardiness, heat tolerance and a classification such as annual or perennial but until industry takes up the cause it is all just information under wraps unless you purchase the reference material and carry it around with you as you shop!
REGIONAL GARDEN GUIDE CLIMATE ZONES
Lifestyle, garden and travel gurus Sunset Publishing and Southern Living Inc. both have book format regional garden guides covering the geographical areas of their circulation focus.
The Sunset Western Garden Book began as a plant selection guide published in 1935. In 1954 the first Sunset Climate Zone maps appeared, dividing its focus area into 13 climate zones. Sunset refined and expanded the maps in 1995 delineating 33 climate zones covering 13 western states and a small part of Canada. The Western Garden Book has been the “go-to” resource for practical gardening information and plant selection for generations of western gardeners.
When I moved to Georgia in 1995, my So Cal girls gave me a subscription to Southern Living and a copy of The Southern Living Garden Book to get me started in my new home. Amazing, this garden reference not only covers areas typically thought of as the South but also Kansas, Missouri, Illinois, Ohio, Pennsylvania, New Jersey, Maryland and Delaware. Everyone must love New Mexico as both books lay claim to this state. I feel badly for the gardeners in the Dakotas, Iowa, the Great Lakes states (2 Ms, 1W and New York) and all those little Eastern seaboard states past New York that they don’t have a publishing conglomerate looking out for them! The Southern Living Garden Book divides its domain into 5 zones: Upper, Middle, Lower, Coastal and Tropical, each followed by the word SOUTH. Well, it is their book after all!
Both these stellar regional garden guides espouse that factors well beyond heat and cold ultimately affect the success or failure of your plant selections. Sunset lists latitude, elevation, ocean influence, continental air influence, mountains, hills, valleys, microclimates and soil as components unique to any geographic region that would make it a welcoming home for a plant with specific botanical needs. Southern Living emphasizes that the relatively short, but sometimes frigid, winters combined with hot and humid summers presenting unique challenges across its southern (and semi-southern) readers gardens.
Each of these guides presents detailed climate information zone by zone and not only gives you a number you can use for reference in identifying plant material’s suitability but also lots of general climatology and geophysical data that is really pretty interesting reading. My Georgia garden was in the Lower South zone which is described as “a region of short winters, early February springs and hot, sticky, often dry and seemingly endless summers with nighttime temperatures usually no lower than the mid-70s.” My California Central Valley garden is in Sunset Zone 8, characterized as a cold-air basin with a long constantly sunny growing season, high summer daytime temperatures and piercing cold winter north winds. We sit right on the edge of Sunset Zone 9–the primary difference being that Zone 9 is a thermal belt sit at the base of foothills whose cold air can flow right into my Zone 8. There goes my opportunity to grow citrus commercially…
If you were to look at the USDA hardiness of my Zone 8a Georgia garden and Zone 9b California garden they look pretty similar, just as my AHS Heat Zone 8 Georgia garden cozies right up against my California garden’s AHS Heat Zone 9. In fact, due to gargantuan differences in humidity, rainfall and soil types these two gardens are very different from each other. The additional factors used by these two regional gardening guides lead me to rely far more on their zone structure than looking at raw numbers of heat and cold only.
It is not totally uncommon to see the Sunset Climate Zones listed on plant tags for things that are grown in California–especially now that Sunset has partnered with growers to market the Sunset Western Garden Plant Collection.
You may also see plants tagged with any combination of the different zone numbers–USDA + AHS Heat Zone as it visible in the ‘Blue Waterfall’ tag above, USDA + Sunset or AHS Heat Zone + Sunset. Hopefully, the tag will tell you what reference is being used not just give you a number! Here’s a test for you below:
You have a number range but whose zones are those? This has to be a Sunset Climate Zone indication because none of the other scales in use now have numbers as high a 24!
Are you ZONED OUT yet? The takeaway here is that we have all kinds of resources to help us plan our gardens using plants which have the greatest possibility of thriving in the setting we provide. Use these resources with the foreknowledge that a zone is not a guarantee. Arm yourself with information, revel in your successes and tell those dead ones that there are more just like them where they came from.