Getting into The Zone – Hardiness, Heat, Moisture, and Altitude

Many gardeners will come across a problem when trying to determine whether or not a plant is suitable for their region. Most plant labels consider a single variable, cold hardiness, and do not take into account other key variables such as heat or moisture. The American Horticultural Society has made a great leap by providing a system for Heat Zones to compliment the well known Hardiness Zones, but there remains to be seen any trend towards using the Heat Zones when indexing plants for nursery sales, and most plant information skips this significant variable.

I have been considering this for a while due to conversations in a popular Facebook Gardening Group after having noticed that areas of Texas and my region of South-Western B.C. share a hardiness zone – yet the overall difference in climates between Texas and B.C. is notable. Relying entirely on Hardiness Zones is clearly not of much advantage considering how many plants will not thrive the same in regions that share only their Hardiness Zone regardless of the “frost resistance” it may or may not have.

So I began to wonder what else should be considered in order to adopt a more efficient system, and the variables that came to mind were the plants threshold for moisture and dryness, as well as altitude – though altitude would be significantly less important in the broad spectrum of plants we grow/buy. Moisture and dryness is not quite as simple either.

At first I was thinking a person could take the rainfall gradient over North America, for a start, and devise a scale similar to theHardiness Zone tailored to average rainfall. The driest climate would be designated 1 and the wettest 10, and this could be transposed onto a map that would show colored regions for each zone designation. But it is not this simple on consideration. Each area, regardless of rainfall, will have varied moisture depending on geography and conditional variables such as ground springs or bog conditions. Though a higher rainfall may increase the odds of a bog condition, it is impossible to use rainfall to determine how much water will sit or accumulate near the ground surface.

The first graphic I created, in a very short time, was intended on describing my thinking,  and is not meant to reflect technical accuracy.

At this time I am still not sure how Moisture/Dry can work most effectively, but first there are some issues with this graphic. The Heat and Hardiness Zones in reality have 12 points, and the pH scale has 14 – I used 10′s for simplicity. The “sun/shade” bar is divided into 5 for convenience, but I would imagine 4 sections for Full Shade, Partial Shade, Partial Sun, and Full Sun would be sufficient.

Either way it describes what I personally think would be more of a benefit to a gardener when they buy a plant, than simply knowing the sun/shade needs. I’ll quickly describe each value; skip this if the graphic is obvious enough. Let’s start with the zones:

  • Heat Zone: As shown in the Heat Zone Map at the top of the page, each region is given a value based on how many days the average temperature is above 30ºC (86ºF). A palm or cactus will do better with a higher value and a plant with a low heat tolerance would do better with a lower value.
  • Hardiness Zone: Most growers are well familiar with this value. The lower your number the colder it gets in your area, and plants have known cold thresholds used to determine their hardiness. The hardiness zones have some known issues that I would recommend Reading on Wikipedia if you are not already familiar with them.
  • Moisture/Dry Zone: I have totally made this up, and I will try to formulate a better way to possible make this a more effective linear system. At this point I am thinking average rainfall would be sufficient for most determination of a hypothetical zone system, but that the “nursery index card” system would be better slightly different. If your map showed high rainfall, say a hypothetical 9, and your plant preferred dryness, it would be easy enough to realize the drawbacks of planting it in your back yard. If the plant is succulent or a water garden plant, then annual rainfall is again a good hint, but a person could alter their substrate to accommodate drainage. Let’s think about this. Feel free to Drop into The Group to discuss it, or Message Me.
  • Altitude Zone: Altitude is not something most of us consider when we are buying plants, or considering the conditions of our region and what plants would thrive the best there. There is certainly though demographics of folks living in high altitudes who need to accommodate their plant choices to their area. One big question would be how many plants actually have a known altitude threshold, or a known optimum altitude. The next question is, if there is not a great deal of information on altitude, do we simply ignore the variable or do we make an effort to understand how it might effect various plants?
Now that we have looked a bit at the zoning, and why it might be applicable or make consumer choices more educated, let’s look then at what might appear with the zones on a nursery index card to assist a gardener in getting the most from their purchase.
  • pH: We all know that plants have a optimal pH, and generally plants will also have a “region of pH” where the plant will survive fine based on the availability of its favorite minerals or its tissue sensitivity to pH conditions. Since most plants pH spectra are known this would not be a difficult thing to include on the nursery card.
  • Sun/Shade: This would be the standard variable already found on nursery cards, generally depicted as circular graphics.

If we put this all together on a nursery card then, it might look a bit like this (I have also added asterix’s to the hypothetical optimums):

As always any input anyone would like to give on the subject is welcome, and I can always be emailed directly if you do not have a Facebook account. Thanks for reading and I will update again with any changes or new considerations.

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