The other day I posted a link to the USDA’s new plant hardiness zones. Although a composite of a number of factors, the zones are generally referenced to determine the minimum temperature at which a plant is likely to remain viable. While these zones are a useful starting point, for growers like me (for whom heat can be a much more limiting factor than cold) they offer quite limited information as to which plants are likely to thrive year-on-year.
To elaborate, I live in zone 8a, now if you look at the hardiness zone maps you will see that zone 8 ranges from the Pacific Northwest to North Carolina to most of the United Kingdom to China to parts of southern Africa. Now when I think of the Pacific Northwest I think of temperate rainforests, moss and lots of ferns; when I think of North Carolina I think of sultry summers, mixed deciduous forests and lush lawns; and when I think of North Central Texas I think of remnant prairie, remnant cross timbers and blazing hot and dry summers (I’m wanting to walk into a freezer just thinking about it!). Obviously, while knowing the minimum temperatures my perennials need to overwinter is useful I’m far more interested in the amount of heat, drought and just downright abuse they can tolerate.
Fortunately, the American Horticultural Society has recognized the limits of the USDA’s plant hardiness zones and developed a complementary set of heat zone maps. As you can see from the AHS’s article, there are still other factors (such as average annual and seasonal precipitation, soil pH and composition, etc.) that must be taken into account when choosing plants, but by taking both hardiness and heat zones into consideration when choosing what to plant we improve the odds of having a thriving landscape (and ultimately making better use of both our natural and personal resources); because even though the saying goes “you’re not a real gardener until you’ve killed hundreds of plants” when it comes down to it I’d really like to pick plants that at least stand chance of making it before I kill them.