Plant Hardiness Zones



By far the most popular way of measuring plant hardiness is the USDA Plant Hardiness Zone Map. This of course applies to the United States only, but maps that are based on similar data exist for other parts of the world. There are yet other scales and systems that attempt to paint a more optimal picture of a plant’s likelihood of thriving such as the AHS’s Heat Zones and Sunset’s eponymous Sunset Zones. For the purposes of this article, we will focus only on the USDA system, since that is the most popular and most likely to be used by nurseries and plant information web sites.

Plant labels

As with plant names, hardiness zones are also, for a lack of a better term, “all over the map.” Web sites that provide plant growing tips are also equally, if not more, unreliable. For example, the Missouri Botanical Garden lists Alpinia Zerumbet (Shell Ginger) as a Zone 8 plant, but I have yet to see it in Portland area (Zone 8b) nurseries or landscapes. In fact, I have only ever seen it sold here as a house plant, sometimes mislabelled as Curcuma ZedoariaSo who is right? Should I plant them in my yard and see what happens? Wouldn’t it be nice if there was a way to quickly see where others in my area have done similar experiments? What is the shell ginger’s true hardiness zone and how can we show it?

Outliers and microclimates

I have seen beautiful cordylines and yuccas grown as far north as Seattle on the West Coast. Yes, I know Seattle’s Puget Sound is a relatively warm microclimate (9a), as is most of the western coastline. Parts of inner Portland have also been touted as flirting with 9a plants.

So what’s missing?

The USDA hardiness zone map is a good enough indicator for the average gardener, but for those who are interested in pushing the envelope further, to gain a true understanding of a plant’s ability to thrive, we need better and more precise tools. We need to show evidence of successful cultivation or the lack thereof, and improve the reliability of these zone maps. A more accurate system has the potential to change the way plants are sold and cultivated in the industry. Scientists can also benefit from an improved system, where they can better track plant distributions and habitats. From the everyday gardener to the plant fanatic, we can all benefit from an authoritative and collaborative plant classification system that is evidence based, backed up by hard data. Improved visualization techniques coupled with an expert knowledge base, these ideas and their benefits are not too far off.

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