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I was recently pondering on how landscapes of cities like San Diego or Phoenix would look like without water supplementations. I was surprised to see quite a bit of trees growing in Phoenix, and thought that most of them must be artificially watered by humans to survive. Is there something equivalent to plant hardiness scale/maps but for water? I guess it would have to take into consideration not only the total precipitation but also distribution, temperature and soil properties.

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  • drop xeriscaping plant selection perhaps with the specific area you are interested in applying this to, as well, into your search engine of choice.
    – Ecnerwal
    Nov 23, 2022 at 15:30

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Drought tolerance does not equal heat tolerance, so a "water hardiness" scale isn't really possible - the two must go together (and also with cold hardiness) when determining what to plant in a particular area. Soil also makes a difference, of course, because sandy soils increase water stress on a plant.

Your best bet on determining a plant's "water hardiness", as you put it, is where it lives in the wild, coupled with its habitat within that area. As an example, a plant native to the Sonoran Desert is likely to be hardy in Phoenix, but there are plants in that desert that live only near rivers. They're able to take the heat but may not be particularly drought tolerant. Some plants native to the northern US Plains states have extremely deep roots (Liatris sp. for example) and so are drought tolerant but are not terribly heat tolerant.

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