So, I live in a pretty warm Zone 7 climate (urban heat island just outside of Philadelphia, PA, USA) and from what I've read, it wouldn't be difficulty from a frost/heat perspective to meet the needs of a pistachio tree. The main issue is, of course, humidity. The areas where Pistachios are grown in California seem to reach a high humidity level of 40 or 50%, whereas in NJ humidity levels consistently get to the high 60s, low 70s year round.
After querying around the web a bit, I haven't found a clear easy answer for what exactly happens to pistachios (and, more generally, to arid-adapted trees and shrubs), when you attempt to plant them in a warmer climate. I assume they are fungal issues relating to moisture levels, but is the issue the moisture level of the soil, or ambient air, or both? Do fungi kill back the trees at an early age, do they die completely, or just enough to regrow but never mature? Or is the issue that the fruit succumbs to disease, and that you'd never actually be able to harvest something? More generally, are folks familiar with the plant biology of arid-adapted plants to know why it succeeds in dry areas and fails in more humid areas (e.g. something with the way cells are constructed or the ways they hold moisture)?
Moreover, is this a more difficult to change aspect of a plant's habit? Plant breeders are constantly at work developing varieties that can survive a zone or two lower temperatures, or resist this or that pest, or even respond differently to varying daylight lengths. Is humidity sensitivity that much different?
I know that's a ton of questions, but that just goes to show how boggled I am!
In areas of high humidity the real concern is air movement.
If you have enough air movement you will not find many issues caused by higher humidity than the plants are accustomed to dealing with.
Many areas might require the installation of fans to achieve the airflow needed to reduce moisture laden air issues like molds and mildews.
Temperatures are the biggest thing to deal with when trying to expand a plant's range on the planet.
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