How rigid are USDA plant hardiness zone designations?

General Fruit Growing
1

I want to use Krymsk 1 Plum rootstock in zone 10, Orange County, CA. That rootstock sounds perfect, except that it’s recommended for zones 1-8. Has anyone here tried it in my area, and what were the results?

Thanks!

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I notice that no one has responded. I don’t have a definitive answer, but I would note that USDA zones are based on the lowest average winter temperature in an area. I’m pretty sure (!?!) that the USDA Zone is irrelevant for a cold hardy variety such as Krymsk 1 Plum in Z10.

On the other hand, I read that Krymsk 1 is susceptible to bacterial canker and exhibits some incompatibility with other stone fruit.

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I also never really understood why some plants have an upper limit on the hardiness zone- there is likely a correlation with chill hours and maximum temperature, but the hardiness zones don’t measure that directly

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Exactly. I thought of mentioning chill hours but I thought that might pertain more to the scion variety than the rootstock.

5

Hi Betty,
I would take the USDA hardiness zones as a very general reference. It’s true that they represent a very long term picture, but I’m finding that things are changing fast. I keep a weather station (and redundant thermometers to compare) and last winter I was in zone 7, despite being in zone 5 according to USDA. Until a year ago their map put me in zone 4! But the change is headed in one direction – warmer. I do use Krymsk here, and so far all seems fine, but you have a much warmer place.
Another system for measuring growing conditions is the Koppen classification. It is more detailed, but I’m not sure how you could go about comparing since few people in the US use it. But if you want to dig into it here is a link. Köppen climate classification - Wikipedia

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Zones are rigid as they are based on 30 years of actual temperature measurements: Average of the annual lowest temperature.

It may be that assigning an upper zone of 8 for the plum means that the winters will not be cold enough (in chill hours) to promote normal fruiting. It is an odd pairing of statistics.

^discussion here