Update 2022:
User whatamidoing has created a more advanced version of this, available here: https://steamcommunity.com/sharedfiles/filedetails/?id=2818728233
There should be no reason to use this mod now, unless you want to grow potatoes in sand (an unintended but acceptable side effect).
Summary:
Allows you to add growing zones to less fertile terrain. Side effect, you can grow potatoes in sand.
Why?
I was playing a desert tribe, and noticed that Saguaro cacti could grow in sand, but I couldn't create a growing zone for sand.
How it works:
The designator tool for Growing Zones checks fertility of the tiles you click to see if you could grow a potato there. Not sure why (probably because potato is the default option).
Rather than figure out how to use Harmony transpilers to patch that method, I added an xPath patcher to set the potato's fertility requirement to 0.01. Much easier, and I don't even mind the side effect.
I did try setting it to 0.00 for the full "anywhere" in the title, but growing things in rock (or concrete) didn't seem right. Even mushrooms don't grow out of rock tiles.
Compatibility:
Should only depend on Core, you don't need Royalty or any mods.
Created in version 1.2 but would work in any version where they haven't changed the location of the potato def or how the XPath PatchOperations work.