In English, we're making a compound adjective so it would be very-old-growth tree.

It's one step short of the German compound noun, and we make it easier to find the fragments...

English is the only language I know of that allows spaces in compound words at all. It's a very peculiar feature of English orthography.

Mandarin written in pinyin comes to mind as another example. Do you discount that because pinyin is not the primary writing system used for that language?

Nah I just didn't know anything about Mandarin really

This sort of thing comes up often for me. I use extended hyphenation to declare precedence: very-old--growth tree.

ancient-growth tree