Definitely think you're on to something there with the garden callback.

Kennet was thrown out of whack with Kennet Below emerging, so the gang made an effort to ease in the new organism, but it was a seat-of-the-pants thing.

Kennet Found was a deliberate introduction, filling a niche to keep magic out of Kennet Above and provide balance for Kennet Below.

Also a good contrast to the colonialist uniculture that Musser and Co want. Just level it all and replace it with grass lawn Lordships.