Is there any way you can add a config option to adjust the speed at which a tree completes a growth cycle? It seems very fast right now, for the purposes of my pack at least.

Does configuring the json based trees myself allow me to adjust their growth time?

Yes, it would. They have a growTimeMultiplier field. You'd simply extract the json files in the types.d folder to your config/bonsaitrees/types.d folder and overwrite/add their growTimeMultiplier field. There is a section about this in the readme.

That has some disadvantages though. If I change the original json in the jar yours still has priority and would overwrite my changes.

At this point I'd recommend waiting a bit until I get the above mentioned features in and put out a new release. Probably sooner (i.e. today) than later (at most Sunday).

Build is uploaded to curse, it takes a moment to get approved.
Enjoy!

Edit: aaaand it's approved.

There is no global modifier for the growtime (yet), but you can configure the json based trees individually.
I'll add that global modifier and a CraftTweaker method for non-json trees.

Would that work for you?

Awesome work! Already tested it out and it works splendidly! Nice job!