Plants from mods such as Biomes O' Plenty do not respond to bonemeal when it is used on them with this mod. No configuration options are provided to add these in either.

Minecraft 1.12.2, mod version 1.2.1

Minecraft doesn't have a generic way to make plants grow, so every plant I want to support needs custom code just for it. For modded plants, wouldn't it make more sense to add this functionality to the mod the plants are from instead of to this one? In particular, the license of Biomes O' Plenty doesn't allow derivative works, so I'm uneasy about adding tight integration with it.

I don't know how to add this functionality to the mod in question. How difficult would it be for me to add it? I am not a programmer. I can't get help from the modmaker in question for it though since he refuses to give support to questions about his mod for older versions of Minecraft.

First of all, you're not allowed to distribute derivative works of Biomes O' Plenty (since it's only "visible source" and not actually open source), so if you made a custom version of it, you'd only be allowed to use it yourself in single player. Anyway, it's not too hard if you know Java at all. Just find the subclass of Block for the kind of plant you want, make it implement the IGrowable interface, and then add the canGrow, canUseBonemeal, and grow methods to it.

By the way, if there's any other mods that you want this to work on that are actually open source, let me know which ones those are and I'll see what I can do.

I know nothing about Java or how to write in it. Guess I'm kind of screwed then.

You could always try asking the author of BoP to release the old versions under a license that does allow distributing derivative works.