I need to know if a plant is harvestable (even plants from other people's mods). Is there a way to do that? I know plants store their growth in their metadata, but how to know if the plant has reached its harvestable stage?

Unfortunately, there is no global meta flag that indicates that a crop has reached maturity, though the vast majority of crops will extend the vanilla BlockCrops class and thus tend to use meta 7 as the max growth stage.

 

I don't have access to the code right now, but if you check the methods in BlockCrops and any interfaces it may implement (e.g. IPlantable or whatever), I seem to recall a method isFullyMature or some such, but it's been a long time so I could be totally wrong. Anyway, if there is such a method, you can check if a Block is an instanceof said class, then cast and call the method.

boolean isMature = false;
if (block instanceof IPlantable) {
  isMature = ((IPlantable)block).isFullyMature(world, x, y, z);
}

That would be more reliable than using metadata directly, but again, I don't have the code in front of me so I'm just making shit up.

There's something similar to that, but I think it's regarding bone meal only (if bone meal can be used). For some stuff like most vanilla crops it works, but there's no way to be sure how ppl will implement that on their mods. The IGrowable interface should have the method you mentioned.

 

I had to do an ugly hack to achieve what I wanted.

I worked on this for my "SmartHarvest" enchantment for tools (including my enchantable ruby hoe):

 

instanceof BlockCrops will be mature at meta=7.

instanceof BlockNetherWart will be mature at meta=3.

 

Can't count on alien mods to use those though. The best we can do is to give them a chance to be rational. If they're not, then that's (usually) their problem. Don't worry about it until somebody gives you details in a comment on your uploaded mod.