Sorry if my formatting is wrong, but there's a bug with the various upgraded watering cans. You can get instantaneous plant growth by auto-clicking whatever block you want to water through a block of water. When you do it in game it makes a lot of bucket filling noises, so I'm fairly sure it's because the watering can is filling up, watering, filling up, repeat.

I'm on version 4.1.8.
Steps to reproduce:

1. Craft a watering can inferium tier or above.
2. Position your cursor so it is looking at whatever block you want to water through a water source block.
3. Click as fast as you can, or turn on an auto-clicker or macro.