• Minecraft version: 1.12.2
• Mod version: growthcraft-4.2.1
• Minecraft Forge version: 14.23.5.2854

I changed the config files as follows :

Percentage that thistle will drop when harvesting tall grass. [range: 0 ~ 100, default: 2]

I:thistleDropChance=0

Add a new with the registry name of the item you want to add to the grassSeed drop table. [default: [growthcraft_rice:rice]]

S:add_grass_drops <
 >

These changes have no effect in-game, so I used Crafttweaker to prevent them from dropping. It seems to work for Rice but Thistle still drop from grass. I run the /ct seeds command to check and indeed everything has been removed from the list as expected so that shouldn't happen.

Thanks for the report ... we'll address this in 4.2.2