Although Melonni obviously knows the fern is poisonous, she knows Carls never learn anything that they don't experience for themselves
Depicted above is the root cause of the problem. When presented with a glowing bush (or indeed, most anything that glows), the natural Carl reaction is "I could make booze out of that!" Consequently, Ghostly Wombat ignored Melonni's evident disdain for the fern and harvested a sample for the Carl distillery. It is a widely-believed fact that Tankarific will drink most anything that makes a sloshing sound when jostled, and she logically concluded that if said substance glowed whilst providing the drunken goodness that he loved so much.
Ignoring Melonni's chuckling in the background, Ghostly enlisted Gwen's help in gathering and transporting the herb back to the kitchen. However, she was soon sidetracked by yet another fascinating bit of herbiage:
Despite several setbacks (including a couple small-scale explosions that had Healing Carl beating her senseless for damaging her precious wyvern painting), refreshments were produced, including a delightful salad that combined the products of both bushes. The dinner a success, Ghostly reveled in the guild's compliments. Until, that is, half an hour later, when several Carls begain experiencing minor digestive problems.