mushrooms
菇 centers on mushrooms as a type of fungus or plant.
菇 combines the grass/plant radical 艹 with 姑, which likely provides the sound. The character is used for mushrooms, a type of plant-like growth.
The grass radical 艹 sits above 姑, like a mushroom growing out of the ground. Think of a mushroom as a little plant umbrella.
For コ, imagine a small mushroom as a tiny 'co'-conut shell: co -> コ, and the mushroom cap looks like a coconut half.
donko (shiitake variety)