Ferns create a really good natural looking landscape and make a sharp contrast between areas that you've cultivated and the wild fields around them. They're great for this. They're a bit oppressive in a planter, but could look good in a larger build, I suppose.
However, there needs to be some way to process them. Rush flooring, bedding, fertilizer via barrels, animal food, anything. Green dye? Farmyard flooring?
The reason for this is that you just accrue them as you clear land and there's nothing to do except have a big box of them and/or use them as an extremely inefficient campfire fuel.
