[header] How to kill ferns [title] Use boiling water as a cheap, natural herbicide. [step] The simplest, most budget-friendly herbicide is hot water. Pour boiling water over a plant's root system and leaves to kill it within several days without leaving a poisonous residue in the soil. 
[substeps] You should need 1-2 gallons (3.8-7.6 l) of water to kill the fern, depending on its size. While you'll only need to pour the water once for most plants, you may need to repeat the process for larger or stubborn ferns.