Making Plantain Oil Choose a dry, sunny day and harvest the plantain in the afternoon (once the dew has dried). Tightly pack a clean, dry jar full of plantain leaves. Cover with olive oil to the top. Place the jar out of direct sunlight and let it sit at room temperature for six weeks. Every day for the first week, top off the oil so that it completely covers the leaves. After six weeks, strain out the plant material. You now have your own green, medicinal plantain oil!