Preserve in seal oil right after you pick them. Sura keeps very well, even all year if stored very cold or frozen. It can be thawed and frozen many times. Eat whenever you have seal oil with meat or fish. The surface of the leaves must be dry before they go into seal oil.
Oil can only handle a certain amount of moisture.
A 20-gallon barrel of oil will easily handle two quarts of fresh leaves. If you let those leaves dry for a day first, the same amount of oil will handle more leaves. If you put too many leaves into oil their moisture will sink and ferment. They can also be dried and used as tea or in soups. Sura can be canned.