Your yams will be ready to harvest after the first frost blackens and kills their leaves. (Sweet potatoes just keep getting fatter until their "tops" die, so don't dig 'em up before they've finished growing. Use a shovel to bring in your potato crop, and remember that these roots extend straight down from the plant's crown. So keep that spade nearly vertical when you dig, or you'll end up cutting a lot of tubers in half!