A version of this story appears in the February 2017 issue of National Geographic magazine. Citrus, in many ways, stands alone. So many cultivated species have come from so few primary ancestors.

That scar is an artifact of a process called grafting, or fusing two separate pieces of wood into a new tree. From citrus to peaches, almost all fruit trees are grafted, and so are pecans.

A gardener took to the r/Citrus subreddit to get some advice on how to revive their dying lemon tree. Being new to citrus gardening, they consulted experts to find a way to ensure the little tree ...