Unless someone like you cares a whole awful lot, nothing is going to get better. It's not. - Dr. Seuss
This is a strange virtue for me to start with, since I'm unable to have children with my partner and live in an apartment too small for gardening, but the alphabet demands an "f" word the ADF virtues include and I will oblige.
I started, like any good nerd, with my textbook: the ADF dedicant handbook defines fertility as: "Bounty of mind, body and spirit, involving creativity, production of objects, food, works of art, etc., an appreciation of the physical, sensual, nurturing." While I expect the inclusion of, say, gardening, baking and animal husbandry, I wasn't necessarily expecting the inclusion of creativity.
The more I think about it, though, the more it makes sense. Ideas are a lot like children - you plant the seed, you let it germinate, you give it form and if you're lucky, it eventually goes onto a life of its own, not dependent of you. (This is the Inception of metaphors, isn't it? A metaphor within a metaphor...) I wrote a book and published it on Amazon and Smashwords. As creative acts go, that sounds pretty low-key. But that book exists independently of me. If Amazon outlives me, that book may go on being available totally independently of my existance.
I think there's also an element here of caring for the next generation, even if my girlfriend and I aren't able to produce a child. The recommended reading for this virtue is The Lorax, which obviously isn't about childrearing - it's about everyone doing what they can to make things better for the people who come after us. Mother Earth can take care of herself, but if we don't do something to help her out, her version of "fertility" may no longer include "hospitable to humans." Therefore, I believe this virtue also encompasses concepts like food justice, making sure that people have access to the things we as druids think are important.
Planting a garden for myself is a fun and spiritual exercise. Planting a garden for my community teaches others why fertility is important and relevant to them, too.
(Also, any religion that sends me to Dr. Seuss for my philosophy has to be doing something right.)