Seeds are used as the currency of the app to begin poetry by choosing “Plant Tree”. The tree will remain forever for you to keep adding verses and once you’ve finished the poetry then you can "cut" the tree to be refunded a seed if there are no other poets in the tree. If you “leave” a tree that you planted then the 2nd Poet to join the tree will become the new Poet 1 with total control of the tree.
1 free seed is given to new users to either plant a tree or request to join another's tree. Seeds are also used to invite poets to your tree and if an invitation or request isn't accepted within 1 week then the seed is refunded. They can be purchased in the Seed Store with a bulk purchase discount and any actions that require seeds can only be performed when the device is online.
Seeds will last you a long time so hopefully you find many trees!