I like to think that once Grace knew about Simon love for trees and nature he asked the eridians to add a small forest to the dome as a welcome gift. However, when they grew closer, he wanted to give Simon something that he could grew himself, and Grace personally gave him biogenetic modificated seeds and little tree sprout that can grow in Erid.
I also like to think that in Eden giving seeds, leaves, or anything related to a tree and nature to someone is a well know, and quite fancy, way to ask someone to marry you, since trees are not only something super weird, but also has a lot of religious meaning for those who grew in Eden. If you give someone something related to a tree and they return the favour, at the eyes of Eden culture, you got married.
So when Grace gave Simon those are seeds and little tree sprout, Simon broke in tears, for the emotion of having his own tree and been seen as spouse material, Grace's spouse. In a rush Simon took a leave of one of the trees of the forest and gave it to Grace, as a way to say that he accepts Grace purpose and hugs him with all he had.
In Eden kisses and anything carnal is seen as a waist of energy, resources and time, so a hug means a lot.
Simon, super emotional and very happy, drowning in his tears, hugging Grace as if he wants to squeeze the life out of it: I promise you that I will be worthy of this honour.
Grace, thinking "wow, he really loves the nature, good for him to have a pasion", trying his best to return the hug without losing his breath, nor squashing the tree sprout: I know, I know, you are already worthy, buddy.
And that is how from Simon perspective their are happily married, while Grace is thinking that having a human friend again in awesome.
