To unlock the Green Thumb trophy in The Sims 4, you will need to reach level 10 in the Gardening skill.
The quickest way to reach level 10 in the Gardening skill is to buy the three Gardening books which will bring you up to level 9, and then build some planters/bonsai trees in your garden and tend to them to gain the final rank. At level 8, you are able to regrow bonsai trees instantly for $50. Cut the trees and then instantly regrow them to gain the final level the quickest.
Once you reach level 10 in the Gardening skill, the trophy will unlock.