The custom of adorning the church with trees, branches, flowers, and grass on Pentecost (Trinity Sunday) goes back to ancient times. The Old Testament Pentecost was the feast of the first harvest (Ex. 23:16). People brought the first fruits of their harvest and flowers into the court of the Temple. In New Testament times, the trees and other plants in the church symbolize the renewal of people through the power of the Holy Spirit which descended in the form of fiery tongues upon all of them.
Additionally, it is the custom of the Orthodox Church for the faithful to bring bouquets of flowers which they hold in their hands, especially in the Divine Liturgy.