The readings for the Third Sunday of Advent celebrate what the Lord has already done, as an indication of the even greater things He will do in the future.
In the first reading, Isaiah writes: "As the earth brings forth its plants and a garden makes its growth spring up, so will the Lord God make justice and praise spring up before all nations."
This allusion to the lushness of the earth's vegetation is a metaphor for the fresh spiritual energy God will pour out upon His people. Both the freshness of the vegetation and the freshness of spiritual rebirth come from the Holy Spirit.