Thomas's imagery emphasizes the explosive nature of this power. In this line, Thomas introduces the creative force in nature. The rhyme scheme in this stanza is ababa. In the first four words of this line, the power that causes growth in nature is revealed as the same force that causes the speaker to grow. Like the flower, the speaker is still in the process of growing.
Green age implies youth since the word green has connotations of spring and renewal.
Throughout the poem, Thomas will combine many seemingly opposite words. After the caesura the pause or break in the rhythm the destructive power is unleashed. Grammatically, the phrase refers back to the force in the first line. However, now it is a destructive power, obliterating trees by their very roots. Thomas makes it clear that the fuse which blasted the flower into existence is also the blast which destroys it.