We already know that Lawrence has a thing for the natural world (check out "Natural Imagery"), and here's another possible reason: the Wragby woods are a big fat allegory for the Garden of Eden. Let's take a look at the evidence:
Lawrence emphasizes that Mellors doesn't see Connie as an individual person but more as a representative of all women: "She was only really a female to him" (10.125). Or this one, when they're in bed together: "the woman gleaming and rapid, beating down the male" (10.347). Or this: "A helpless silence fell between the man and the woman" (12.15). See a pattern? Adam and Eve are traditionally representatives of "men" and "women" respectively, and this sounds like Lawrence is drawing on that tradition.
And it's not just flowers—it's a whole celebration of flora:
He had brought columbines and campions, and new-mown hay, and oak-tufts and honeysuckle in small bud. He fastened fluffy young oak-sprays round her breasts, sticking in tufts of bluebells and campion: and in her navel he poised a pink campion flower, and in her maiden-hair were forget-me-nots and woodruff. 'That's you in all your glory!' he said. 'Lady Jane, at her wedding with John Thomas.' And he stuck flowers in the hair of his own body, and wound a bit of creeping-jenny round his penis, and stuck a single bell of a hyacinth in his navel. She watched him with amusement, his odd intentness. And she pushed a campion flower in his moustache, where it stuck, dangling under his nose. (15.145-57)
Naked except for the natural bounty of the woods, Mellors and Connie have non-shameful, non-sinful, truly awesome, world-restoring sex. Still not convinced that the flowers matter? Check this out: in Milton's Paradise Lost, which is all about Adam and Eve in the garden, Adam weaves Eve a garland right before she comes back to give him the forbidden fruit.
As Connie walks in the woods one day, she sees bracken "lifting its brown curled heads, like legions of young snakes with a new secret to whisper to Eve" (13.80). That is, to whisper to her, Connie. And later in the rainstorm, Connie thinks that it's "like being in a little ark in the Flood" (15.50). And not too much later, Mellors suggests that "All the modern lot get their real kick out of killing the old human feeling out of man, making mincemeat of the old Adam and the old Eve" (15.60).
These passages drive home Lawrence's (possible) point: Connie and Mellors are like a new Adam and Eve, charting a voyage into an unknown world. Only rather than being cast out of the garden, it's almost as though they've found their way back to the garden—at least temporarily.