Picking flowers is not a sin. And it’s even mentioned as one of the Bridegroom’s activities in the Song of Songs:
“My beloved has gone down to his garden, to the beds of balsam, to pasture his flock in the gardens and gather lilies. I am my beloved’s and my beloved is mine - he who pastures his flock among the lilies.”
(Songs 6:2-3)
The Bridegroom is God, of course.
Obviously it is not wrong to admire the beauty of flowers, and apparently lilies are a particular favorite of God, seeing as Our Lord (who is the Bridegroom) pointed their beauty out in both the Gospels of Luke and Matthew:
“Consider katanoesate] the lilies - how they grow. They do not labor or spin. But I say to you - not even Solomon in all his glory was clothed like one of these.”
(Luke 12:27)
“And why are you anxious about clothing? Study katamathete] the lilies of the field - how they grow. They do not labor or spin. But I say to you that not even Solomon in all his glory was clothed like one of these.”
(Matthew 6:28-29)
(The Luke passage is interesting, because it shows again that Jesus is concerned about carrion birds as well as the more gentle sorts of birds. Luke 12:24 is not about the generic “birds of the air,” or the more specific “sparrows,” but instead mentions “ravens.”
korakas]
“Consider the ravens. For they do not sow or reap. There is no storehouse or barn for them, and God feeds them. How much more precious you are than the birds!”
(Luke 12:24)
Of course, ravens eat other stuff besides carrion; they’re omnivorous birds and very clever. But since Jesus is also the one who pointed out that
"Wherever the body is, there the vultures will gather together," (Lk. 17:37, Mt. 24:27) I thought it was pretty funny for Him to talk about providing food for ravens. And indeed, the Psalms also talk specifically about God making sure that “bad” prey animals like baby lions get food, as well as the species that Israel’s farmers and herders would have liked. The OT Bible writers may not have known explicitly about ecology, but they did know that all of Creation was created and loved by God. And of course Jesus, being God, shows the same care and interest in ravens as in sparrows and lilies.)