How do we know that this "show" of daffodils had a long-lasting impression on the poet in "I Wandered Lonely as a Cloud"? What do you think Wordsworth is suggesting about the power and beauty of nature in the last stanza?

The poem "I Wandered Lonely as a Cloud" by William Wordsworth describes a beautiful field of daffodils and the effect these daffodils have on the speaker. It is obvious by the language the speaker uses that the daffodils completely capture his mood and attention when he sees them. He describes the "golden daffodils" as "fluttering and dancing in the breeze" and says they were "tossing their heads in sprightly dance." The speaker considers the daffodils more beautiful than the lake bedside him, saying the daffodils "out-did the sparkling waves in glee."
The encounter the speaker has with the daffodils stays with him long after his walk by the lake. He speaks of the "wealth the show to me had brought," placing value on his experience of admiring the many flowers. Even after an unknown amount of time, the speaker still remembers the beauty of the daffodils on that day, and this memory brings him joy when he is feeling down. The speaker describes his experience this way:

For oft, when on my couch I lieIn vacant or in pensive mood,They flash upon that inward eyeWhich is the bliss of solitude;And then my heart with pleasure fills,And dances with the daffodils.

The mere thought of the beautiful daffodils dancing in the wind fills his heart with joy and improves his melancholy mood.
Within the last stanza, the speaker shows his appreciation for the beauty and power of nature that he was able to experience. This power of nature lies in his memory of the colorful dancing flowers. In his mind's eye, he is able to conjure up a picture of the carefree dancing daffodils, and this picture immediately begins to cure his pensive mood. His heart is then free to feel happiness and start a dance of its own, as he states, "And then my heart with pleasure fills, / And dances with the daffodils."