"The Rhodora" is a perfect example of transcendental poetry because it connects the real with the ideal and because it is self-reflective, a meditation on the act of poetic composition.
Transcendentalism is German in origin, and it was the German philosopher Friedrich Schlegel who provided the best-known definition of transcendental poetry. Schlegel said in his Dialogue on Poetry that this school of poetry connects the ideal with the real.
In "The Rhodora," the speaker describes the pure and perfect beauty of a flower hidden deep within the woods, a beauty which "is its own excuse for Being." This phrase can be read in at least two ways, signifying that beauty is a sufficient reason for the flower's existence or making the larger claim that its presence gives meaning to existence in general. In both cases, it connects real existence to ideal beauty.
"The Rhodora" also tells the story of how it came to be composed and is therefore a meditation on how poetry is written. This also conforms to Schlegel's requirement that transcendental poetry "portray itself with each of its portrayals ... it should be poetry and the poetry of poetry."