Our national flower...

... has been the rose since 1986, when Reagan signed it into law... I did not know that! There had been a movement– championed by seed mogul David Burpee and Senator Everett Dirksen– to name the marigold, but obviously that failed. In 1962 Katharine S. White made a good case against both the rose and the marigold, and she had a much better nominee, although it ultimately fared no better than the marigold against the rose:

Feeling as I do about the marigold as the flower of the United States, I've decided to start a flower lobby of my own. I thought of nominating the moccasin flower (or lady's-slipper), our handsomest native orchid—it would be a nice tribute to our earliest inhabitants—but the moccasin flower does not grow in all our states. May I therefore suggest the humble but glorious goldenrod, which grows wild in every state of the Union[*] and spreads its cheerful gold in August and September from Maine to the Pacific, from Canada to Florida and New Mexico? It is as sturdy and as various as our population; there is delicate dwarf goldenrod, silver goldenrod, tall yellow goldenrod in a multitude of forms and shapes—spikes, plumes, and panicles of native gold. Hortus Second lists fifty-five species, only one of which is not native to the United States, and one or another of these fifty-four grows in every sort of soil. Descend into a bog and there, growing wild, is goldenrod; climb a mountain and there, between the crevices of boulders, is goldenrod; follow the shore of the sea and goldenrod gleams along the edge of the sands; drive along our highways from coast to coast in August and September and the fields and ditches are bright with goldenrod, unless the state you are driving through has destroyed them with chemical sprays. The very ubiquity of the flower has given it a bad name as an irritant to hay-fever victims, but I've recently read that it is the ragweed and flowering grasses growing alongside goldenrod that are the villains during the late-summer hay-fever season. The goldenrod also has the great advantage—if it were to be our national flower– of owing nothing to man, of enriching no seed company, or companies, and of being as wild as our national bird, the eagle. Canaries, like marigolds, presumably thrive in all fifty states, yet no one would dream of nominating the canary as the national bird. 

 – Katharine S. White, Onward and Upward in the Garden

The goldenrod I planted in my yard some years ago is just getting started on its graceful, cheery, and abundant annual bloom. I look forward to it every year.

* Perhaps with the exception of Hawaii, methinks, where it is an introduced non-native.