Because the flower remains hidden, it was originally believed that the tree did not flower at all or extremely rarely. Thus… a rare udumbara flower, blooming only "once every three thousands years", was interpreted as heralding the advent of a cakravartin (ideal universal ruler) or a buddha.
A reference might be used (as in the Tale of Genji, a monument of Japanese literature) to praise a ruler, e.g. saying « At last I have seen the udumbara flower… »
Post Scriptum: if you wish to read the Tale of Genji in English, I strongly recommend Royall Tyler's version (www.amazon.com/gp/product/014243714X/ref=as_li_tl?ie=UTF8&camp=1789&creative=390957&creativeASIN=014243714X&linkCode=as2&tag=koanmu03-20&linkId=5PU6XFT6MTNOT64E) over Edward Seidensticker's simpler rendering.
In French, René Sieffert's version is similar to Royall Tyler's, with magnificent illustrations (www.amazon.com/gp/product/2903656460/ref=as_li_tl?ie=UTF8&camp=1789&creative=390957&creativeASIN=2903656460&linkCode=as2&tag=koanmu03-20&linkId=WWDXLY3U6HUVKJOF).
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Udumbara (Buddhism) - Wikipedia, the free encyclopedia