In episode 1 we see Holston place a vase of flowers in front of the mirror in his office apartment, close the vent (where we later learn he left George’s case file for Juliette), then walk to his office, sit down in front of his computer and write the note to "double the flowers".

So he places the flowers while in the midst of hiding a file for Jules, then leaves her a note about them in a different location.

I’ve seen theories there’s a hidden camera in the mirror which the flowers are intended to block, but there are issues with this: firstly, he just placed the flowers, so he didn’t previously think they were needed; and secondly, if there were a need for more flowers, he would have added them himself instead of leaving a note to Juliette do so. I don’t think the note refers to the actual flowers at all—he just left the flowers to conceal the meaning of the note, to make the note look like an innocuous reminder to decorate.

My theory is that the note is really a clue to give Juliette the passphrase to access the files recovered by George and Allison: somewhere else she’ll find an incomplete passphrase that includes the words “flowers” and “mirror”, but to get the full passphrase she’ll need to add the word “flowers” a second time.