The rose used to be on Bougainville's grave, but I asked Odile Masquelier to check when she mentioned it in an article, and it's no longer there. From the Journal of Heritage Roses in Australia summer 2011, 33.4, pp 26-30 (you can find it via Trove), reprinted in translation from Le Journal des Roses Anciennes en France, no. 16, Autumn 2010.
"Push hard, because it has rained",  kindly observed the keeper, whom I knew and who had handed me the key of St. Peter’s cemetery in Montmartre. In this green and quiet haven, whose entrance is closed to the public, a few gravestones are still looked after and, on one of them, the inscription reads: “To the Memory of Louis-Antoine, Count de Bougainville, 1729 – 1811”. On each corner of that grave I reverently planted a rose of the ‘Bougainville’ variety¹ created in 1823 by my grandmother, daughter-in-law of Scipion Cochet (the famous ‘Maman Cochet’)".
The person thus speaking of the Admiral is none other than P Charles Cochet, nephew of the founder of the ‘Journal des Roses’, Scipion Cochet.