Green’s Dictionary of Slang

1. (UK Und.) members of the aristocracy [‘the flowers of society’].

1821		Egan Life in London (1869) xiii: [explanation of the frontispiece] The King on his throne, giving an audience to his subjects, and which, for derivation, the ‘universities must admit is “the top of the tree!” (The roses, pinks, and tulips: the flowers of society) [from the explication of the frontispiece].’.
1835		G. Kent Modern Flash Dict. 28: Roses – nobility.
1848		Flash Dict. in Sinks of London Laid Open [as cit. 1835].

2. the menstrual period [pun on flowers n. (1)].

1922

Joyce Ulysses 76: Such a bad headache. Has her roses probably.

3. (drugs) amphetamine [play on red n. (12)].

1967		J.B. Williams Narcotics and Hallucinogens.
1969		R.R. Lingeman Drugs from A to Z (1970) 221: rose benzedrine tablets.
1980		‘Gloss. of Drug Terms’ National Instit. Drug Abuse.
2001		ONDCP Street Terms 18: Roses — Amphetamine.