This section is from "The Horticulturist, And Journal Of Rural Art And Rural Taste", by P. Barry, A. J. Downing, J. Jay Smith, Peter B. Mead, F. W. Woodward, Henry T. Williams. Also available from Amazon: Horticulturist and Journal of Rural Art and Rural Taste.
The origin of this custom seems to date back as far as the 16th century, where it is mentioned by Newton in his "Herball to the Bible in 1587," as follows: "I will heere adde a common country custom that is used to be done with the rose. When pleasant and merry companions doe friendly meete together to make goode cheere, as soone as their feast or banket is ended, they give faithfull promise mutually one to another, that whatsoever hath been merrily spoken by any in that assembly, should be wrapped up in silence and not to be carried out of the doores. For the assurance and performance whereof, the tearme which they use is, that all things there saide must be taken as spoken under the Rose. Whereupon they use in their parlours and dining roomes to hang roses over their tables, to put the companie in memorie of secresie, and not rashly or indiscreetly to clatter and blab out what they heare. Likewise if they chaunce to shew any tricks, wanton, unshame-fast, immodest, or irreverent behaviour, either by word or deed, they protesting that all was spoken under the rose, do give a strait charge and pass a covenant of silence and secrecy with the hearers, that the same shall not be blowne abroad, nor tattled in the streets among any others,"