"A Garden for Cutting" by Margaret Parke states: "Certain plants require an extra step. The cut stems of poppies, hollyhocks, hibiscus, and oleander, for instance, ooze a milky or yellowish latex. If the ends of the stems are untreated, this sap will coagulate and clog the stem's water-conducting tubes. To prevent this, make a few slits in the stem end and sear it for about half a minute over a candle or gas flame, or dip it into a few inches of boiling water for about a minute. Take care to apply heat only briefly lest too many of the water conducting cells be damaged in the process. Finish conditioning in a deep, water-filled container."
Cut hollyhocks when the bottom third of the florets have opened. Once the stem is cut, more florets will open; the entire stem should last ten days."