I think I've found the perfect tool—or crutch—for the times when I need to get my nose out of my books, or my hands out of the dirt, and actually, like, talk with other gardeners about plants. I've come upon this book: The New Garden Encyclopedia (—or maybe I should say The "New" Garden Encyclopedia, since it's from 1941), and plan on using it as my reference for (and arbiter of disputes regarding) botanical name pronunciation. The Encyclopedia is a plumped-up dictionary, with concise entries alphabetically ordered. My copy has a mildew-mottled spine (gah, I am such a sucker for these old books), and sits nicely in the hand with its compact (6" x 9") page size. There's a lot in its 1139 pages, and it's ideal for surfing: if you open the book to any page at random, you'll find something interesting, which leads you to something else that's interesting, and so on, and so on (Mom...Mom! Supper's burning!).
Each plant name is accompanied by a pronunciation guide. Test drive: Hydrangea (hy-dran´-je-ah). (Good: "dran", not "drain".) Hellebore (hel´-ee-bor). (Hmm, don't think I've ever said that with a long e before.) Clethra (klee´-thrah). (Oh, I get it—like "ur-e-th—". Ahem. Moving right along...) Narcissus (nahr-sis´-us). Whew...I won't have to use that affected hard "c". And let's not forget clematis: Encyclopedia says klem´-a-tis. Well, that settles it. I'll similarly defer to the Encyclopedia on "-ceae" suffixed family names: see-ee it is. I'll even buckle under and accept ber-jee´-ni-ah for Bergenia, named after a fellow called Bergen (as in ber-gen). But thank goodness for the sensible pronunciation of Weigela (wy-gee´-lah). That one had puzzled me for a while, with lots of folks out there saying "Wy-gee´-lee-ah) [but it's not spelled Weigelia! or Weigelea!] or even "Wy´-zhu-lah" [just before zhuzhing one's sleeves, I suppose].
The Encyclopedia covers genus names extensively, but not species names, so here's a site I'm using as a supplement: Landscape Plants of the Upper Midwest, produced by our very own UW, where you can hear the plant name pronounced at the press of a "play" button. This was helpful for Cotinus coggygria (cog-gy´-gree-ah), and will save me from having to call sweet autumn clematis "Clematis maximumblemumble" (Clematis maximowicziana...kindly disregard the cle-mat´-is).
(Of course, no criticism is intended, whatsoever, for folks who say any of these names in a different way... my goal being only to gain the confidence to articulate botanical names as elegantly as Mr. Giles.)
The New Garden Encyclopedia: A Complete, Practical and Convenient Guide to Every Detail of Gardening, Illustrated with 250 Halftones and More Than 500 Line Drawings Made Expressly for This Work. "Written by a group of horticultural experts [including the famed rhododendron expert and hybridizer Clement G. Bowers, and our old friend G.A. Stevens] and edited by E. L. D. Seymour, B.S.A." Wm. H. Wise & Co., 1941. Out of print, but this Bookfinder.com listing shows many copies available through various sellers.