When translating or making notes, I always look for primary sources, so my main source for the meaning of clematis was The Language of Flowers by Kate Greenway, a very popular artist at the time, published in 1884, 10 years before Bird's trip. http://gutenberg.org/ebooks/31591
It's well-known and simple enough to assume this was the basic knowledge people of the era had regarding flowers. I'll admit though that it's possible there was a different book which listed artifice for clematis that has fallen into obscurity in the modern era. Most websites don't list their sources for flowe language meanings, so I can't be sure of the age/accuracy of the sites.
The underlying issue is that I can't be sure of what sources Sasa-sensei is using, or if he can even read English well enough to understand primary sources. So there's a very fine line here between faithful to the Japanese and faithful to the source material. When there's enough of a disconnect, I tend to point it out in my notes. (I'm still not sure what the hell was up with that egg wine in chapter 18)