::Hey Michael, thanks for your work on this one. I'm not qualified enough in this area to give this a full GA review, but I can give you a few comments about this one after a quick glance over it. I think it looks good overall, but here are a few areas that a GA reviewer might ask for further work on. 
 ::*All quotations need an inline citation per criterion 2b (full criteria at WP:GA?), so the quotations in "Etymology" will need sourcing 
 ::*This may still need a bit of copyediting. For example, " but in 1795 William Curtis in his Botanical Magazine that magazine, described" seems to have extra words ("that magazine"), and it's not clear where the quotation that begins "I have named" ends. 
 ::*Judgments like "The double flowers from Japan are particularly beautiful" should probably be attributed to a particular author intext for neutrality reasons. (criterion 4) 
 ::*Use of numerous very short sections, as in "Propagation", is discouraged by WP:LAYOUT (criterion 1b) 
 ::*The lead should ideally provide a summary of the article's main aspects, touching on each section (WP:LEAD, criterion 1b) 
 ::*As a minor stylistic point unrelated to GA, there shouldn't be a space between a punctuation mark and an inline citation.  
 ::I don't mean any of this to sound negative, though; I really appreciate the work, and hope this becomes a GA very soon!