Isidore often illuminates the thinking behind medieval Latin imagery (especially to the 13thC). The first full English translation (published by Cambridge Uni Press in 2006) is now online as a pdf.
No help understanding the thinking behind the Voynich plants, though. 🙂

And the flowers kind of look like cups used for pouring water, or perhaps the leaves as upturned vessels..

But from there on it’s shaky. An aspect of John the Baptist is his rough coat, which may be reflected in the leaves, but that doesn’t explain much about their shape. And he usually has a staff, but not the one with a crook. And then what’s with the amount of flowers…

The cut flower stems may just be there because there was actually no space to draw them, so that’s another complication.