Following Boyd´s fascinating text on the "Inseparable Fates" chapter of his " Nabokov´s Ada" I came across once more a very complete and complex analysis about the mulberry-soap reference. 
Soon later I found " Lucete hides among the burnberry bushes" and has her shorts "stained with burnberry purple".
I remember Boyd explaining that a brook is a "burn" ( page 141) .
 
I have not been able to locate any "burnberry" amidst the botanical references I could acess. Are there indeed " burnberries" and "burnberry bushes" as real plants?
 
If not, would those plants be an indirect way of introducing the "here we go round the mulberry bush" theme? 
If it happens to be so ( burnberry as another way of writing about mulberry ) we would once again find those curious exchanges bt word sounds  in  Nabokov...
 
Could any botanist in the list help ? 
  Jansy