Sorry, Hilary, I'm afraid there isn't. However the main search facility can work as a very effective stand-in for an index. If you click on the HOME tab (upper left on the screen), enter the plant name - or significant part of that name - that you would have looked up in an index in the box beside the Search button on the right, then click on Search, you'll find all/any references to that plant right away.
I'm going to shift this topic to the section on How The Forum Works.