OK...
First of all, I was wrong about the iris count being correct (once there are too many items, I can lose place when I am counting).
After a few recounts, I verified that I had 20 iris, but the total showed as 21 in the iris category on the left side list.
I removed one iris, so I now have 19. (I counted the irises again to verify this.) However, the left side list was unchanged and STILL lists 21 irises in the iris category.
Somewhere in the code, the category total is not being decremented when items are deleted from the overall plant list. (And yes, the iris that I deleted had been tagged as being in the iris category.)
