Are you claiming that your buddleia drownded?
I've seen them growing in standing water.... more water... usually means more seedlings.
As much of water-pigs as these things are.... very surprised if they died from too much rain...
nematodes seems far more likely.
I would suggest trying vitex instead.
Vitex is very pretty, and drought hardy, and I haven't seen any problems with nematode issues either.
As to drowning?
I can't say.
