Hiya Brian. I'm not an expert, but I always assumed that marigolds had to be alive and in the presence of parasitic nematodes to be of any use (well, besides being a trap crop for white fly, in my experience). I got curious after reading this thread, so decided to try to find some scientific info. I found this paper, which seems to have a lot of good info: http://scholarspace.manoa.hawaii.edu/bitstream/handle/10125/12417/PD-35.pdf?sequence=1
Quote from the paper:
"Nematicidal compounds apparently permeate from marigolds’ root tissues into nematodes attached to the root, but they are also believed to kill nematodes found in the rhizosphere, the soil near marigold roots. Thus, marigold is believed to be most effective in suppressing plant-parasitic nematodes when actively growing, but it is not as effective when incorporated as crop residues or root extracts."
So maybe what John said is right on the money- harvest the seeds and plant those marigolds elsewhere!