Too late for nematodes. Find and squish works well, better then drowning which is quite ineffective as Arlyn stated. Sometimes they are obvious in the fan so just need to press the leaves together and say adieu. When you find any in a rhizome just run a wire in the hole and the thing won't survive. Then let the wound air dry in the sun if possible. I leave a rhizome like that out of the ground with just roots in the soil for the rest of the summer most times. Ignoring the buggers will allow things to get worse as they go into their adult stage and lay eggs.
