Hi Gavin, I know I have them because all of my crops from all of my beds did horrible growing really slow if at all. When i pulled everything out, all the roots had huge lumpy knots on them. So, for the 1st part of the summer I stopped waterimg the beds, letting them dry out slightly, then I densely planted marigolds (from seed) completley covering the beds. I let them go to flower and then seed so i could collect them. In the beginning of this year, I chopped the Marigold plants up and mixed them into the top layer of soil. I then put down a fresh 3" layer of soil & compost/manure. And started to plant in that.I have marigolds in my bed at the moment. How do you know that you have an issue with nematodes?