I've only just learned to do this as I'm in no way a lepidoptera specialist.
The Knotgrass plant is in the same family as the sorrels - the dock family - and like Common Dock Knotgrass plant has a strong root. It grows as a weed among stones often in gardens and seems as if it has a root as strong as string tied fast to something underground. I suppose that's where its name comes from. I'll try to get a pic of a plant and post it. Its wonderful scientific name is Polygonum aviculare.