From experience, thistle will pop up through landscape fabric with no problem. A tiny missed piece of root will become a new plant -- or a new colony. A friend told me about an extension service experiment where they enclosed a one inch piece of root inside a 120 gallon black plastic tub of soil -- no light, no holes for water to get in (although it must not have been bone dry soil), and they figured the black plastic would heat up in the sun and kill anything inside. One year later, they opened the container, and it was filled with thistle roots. I applaud your dedication to using organic methods, but I had to resort to chemical warfare... and ordinary weed killer did nothing. Brush killer works but can take several applications. I have a weed torch that I use in some areas, and that has worked on thistle, but it often regrows a few times before giving up.
