I had a beautiful flower garden. Last year I noticed thistles in the garden. They seemed to have come from no where. This year is a full invasion of thistles crowding out all my flower plants, many lilies, etc. How do I get rid of the thistles and restore my flowers,? The thistle and strange looking plants appeared after a tree had fallen next to the garden and the city came and took it away, digging down deep to remove it..