Q: What is this ornery weed trying to take over my garden. When it started last year, I first thought it was my ‘Black and Blue’ sage but soon discovered it was not. It has a square stem and smooth leaf edges. No discernible fragrance on a hot afternoon.
A: It’s Japanese chaff flower, Achyranthes japonica. It is a perennial plant so thorough and persistent control is necessary. As you have found, it is very invasive. Pull seedlings by hand and spray older plants with glyphosate (click for sources).