It's lady's bedstraw, Galium verum. It's a dual duty herb, used as a natural flea repellent and as a dye plant.
It has tall beautiful, honey scented flowers - 24-30" tall depending on how happy it is. The roots form a dense creeping mat and the leaves are a spiky version of typical galium leaves.
It was dried and stuffed into mattresses traditionally, hence the common name - Lady's Bedstraw.
This is one natural dye we never tried to use because it's a bit seasonal. The roots have to be dug up at just the right time for the best color. Then they have to be chopped finely. Here's a great write up from someone who has tried it.