Flower:

Many-flowered branching clusters arising from leaf axils in the upper plant. Flowers are about 1/8 inch across, yellow with 4 petals, occasionally 3 or 5, longer than wide and pointed or blunt at the tip, and 4 yellow stamens.

Leaves and stems:

Leaves are whorled in groups of 8 to 12, very narrow, ½ to 1½ inches long, up to 1/8 inch wide, variously covered in short hairs, sharply pointed at the tip. Leaf edges may be slightly rolled under and the upper surface is typically glossy. Stems are 4-sided with rounded angles, covered in very short hairs to varying degrees, rarely hairless. The lower plant is rather leafy, the upper can be many-branched.

Notes:

The only yellow bestraw in Minnesota, this may be an up-and-coming invasive weed. It's been described as “adventive”, which means it is not yet completely naturalized, but the colony we came across on a county highway looked well established to us, creating quite the mono-culture, spreading via rhizomes as well as seed. That particular colony was first documented in 1998 and shows no sign of diminishing. Yellow Bedstraw is present all across the northern half of the US and southern Canada. We shall see how this species fares, but it might be best to eradicate it before it does have a chance to become invasive.