Bumblebees love Russian comfrey. The plant is frequently recommended as a bumblebee magnet. I often find four or more different varieties of bumblebees in the comfrey along with many other pollinators who also like the plant.
My son Geoff introduced me to Russian comfrey (Symphytum × uplandicum) over 20 years ago when he brought home a few roots from the farm where he was working. It likes it in my garden and I have to work to keep it from spreading too far. Here is the largest stand, which I keep next to the compost area.
Comfrey can be harvested, totally cut down to use as mulch or a compost additive, up to four times a year. I’ll cut these plants down, soon. The flowers are already past their peak. I’ll give the bees a few more days at it, then I’ll put most of this stand into the paths as a weed mulch.