Apparently, this is "true" comfrey, NOT "russican comfrey". In permaculture, this is often planted under fruit trees because it helps the fruit trees. Comfrey is sometimes called "knit bone" because of how effective it is for mending bones. Apparently the FDA is coming down on comfrey because it has dangerous alkaloids in it. It has about 1/100th the dangerous alkaloids that beer has. https://www.youtube.com/paulwheaton12#p/u/0/dogNx-z7Oig
The thing about the toxicity of internal comfrey use is that the quantity of dangerous alkaloids varies from plant to plant* and you have no way of knowing what the content is in the plant you are using (unless you get it tested). Pyrrolizidine alkaloids do cause damage in humans, and it's not obvious (they cause an occult damage of the liver over years that eventually cause the liver to fail, but you don't know the damage is occuring until the liver fails). There are very experienced herbalists who won't use it based on understanding the actual science. Of course there are also very experienced herbalists who will use it, based on understanding the actual science in the context of actual practice in the real world. This is the best explanation of the controversy I've come across - https://www.comfreycentral.com/ This is also good - https://www.herbcraft.org/hoffmanncomfrey.html *and probably from species to species, but it's unclear which species are the problematic ones. Aside to all that, I'd like to know why that farmer thinks growing comfrey from seed is a good idea. Is it going to become a problem, the self seeding? Does the seeding variety also produce rapidly from disturbed root, so that if you try and dig it out you end up with lots more plants?
I just cut my comfrey plants off about an inch below the ground and planted them where I wanted them to be..pretty much forever. As I was doing it I was praying that the variety I have was the sterile Russian variety, as the last thing I need is the seeds striking when I use the comfrey as fertilizer in the garden. The plants have a mass of flowers most of the year and if those seeds weren't sterile my place and everyone down stream would be drowning in comfrey.