I have no intention of growing these seed, I bought the pack to see what warnings if any were on it about how invasive this particular plant is, there are none.
I visited the Thompson and Morgan web site and did a search for Comfrey under more information there is a warning that Comfrey is harmful if eaten and that contact with the foliage may irritate the skin and eyes, with a suggestion to use gloves when harvesting. But there is no warning on how invasive this plant can be.
I then thought I would see how many other seed suppliers, provide comfrey seeds and if they actually warn the purchaser about it's invasive nature. Out of all the seed suppliers listed on the left panel of my blog only D.T.Brown, Just Seeds, Premier Seeds Direct, Nicky's Nursery and Seedaholic.com supply comfrey seeds.
Of those companies Just Seeds are selling packs from Thompson and Morgan none of them give a warning about how invasive Comfrey is apart from Seedaholic.com that state under the cultivation heading "Take root cuttings (about 2 inches long) and away you go again. Be careful as the bits left over will happily root wherever they fall". However they completely fail to mention that the plant grown from seed is not sterile and if allowed to flower and go to seed, will spread like wild fire.
The plot next to mine has comfrey all over it, inside the greenhouse and the polytunnel, and is not Bocking 14, the current tenant had no idea what Comfrey was they thought it was just an annoying weed they could not get rid off, until I educated them, what it could be used for and the need to keep it cut and not let it flower and go to seed and spread anymore, as what they have on their plot is quite obviously not sterile.
I feel that providers of comfrey seed have a moral obligation to warn their customers about just how invasive this plant can be. I find it completely irresponsible of any company selling Comfrey seeds not to explain how invasive this plant can be, if grown from seed and not properly and carefully managed.
I salute all the seed companies that don't sell comfrey seeds.
So Bottom line, whatever you do DON'T GROW COMFREY FROM A PACK OF SEEDS
So what is the alternative? "Bocking 14" is a cultivar of Russian Comfrey, this strain was developed during the 1950s by Lawrence D Hills, and is the preferred type as it is sterile and can only be grown from root cuttings so it's not going to take over your plot from seeds spreading.
Sources for Bocking 14 root cuttings in the UK are
Sources for Bocking 14 root cuttings in the USA