Is sterile comfrey a risk plant?

Over the years, I have talked warmly about the sterile comfrey's many benefits. It is enormously vigorous and seems to have the ability to kick-start the dynamics of farming systems through its nutrient-accumulating ability, why the i Forest Garden: growing edible everywhere ended up in the glorious list of accelerator plants. It is a favorite among bumblebees, can keep weeds in check and at the same time cannot spread by seeds. For a while we sold a few thousand root cuttings of the named varieties 'Bocking 4' and 'Bocking 14' per year to cover some of the seemingly never-ending demand for this smash plant. Recently, however, some question marks have started to appear about comfrey.

And I don't mean that it will be bigger than many people think. In fertile places it can be 150 cm high and almost as wide - nothing for the small garden. But I mean the fact that comfrey, just like many other species in the family Boraginaceae contains so-called pyrrolizidine alkaloids (PA). The term is a collective name for a group of highly toxic alkaloids that damage the liver both acutely and long-term (then often in the form of liver cancer with repeated intake of high doses of PA). Within the EU, there are limit values ​​for the content of PA in food and dietary supplements, and usually elevated levels can be derived from contamination from all kinds of weeds that end up in, for example, herbal tea, spices, honey or certain dietary supplements. Except for plants in the genus Symphytum which comfrey belongs to, among other things, stands (Jacobea vulgaris) and its relatives, gurkur (Borago officinalis), horseshoe (Tussilago grandpa) and various pea plants (Leguminaceae) which contain high levels of PA.

During the work on my books, I received several tips on making green smoothies on comfrey leaves, but it is therefore really not recommended. What is the problem then for those who do not want to eat their comfrey plants? Already, it has been discovered that there is a mechanism for the transfer of PA from one living plant to another. There are not many studies on this phenomenon and that most cited is a study where sorghum was grown together with parsley, mint, lemon balm, Indian cress and chamomile. The control group that had no ground contact with the stall contained no PA at all, while the levels in the crops that were co-cultivated with stalls became very high, even over the limit values for certain products (for unclear reasons, the limit values ​​vary between, for example, black tea and herbal tea). However, the fact that a lot of stands were grown very close to the leafy crops is also part of the matter (as far as possible 20 cm from them) in the experiment and that it was probably about getting evidence for the mechanism itself, rather than mimicking realistic conditions.

I am not aware of any studies where comfrey has been co-cultivated with herbs, but based on the precautionary principle, my conclusion is still that you should be careful with growing comfrey (and other plants with high PA content) together with leafy vegetables and herbs used in tea or as seasoning. Even the fertilizer water that many people make from comfrey leaves should perhaps not be used on leafy vegetables and herbs. At the same time, the toxic alkaloids break down quickly if they are composted in a normal compost – after three months of composting in a regular closed plastic compost container each 99% of PA degraded, so it is still possible to use comfrey as soil improvement after composting.

Any transfer of PA to fruit, berries or nuts don't seem to be a problem at all. So feel free to continue growing comfrey near trees and bushes. Sterile comfrey is and remains an unbeatable accelerator plant that can produce more green mass than most other herbs in a season, without the risk of becoming invasive.

Thanks to Kristina Törnblom and Vanja Ekerhult who brought this to my attention!