The following are the distinguishing traits I've encountered for Common, Russian and Prickly comfrey when compared to each other:
*Common - Less vigorous, shorter, narrower leaves, flowers tending to be creamy yellow, but also found in white, red or purple, leave edges attaching to the stem and running down it like "wings", fertile seed produced allowing for natural spread
*Russian - Very vigorous, taller, wider leaves, flowers tending to be blue, purple or reddish purple, leaves lacking "wings" where attaching to the stems, but sometimes with slight "wings" in certain specimens, supposedly sterile (but see my note below) and only spreading by root division
*Prickly - The tallest of the three, seems to play better with other plants (less dense and smothering), flowers tending to be blue while buds can be more pink, leaves lacking "wings" where attached to stems or with very little "wings", spreads by seed (not excessively) as well as by root divisions.
Out of all these traits it seems that the most commonly mentioned trait across the internet to distinguish these types is that Common and Prickly (prickly usually not even being mentioned) spread by seed and the hybrid Russian does not. This is similar to how a horse and a donkey can mate and produce a mule, but mules are considered sterile... usually... Although we know that mismatched chromosome counts in offspring resulting from inter-species breeding often results in sterility of offspring in animals, the same isn't as reliably true for plants. In fact nowadays you can buy strawberries with pink flowers that were the result of plants in two separate genera and with different chromosome counts being hybridized and then back-crossed with strawberries to produce plants with mostly strawberry type characteristics but with the pink flower color of the non-strawberry parent. My point to all of this is that I think the idea that ALL Russian comfrey strains are sterile doesn't seem well founded. It seems that most of the info available about the Russian type stems from info that's actually specifically about the Bocking cultivars (google them for more info). There are two Bocking cultivars that are widely distributed from when they were selected in the 1950's. The claim about Russian comfrey being sterile may be true for the two most popular cultivars of Russian comfrey since selecting for sterile seed would have been part of the process. However, I suspect that Russian comfrey in general may not be fully sterile as an overall variable population. I found an old identification key for comfrey that listed traits for identifying and distinguishing Russian comfrey and its two parent species as well as the varying back-crosses between the hybrid and one parent species or the other. A Finnish website also referenced the Russian type readily back-crossing with either parent making identification very challenging. I also encountered a reference to Russian comfrey (in general) possessing fertile pollen.
Until recently, I was under the impression that all the comfrey in my location seemed to be the Russian type and assumed it was all one singular clone, and were only spread around through soil disturbance. I've never had a problem with comfrey seedlings popping up where roots weren't accidentally/intentionally moved to. Now, after closer observation I have found genetic variation in some parts of the population, and now I'm wondering if there might actually be some limited genetic variation within the population as well as a separate distinct population sharing the nearby area.
|The main clonal colony|
At the time I'm writing this, the comfrey is in full bloom (which is beautiful), and I can see that the vast majority in the main patches where it has spread through soil disturbance look identical in every way right down to the exact color of the flowers... Except... Then I noticed variations in the flower color and shape in a select few individuals at the edge of the colony and elsewhere in very sparse colonies within about a quarter mile or so of the main patches. While the main clonal type is incredibly vigorous and uniform with rounded bell shaped pinkish flowers I found two specimens with flowers so pale with just a hint of blue/pink that they were close to white. I also found one with a much purer and more saturated pink color. These first variations still had the same flower bud & bloom shape as the main clonal colonies and showed the same level of vigor. Then I started noticing that here and there along the road outside of the main area where the comfrey grows there were sparsely scattered specimens with much bluer flowers often streaked blue/pink. When still in bud, the main clonal type have roundish buds with only a soft point, whereas the bluer flowered types were consistently displaying more pointed and narrow flower buds which opened into slightly narrower flowers. I'm beginning to suspect these individuals may be either Prickly comfrey or Russian comfrey back-crossed to Prickly comfrey.
As a side note, I saw on a Facebook gardening post where a lady asked why her comfrey grown from a root cutting had a different flower color than the parent plant the cutting was taken from. This reminded me of how African violets can be grown clonally from leaf cuttings, but the progeny do not always end up blooming with the same color as the parents despite being genetic clones. I wonder if this phenotypic color change could be an explanation for the rare variants I found that otherwise seem the same as my large clonal patches... There is so much to wonder about that it may never all be known...
This may be about as well as I'll ever figure things out for my comfrey, but I will continue to observe and learn. I'm not sure that I would be able to afford the genetic testing required to satisfy my curiosities. :)
I am posting this in hopes that my opinions will be challenged and refined. Maybe I've got everything all wrong. Your thoughts and input are welcome.
Follow-up: After scouring the comfrey population for variants I dug a chunk out of my favorite four plants to grow on side by side for comparison. I had to cut them back a bit for transplant, but after about a week two of them are blooming again. He're the kicker... The two that are blooming now were the two with the bluest flowers I could find. Guess what color they are now... Pure beautiful pink. It will be interesting to see if they turn back to blue later or if the color is actually affected by the change in soil they are growing in rather than genetic variation or mutation. Perhaps they're like hydrangeas?
|These guys are representative of the majority of the population. The following show variation, but are by far in the minority. Some of the following pictures simply are to show the normal range of leaves and stems as well.|
These two may be the Prickly comfrey or some back-cross with it.
|Notice the cluster at the bottom of the picture. It's buds are much more pointy with a greater color change between bud and bloom.|