There are several species of Comfrey, but there are two that you might come across in woodlands or shady locations. One is Creeping Comfrey, (Symphytum grandiflorum) This flowers quite early in the year I have seen it flowering in mid March, but it then continues to flower right the way through Spring and Summer. Then there is Tuberous Comfrey ( Symphytum tuberosum) and this comes into flower in June and July. The photograph above is Common Comfrey.
I have seen Creeping Comfrey also known as Dwarf Comfrey growing in some quite shady places, often close to water, it is quite low growing and carpets the ground. It has rhizomes which gradually spread out so the initial small patch can spread and become quite invasive. The flowers are arranged in a one sided spike as with all the Comfrey species however there are less flowers in the spike than with the majority of species. Common Comfrey and Russian Comfrey which you might well be familiar with as garden species typically have 10 to 15 individual flowers. Also the flowers of this species are cream to white with and pinky/purple when in bud.
Another distinguishing characteristic of this species is that the leaves have a definite stalk (petiole) so the leaf blade does not run down to and fuse with the stalk.
It is not a native species but has become naturalised, originally it came from the Caucasus, it is very much restricted at the present to the southern counties of the UK.
Tuberous Comfrey is not so common in the south of the UK it is most likely to be encountered in Scotland , where it favours shady places like damp woodlands but you might find it growing in hedgebanks and roadside verges. It flowers later in the year, during June and July and the key identification points are that the flowers are creamy white and the leaf blade continues down the stem producing a winged effect.
The common species of Comfrey are Russian Comfrey which many people grow in their garden and Common Comfrey which you will most likely find growing close to fresh water. It is well known that Comfrey can be used as a fertiliser. I had some growing in my garden in Norfolk and used to regularly harvest the leaves and plunge them into a large bucket of water. Six weeks later it was ripe and stinky. A rich brown liquid which I would carefully pour into a watering can, just some of it and then dilute it with rain water from the but so that it was about 10% strength. This was then tea colour and still quite smelly and I would water it onto crops like tomatoes, aubergines and green peppers which I grew quite a lot of and I don’t know how much it was down to the fertiliser but the crops were quite prolific.