In the following account I will use Arctium pubens for brevity and clarity, not A. minus subsp. pubens. I think really we should all be treating this plant as a species for recording purposes, lets call it 'Confused Burdock' for want of a common name. I'm not suggesting there are not challenges here (more later), and this species is a PhD study in itself BUT lumping with Lesser Burdock (Arctium minus) is not helpful and is a major part of the problem. My rationale is:
1) Arctium pubens is probably the most common species over much of England and perhaps the UK as a whole, but too much data is being lost under the agg.
2) Lets be honest it (with notable exceptions, see photos) looks reasonably distinct, and to treat it as a form of Lesser Burdock for recording purposes is unhelpful and potentially dangerous (in the same way, for example, that it is untenable not to segregate the native and non-native forms of Lamiastrum when recording - think of the implications for nature conservation and scientific understanding). I suspect that Wood Burdock may be over-recorded because many botanists find but are unaware of Arctium pubens and how much it typically differs from Lesser Burdock. They can also quite clearly see it is not good Lesser Burdock, so 2 + 2 = 5 it must be Wood Burdock. Lumping with Lesser Burdock implies a level of similarity that usually is not there in practice.
3) All botanists are more than capable of, and should be, recording burdock taxa to species level when in flower - don't let the variability of Arctium pubens deter you. I hope this account helps with this, most species are readily picked out at distance (from a car even) once known (but in some cases an ID may need refining to Arctium pubens on close examination). If you don't record to species then I see little value in recording the aggregate, its just perpetuating the historic problem and taking us further from a solution.
4) I really don't understand why Arctium pubens is considered closer to Lesser Burdock than it is to any of the other species, nor how it has derived certain traits (particularly the often exaggerated pubescence) from its putative parents.
Before getting to the photos, it's worth giving quick consideration to the one character that can be quickly used to split Arctium pubens from Lesser and Wood Burdocks. The former has peduncles greater than 8mm, the latter two have peduncles up to 8mm with capitula more usually +/- sessile. This comes back to my point about the poor quality of published descriptions. This trait relates to the distal part (upper part away from the middle) of the branch, but what is meant by distal is never defined i.e. is it immediately just above the mid-point or if not how far up? At a certain point down the branch the peduncles start 'trying' to be branches (you can't have both sessile capitula AND a raceme in Lesser and Wood Burdock without allowing for non-sessile stalked capitula) and clearly if you measure these you will quickly go wrong. For the last few years I have been concentrating on the top 15 cm or so of the branch, and this works well for all species except Wood Burdock. My eureka moment was realising distal would be better phrased as the terminal part of the branch. It may be that distal in Lesser Burdock and Wood Burdock are not quite the same thing, this may sound like a fudge to suit me but I feel that this comes back to age old problem of writing keys to separate taxa. Quite often it is very difficult to explain how taxa reliably differ in a key, but the human eye can tell the difference at a glance. This is the case here and the photos below will show this. Wood Burdock in particularly is a completely different beast from Lesser Burdock and Arctium pubens.
So lets start with Wood Burdock, as its a plant yet to feature on the blog, and then work through the spectrum from sessile species to the corymbose pedunculate Greater Burdock (Arctium lappa).
Wood Burdock (Arctium nemorosum)
Sell & Murrell are probably right to rename this Northern Burdock, records from the south need revisiting. As I have noted previously, I have searched in vain for this species in my Yorkshire VC, and have only found Arctium pubens. Note the relatively large capitula and the terminal clusters of sessile capitula at the tips of the main and side branches. This is the key jizz of this species. The Swaledale (you will also find Arctium pubens in Swaledale) plants are all uniformly short and I wonder if this is typical of the species, if so this is also useful jizz. Unfortunately I did not photograph the plant as a whole, this would have been helpful as it would have shown how the morphology of each branch contributes to the look of the plant as a whole.
Note also how quickly long peduncles start to appear below the terminal cluster - so the ambiguity in use of the word 'distal' is not helpful. This species seems a bit of a scruff-bag - the ugly duckling of the group.
Final bit of advice - don't trust Google!
Lesser Burdock (Arctium minus)
This is the plant of the south of Great Britain, see Sell & Murrell for their detailed views on distribution. Capitula are +/- sessile but in a different way to Wood Burdock, with the terminal capitula usually solitary (but not necessarily remote).
This is were the fun begins and this species needs urgent genetic study to determine its origins, relationships, and whether it hides a 'real' species within a hybrid swarm. Also research based around whether any of the various geographic variants merit description at the rank of variety or forma. There are distinct forms that are consistent over large areas, indicating that they are capable of breeding true and maintaining themselves. There is variation in capitula size, branching pattern (corymbose to racemose), degree of pubescence, and degree of browny-red pigmentation. Its often simpler to start with 'have I got Arctium pubens, if not why not, therefore it must be species X?'. This is best achieved by looking at the peduncles.
The holy grail species! Inflorescence approaching corymbose. Completely unmistakable, based on my two encounters with this rare plant, with the combination of brilliant white woolly capitula and rich purple flowers. There are supposedly less hairy variants in Europe - hmmm, I can't comment but if Continental botanists face the same problems as UK botanists then perhaps this statement is best treated as equivocal. Don't believe the key in Sell & Murrell - a case of over-emphasising one trait that is not reliable - the inner involucral bracts can be hooked. Trust your eyes instead.
Greater Burdock (Arctium lappa)
If this species is rare in your area and you are not familiar with it be careful, some forms of Arctium pubens have inflorescences and capitula redolent of this species. That said it is unlikely to cause problems. Large plants with a corymbose inflorescence and green glabrous or very sparsely pubescent capitula. Note the solid petiole is only of use for separating from Lesser and Wood Burdocks. It is not reliable for Arctium pubens.