Arctium in the Northeastern US -- what I've learned so far
Bottom line up front: It would help narrow the IDs of Arctium in the northeastern US if we would 1) measure the involucre when present 2) photograph a cross section of a lower petiole to show if hollow or solid 3) get a good photo showing the structure of the inflorescence
BONAP lists three species in the northeast http://bonap.net/Napa/TaxonMaps/Genus/County/Arctium
Flora of North America:
Arctium http://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=102484
Arctium lappa http://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=200023153
Arctium minus http://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=242416085
Arctium tomentosum http://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=200023154
Per the FNA Arctium key linked above, both lappa and tomentosum have corymbose inflorescences . In fruit, they might be distinguished by measuring the size of the fruit? BONAP has the range of tomentosum in the northeastern US as much more limited than that of lappa. This key makes no mention of petiole laxity, leaf shape, or stem grooves.
Go Botany:
Arctium https://gobotany.nativeplanttrust.org/genus/arctium/
Arctium lappa https://gobotany.nativeplanttrust.org/species/arctium/lappa/?key=dichotomous
Arctium minus https://gobotany.nativeplanttrust.org/species/arctium/minus/?key=dichotomous
Arctium tomentosum https://gobotany.nativeplanttrust.org/species/arctium/tomentosum/?key=dichotomous
This key mentions petiole laxity and leaf shape as distinguishing features, noting that lappa has weakly angled petioles and acute leaf apices.
Newcomb's Wildflower Guide lists lappa and minus only. It distinguishes the species by size of flower heads, length of stalks ("long" or "short"), petioles solid or hollow, and noting that lappa's petioles are "deeply grooved."
Clemants and Gracie lists only lappa and minus. It notes that "the major differences between [lappa and minus] are the inflorescence type and the size of the heads." It refers to the lappa inflorescence as "flat topped," but the photo shows a corymb with flowers at different levels.
"Wildflowers of New England" by Elliman notes that petioles of lappa and minus are "usually" solid or hollow, respectively. It distinguishes leaf size and shape, as well as structure of inflorescence, length of petioles, and size of flower and fruit. It does not cover tomentosum.
Royer and Dickinson "Weeds of the Northern US and Canada" cover all three species. It differentiates the three based on flower head size, the presence of hairs on bracts in tomentosum, and one more feature not found elsewhere: they state that at maturity, lappa's outer bracts spread and release seeds, but minus bracts do not spread and release seeds at maturity. Confusingly, this reference states that minus is used for food, while all other references indicate that lappa is the burdock most often cultivated.
"Peterson's Wildflowers Northeastern/North Central North America" has all three. In addition to flower stalk length, inflorescence structure, and size of bur, it mentions that lower leaves of minus have unfurrowed stalks, and lower leaves of lappa of groove on upper surface of stalks.
