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Here are more images of a “mystery Stachys” growing at Edgewood County Park that has interested many iNatters recently…for a summary, see observation 172567968 by @sandy_b. Part of the “mystery” here is that Edgewood Stachys plants like the ones seen here (which typically grow in wet, flat areas at the grassland/scrub ecotone) seem quite distinct from the other earlier-blooming Stachys commonly found in oak woodland habitat nearby…but both seem to key to S. rigida.

One aim of this iNat observation is to add some higher-resolution images of vestiture (or "indumentum"), i.e. the form & variety of hairs (or "trichomes”) present on the plants. There appear to be two main types of hairs here:

1) relatively conspicuous, long (≈1-2 mm), whitish-translucent, non-glandular hairs that are straightish-to-slightly-wavy and (mostly) spreading (or somewhat inflexed or reflexed). I would tend to call these hairs “silky”…a descriptive term employed by authors of a number of floras, though I’m not completely sure of the precise nuances they may have intended by that term?

2) less conspicuous, much shorter (≈0.5 mm), narrower, and often somewhat denser, gland-tipped hairs, that are strictly straight and spreading.

Both the above types of hairs are present on stems, leaves, calyx lobes…even on the outer surface of the corolla (especially on the upper lip). There may also be sessile glands (= tiny, stalkless, spherical blobs of resinous goo)…but it’s hard to tell for sure, as a stalk may appear absent if it's very short, or one is looking "straight down on it" (versus a transverse/profile view).

======= Species ID challenges =======

Obtaining an unambiguous species (or ssp. or var.) placement has been elusive, due to what I think is likely considerable intraspecies variability for a number of characters in the “entity” here (whichever name one may choose to refer to it). But to start, we can eliminate a number of initial candidates: i.e. …

S. bullata has a horizontal ring of hairs inside the base of the corolla tube (see Fig. 4390 here)…whereas the ring here is oblique (see the longitudinally split-open corolla in the 1st photo above).
S. pycnantha has a relatively short inflorescence of continuously-packed flowers (essentially no interrupted whorls) with just a single pair of fairly-conspicuous opposite bracts at the base of the inflorescence…whereas the plants here have longer inflorescences whose whorls, while quite congested above, have several well-spaced whorls on the lower inflorescence – and each individual whorl has its own sizeable pair of opposite bracts...though they do diminish in size upward.
S. albens has heftier stems, with more tangled & cobwebby hairs that are often subequal in length to the stem width (see CalPhotos images here)…whereas in the plants here the stems are narrower, and have shorter straighter hairs.

The plants here lie somewhere between S. rigida and S. ajugoides…but many of us are finding it difficult to place a consistent name on them due to what appear to be ambiguous circumscriptions of various characters suites attributed to the two species. For instance, the primary key character used to separate rigida and ajugoides in numerous regional Floras (e.g. Jepson eFlora, Marin Flora, Plants of Monterey County, Munz(1959), Abrams(1951)) involves the leaf bases:

1) leaf bases narrowed (or wedge-shaped, cuneate)….ajugoides

2) leaf bases rounded, truncate, or cordate…..rigida

…under which these plants would go to S. rigida. But considering other characters for each species (e.g. presence vs. absence of “silky” hairs, relative congestion of upper flower whorls, glandularity, etc.) suggests ajugoides. These two species seem to have a fairly long taxonomic history of “mix & match” ambiguity as evidenced by past & present synonymy (e.g. see the "Synonyms" lines in the Jeps descriptions for S. rigida var. rigida and S. rigida var. quercetorum, and the "Unabridged Synonyms" line for S. ajugoides). In the circumscription of Jepson(1943), only S. ajugoides was recognized (see key here), and Jepson gave an interesting & relevant discussion of variability in the ‘complex’ (see last paragraph on pg. 425 here and its continuation on the following page). Using Jepson’s 1943 treatment this would appear to go to S. ajugoides var. rigida…a somewhat amusing & apropos combination of the two epithets in question ;-). That name is currently synonymized under S. rigida var. rigida.

If one follows the Stachys key in the Jepson eFlora…and tentatively ignores the reference to “silky” hairs at couplet 6…these plants go to S. rigida var. rigida. This plant appeared to have some stems > 1 meter, which per couplet 11 would reinforce the choice of var. rigida. As @randomtruth suggested, perhaps the other distinctly-different earlier-flowering, woodland Stachys at Edgewood that locals there previously referred to S. rigida might correspond to the other variety, S. rigida var. quercetorum? But my hesitations regarding ambiguities mentioned earlier still linger.

@andyjones1 noted these plants seem a good fit with Amos Heller’s S. ramosa…and, overall, the protologue does indeed agree well...though it indicates the corollas have “the tube exserted 2 mm beyond the calyx" and are "densely bearded at the middle on the inside with a horizontal ring of hairs”…whereas in a number of the whorls I looked at here the corolla tubes were not exserted from the calyx (though note the exsertion in the 5th photo here) , and the interior ring of hairs here is oblique and lies near the bottom of the corolla along the “crease” delineating the spur (or “pouch”)…again, see the 1st photo (and also the dried corolla at lower right of the 3rd photo, which shows well the "crease" of the spur/pouch). A small amount of variation in exsertion of the corolla tube seems plausible to me...though I'd expect the position & angle of the "hair ring" interior to the corolla to be a fairly stable character. Images for two of Heller's isotypes for S. ramosa...here and here...were subsequently determined, respectively, as Stachys ajugoides var. rigida in 1992 by Barrett Anderson and Stachys rigida ssp. quercetorum in 1931 by Carl Epling. Currently, both the Jepson eFlora and the World Flora Online list S. ramosa as a synonym under S. ajugoides.

The following two papers may offer insights towards resolving the ID of our “mystery plant”:

EPLING, C.C. (1934). Preliminary revision of American *Stachys*. Feddes Repert. Spec. Nov. Regni Veg. 80:1–75.

MULLIGAN G.A. AND D.B. MUNRO (1989). Taxonomy of North American species of *Stachys* (Labiatae) found north of Mexico. Naturaliste Canad. 116:35–51

I haven’t been able to access them yet.