Many of these violets were noted on a field trip with fellow CNPS members Loretta Brooks & Chuck Heimstadt...but none of us knew the species. They were growing near a creek in soil covered with leaf-litter, under the shade of a mixed-forest composed mainly of oaks, douglas fir, bay trees, and willows.

To determine the species, I began with the Jepson eFlora key to Viola, where the following characters were invoked (all visible in the photos here and/or checked in the field): stem 0; internodes 0; rhizomes not especially thick & fleshy; plants with green leafy stolons; leaves round, crenate; petals lilac.

But, as so often happens, ambiguities arose in interpreting some of the characters presented in the key. In particular, for plants of this population some stolons were green & leafy (see right-of-center here...click image after it loads to enlarge); while other stolons were paler, having no leaves and only feeble root-leaders at their nodes (see center of image here). And at couplet 3 of the Jepson key, one needs to determine whether the fruit is puberulent or glabrous...but no fruit was found. The upshot of these ambiguities was that the ID here seemed to boil down to either V. odorata or V. palustris.

To clarify the choice, I consulted the Flora of North America Viola key which went smoothly up to couplet 13, where the break involves two length ranges for the petal spur...with the cut-off of 3 mm being just about what the spurs measure here (of course! :-). Going ''both ways'' at couplet 13 led to the same candidates as in the Jepson treatment: V. odorata or V. palustris...as detailed below:

1. Taking the 1st choice leads to a ''widowed lead'' at couplet 14 (i.e. no opposing choice is provided...presumably an editing error):

   1. On the one hand, going with (fruit) ''capsules puberulent'' leads immediately to V. odorata.
   2. On the other hand, the (erroneously omitted) alternative (presumably ''capsules glabrous'', leading to couplet 15) yields potential candidates selkirki and japonica...but both can be eliminated by studying their detailed FNA descriptions: e.g. due to having corolla spurs too long; lacking 'bearded' lateral petal bases (which are present here, see this photo); and being way out of range (e.g. Washington state, Yukon, the midwest & east coast, etc.).

2. Next, returning to the 2nd choice at couplet 13 in the FNA key leads without ambiguity to couplet 22 which yields V. palustris, since the bracteoles can be seen to be below middle of peduncle (cf. left-of-center in this image); and the leaf blade margins are crenulate. However that couplet also indicates the leaves of V. palustris are eciliate...whereas the leaves in our plants here can be seen to be ciliate (see photo here...click after image loads to enlarge). Also, the FNA description of V. palustris indicates its petioles, peduncles, and leaves are glabrous...whereas the FNA description of V. odorata indicates its petioles, peduncles, and leaves are purberulent...which is the case with our plants (also, cf. the images here and here). Futhermore, the FNA descriptions for V. palustris and V. odorata indicate the the number of basal leaves are ''2–4'' vs. ''5–10'', respectively...which clearly points to the latter for our plants. In summary, studying & reconciling the keys & descriptions in both the Jepson eFlora and FNA treatments leads to a clear ID of V. odorata for these plants.

The location here seemed fairly wild and undisturbed, though not far (~1/2 mile) from roads and human developments (e.g. rural homes). The Flora of North America discussion states that: ''Viola odorata is sometimes found in remote locations not easily explained by anthropogenic influence'', but that ''It is native to Eurasia and assumed to be introduced in North America where it is usually found in areas associated with human habitation, including parks, lawns, and roadsides.''

Note that V. odorata does not appear in the 2011 ''Checklist of the Vascular Plants of San Mateo and Santa Clara Counties, California'' by Toni Correlli, and zooming-in on the inset map at the bottom of the Jepson eFlora V. odorata page, I saw that the nearest voucher currently appearing there is a 1975 collection from Henry Cowell State Park, 18 air miles to the SSE in Santa Cruz County. However, doing an independent search of the California Consortium of Herbaria web site, I did find a single 1935 San Mateo County record for V. odoroata from similar habitat to our site...and about 15 air miles to the northwest.

Since this species was not listed for San Mateo County in Toni's checklist, nor in the records displayed on the Jepson eFlora inset map...I decided to cross-check my ID using a number of other regional floras:

• In the classic 1968 A California Flora by Philip Munz, the plants here again key to V. odorata...with the terminal couplet there separating it from V. palustris by (among other things) ''plant pubescent'' vs. ''plant glabrous''.
• In the 1997 An Illustrated Field Key to the Flowering Plants of Monterey County by Mary Ann Matthews (AKA ''Corky'')...these plants again go to V. odorata
• In the 2013 Annotated Checklist of the Vascular Plants of Santa Cruz County by Dylan Neubauer, the following note appears: ' Viola odorata: ''Both violet and white-fld, scented cultivars have persisted as garden escapes along Bear and Laguna creeks since at least the 1960s. There is a 1975 record from Henry Cowell Redwoods S. P.'' —Linda Willis '
• No mention of V. odorata is made in:
  • the 2003 Plants of the San Francisco Bay area by Biedleman & Kozloff
  • the 2007 Marin Flora by Howell, Almeda, Follette, & Best
  • the 1990 A Flora of the San Bruno Mountains by McClintock, Reeberg, & Knight (note the 1968 version can be read online here).
• In the 1951 Volume III of the classic Illustrated Flora of the Pacific States: Washington, Oregon, and California by Leroy Abrams, there is just a single mention of V. odorata on pg 124...where it's cited as the type species for the genus Viola.