This post has been waiting for a long time! I am again using Hitchcock and Cronquist's Flora of the Pacific Northwest and this time am starting at the Cardamine key. Thanks again to Kitty for looking it over and providing the commentary in italics.
The leaves are compound so we take 1b and the petals are small and white leading us through 4b to Group II.
It isn't always clear to me what counts as taprooted, but the petals do not approach 7mm, I believe the plant is annual/biennial and it is definitely weedy so I will go with 12b.
Seems like the length of the petals is the most straight forward character at this couplet. I felt compelled to look up taproot in a handy little book, “How to Identify Plants” by Harrington and Durrell. Their definition was pretty straight forward; (a thick tapering root, that can be slender or as thick as a beet). So how to decide if something is a taproot if it is slender? I thought that the discussion of annual root systems in the same book offered a bit of useful information “ …seldom much enlarged, has no special food storage structures and usually merges into the stem without a break caused by scars or constrictions” In other words, I’d look for a scar of constriction to help decide about the nature of a root.
This last bit is a little more fiddly, but not too bad. When I count the seeds through the silique in the above picture, it looks like there are 11, but since there will be a similar number on the other side, that gives 22 seeds for the silique. None of the siliques appeared to have significantly more than that where as option 14b calls for 24-40 seeds. Also when I enlarge the image of the whole plant I can see that the lateral stem leaves appear relatively egg shaped rather than lanceolate. This leads me to choose 14a: C. oligosperma. We can go further in this key and since the plant is growing at low elevation and would not be surprising to find as a garden weed it is likely var. oligosperma.
Seems like this one worked out to species easier than I thought it might. The distinction between C. oligosperma and C. pennsylvanica seems pretty straightforward as long as the fruit are available.
I tried the key in Cruciferae of continental North America that was posted anonymously. I used the key to Cardamine occurring north of Mexico and followed a pretty straightforward path until couplet #24 which required a bit of wiggling. In order to get to C. oligosperma, the plant should have abundant basal leaves in the form of a rosette and spreading hairs on the stem bases and petioles. The basal leaves don’t look particularly abundant, but there are at least a few basal leaves. I’m willing to describe the basal leaves as forming a rosette (although weak). Seems like we established that there were a few simple hairs in Part II of this exercise..
If I take the path that leads me away from C. oligosperma, I can eliminate from consideration the species because of geographical distribution (e.g. C. debilis) or number of seeds (e.g.C. pennsylvanica) or leaf shape and abundance (C. parviflora). The only species remaining is C. flexuosa. I’m having a bit of trouble eliminating this last species from contention. I think that the number of seeds is probably a good characteristic, but am having trouble finding the number of seeds per silique in flexuosa. One clue is that this flora talks about the similarity of flexuosa and pennsylavanica and a discussion of a bitter cress found in West Vancouver BC. http://www.ubcbotanicalgarden.org/forums/showthread.php?t=22568
Although tempted by flexuosa, I think I’ll stick with oligosperma