There is a clump of thistles in the median between the W&OD bike path and the W&OD gravel path north of the Dulles Toll Road bridge. These have been blooming for about a month now. This picture was taken in the evening.
A closer view of the same plant, taken that morning. Based on the criteria in
Newcomb's Wildflower Guide, this can be identified as field thistle (
Cirsium discolor). See also this
description from Illinois. The leaves are whitish underneath, and the bracts (the green round golfball-like structure underneath the flowers) is not spiked as in bull thistle (
C. vulgare). Moreover, the flowers are not fragrant, and thus this is not pasture thistle (
C. pumilum). And the leaves are spiky and extend all the way up to the bracts, so this is not Canadian thistle (
C. arvense). Field thistle is native, unlike some of the other species.
I believe this unassuming little weed, on the north side of the W&OD bike path between American Dream Way and Isaac Newton Square, is common ragweed (
Ambrosia artemisiifolia), the bane of allergy sufferers this time of the year. Compare this
Virginia Tech weed guide and this
image page from Oregon.
Here is a closer view. Although somewhat discolored by the effects of the camera flash, one can see both the somewhat bent spike of flowers as well as the characteristic deeply dissected leaf pointing down from the stem in the middle of the image.
I also had some success with distinguishing species in a genus. As
previously noted, most of the thoroughworts along the W&OD bike path are late-flowering thoroughworts (
Eupatorium serotinum). This image highlights the petiolated lanceolate leaves that are the distinguishing characteristic of this species.
But along the bike path south of the Dulles Toll Road I also found a couple of specimens of this smaller plant, with very narrow sessile leaves that do not increase in size as one goes down the stem. This appears to be hyssop-leaved boneset (
Eupatorium hyssopifolium). Compare this
page from the Connecticut Botanical Society.
And then there is this species, which occurs in a clump of winged sumac between the Access National Bank building at 1800 Robert Fulton Drive and the W&OD bike path just north of Sunrise Valley Drive. This species has sessile leaves that are a bit blunter than those in
E. serotinum and become more noticeably toothed as one goes down the stem. Of the species listed in
Newcomb's Wildflower Guide, this most closely answers to rough boneset (
Eupatorium pilosum), though it might also be one of the other sessile-leaved thoroughworts, such as round-leaved boneset (
Eupatorium rotundifolium).
