This time we will examine two similar species of Thoroughwort (sometimes called boneset) – Hyssop-leaf Thoroughwort (Eupatorium hyssopifolium L.) and Torrey’s Thoroughwort (Eupatorium torreyanum Short & Peter). The synonym Eupatorium hyssopifolium L. var. laciniatum A. Gray is sometimes applied to Torrey’s Thoroughwort.
The difference between the two species is subtle, but distinct and is mainly seen on the leaves.
The shape of principal leaves on the Eupatorium hyssopifolium are described as linear to narrowly lanceolate. Additionally, they are generally shorter (2 to 7 cm long) and narrower (1-5 mm wide) than the Eupatorium torreyanum, and are 10 to 40 times longer than wide. Also, the margins of their leaves are mostly entire or obscurely toothed.
On the other hand, the principal leaves on the Eupatorium torreyanum are described as lanceolate, 5 to 12 cm long and 5 to about 10 mm wide. The length to width ratio is 6 to 15. The margins of the leaves are conspicuously and divergently toothed.
Hyssop-leaf Thoroughwort naturally occurs in Eastern USA from Massachusetts on the north, Georgia on the south and Tennessee on the west. It is common throughout Maryland and on the Refuge.
Botanists have found Torrey’s Thoroughwort from New York on the north, to Florida on the south, and to Ohio, Tennessee, and Louisiana on the West. It is considered to be a viable hybrid between Hyssop-leaf Thoroughwort and another species of Eupatorium. The identity of the other parent species is a matter of controversy and requires further research.
Interestingly, Torrey’s Thoroughwort is listed as occurring in Maryland, yet the Maryland Biodiversity Project (MBP) and the Maryland Plant Atlas (MPA) have no report (as of 5 September 2016). Maybe this is the first known report of this species, or previous reports of it just have not been documented yet. In any event, this species is often overlooked because of its co-occurrence with Hyssop-leaf Thoroughwort, and observations of it could be easily dismissed because it is so similar in appearance.
Some botanists elevate Torrey’s Thoroughwort to a species level, others subordinate it as a variety of Hyssop-leaf Thoroughwort, and others have lumped these two together into one species. Whatever its taxonomical status, Torrey’s Thoroughwort is definitely a distinct botanical entity from Hyssop-leaf Thoroughwort.