Field Thistle (Cirsium discolor) 9/19/2009 Eastern Neck NWR, MD

-stems not spiny
-often heavily branched in the upper plant
https://www.minnesotawildflowers.info/flower/field-thistle
Tall Thistle (Cirsium altissimum)
-upper, and often lower, leaves are not lobed and more coarsely toothed
-"non-native thistles are distinguished by their sharp spines, especially along the stems.
-While most references give a height of Field Thistle up to 7 feet, we've seen plants rather taller."

https://www.minnesotawildflowers.info/flower/field-thistle
Field Thistle (Cirsium discolor)
stems not spiny
often heavily branched in the upper plant
whereas:
Tall Thistle (Cirsium altissimum)
upper, and often lower, leaves are not lobed and more coarsely toothed
"non-native thistles are distinguished by their sharp spines, especially along the stems. While most references give a height of Field Thistle up to 7 feet, we've seen plants rather taller."