Can you post a photo of the foliage?

Unfortunately, no.

For each plant there are 1-5 stems coming out of the ground. Each stem separates into three. At the end of each of the three "stemlets" it divides again into three. At the end of these there is a leaf. So a total of 9 leaves per stem. The leaves are a light to medium green and smooth not hairy. They are oblong in shape and not serrated but there are two very slight indentations so technically I guess it is lobed and the magic number three shows up again. No sign of flowers or seedheads.

The site has been disturbed; it's a woodlands area that has been overrun by buckthorn and is now being turned into a city park. Heavy machinery has been through and some trees have been taken down and chipped up, the remains being strewn around the open area. I found these plants around the edges.

Does that help? Are there woodland rues that have very fleshy roots?

Thalictrum thalictroides has tuberous roots

http://www.missouriplants.com/Whiteopp/Anemonella_thalictroides_page.html

As far as Minnesota goes, there are only 5 species of Thalictrum.
http://plants.usda.gov/java/profile?symbol=THALI2

dasycarpum
dioicum
revolutum
thalictroides
venulosum

You might Google images of the others to see if they tie in with your description.

Here is a link that might be useful: Thalictrum thalictroides