I had the very fortunate opportunity to head out into the field with one of the more respected Biologists in the Midwest region.  Dr. Andrew Hipp is the senior scientist in plant systematics at the Morton Arboretum in Lisle, IL.

Below are some examples of how closely related Sedges can be (via: http://michiganflora.net):

Starting from top left and going clockwise: Cx tenera, Cx bebbii, Cx normalis, and Cx cristatella.

Sedge species are usually a good indicator of a wetland, or saturated soil.  They represent the cyperaceae family which holds a large variety of species. Sedges usually have triangular stems, and this is probably the easiest ID attribute.  

Andrew was one of the collaborator's at the Field Museum's herbarium downtown Chicago.  They have a fantastic article (for the Chicago region) online found here:  http://fm2.fieldmuseum.org/plantguides/guide_pdfs/384.pdf. 

They list out 8 groups for Carex Sp.

1.  One headed 

2.  Hairy 

3.  Large headed 

4.  Beaked bottlebrushed

5.  Beak-less

6.  Female-tipped 

7.  Fox and woodland star

8.  Oval

These are external observations that can be made to lead to you to a certain group.  

Now you begin to break down the plant from the outside in.  Knowledge of the Sedge's structure is vital, and this allows navigation throughout a key to correctly ID the individual.  Each part of the plant listed below will have a distinct characteristic that will differ.  The perigynium is how we identified the difference between the normalis, bebbii, and tenera listed above.  

We found 23 different sedge sp out in the field with Andrew within a four hour time period.  I'm sure if we had more time, we'd find a few more.  Not too many surprises with what we found.  Carex crawfordii was found, which is usually a more northern plant, so that was interesting.  With all the precip we've gotten so far this year, our wetlands are blooming, bursting, and feeling good.