Ok - here's a definition of yellow nutsedge: Perennial sedge. Yellow spikelettes at top of mature plants. Triangular stem. Reproduces by seed and rhizomes.
We belong to a group of farm folks out in Iowa - don't go there and ask why ;) Anyhow, this was a topic of discussion awhile ago and here were the comments in part: If you thought it was GRASS, you were not alone. Many farmers made the same mistake last summer. Yellow nutsedge is SEDGE and NOT a grass. You can NOT kill it with a grass control herbicide.
Normally, we think of weeds being either a grass or a broadleaf weed. Yellow nutsedge is NEITHER of those. It is a member of the THIRD class of weeds: the sedges. Yellow nutsedge is a perennial weed. It can grow from seed OR from tubers. You can FIND tubers if you dig up the roots on a nutsedge plant. They are brown, round bulbs attached to the roots. You can also see that yellow nutsedge spreads like many of the other perennial weeds do, by RHIZOMES. Yellow nutsedge can ALSO grow from seed. There are clusters of seed at the top of each mature plant. The seed clusters are supported by three, leaf-like structures. The seeds are little yellow spikelettes, and they make yellow nutsedge easy to identify. But the most unusual characteristic about yellow nutsedge is its stem. Yellow nutsedge has a triangular stem. If you feel the plant at the base of the stem and roll it between your fingers, you can feel that it has three sides. A triangular stem means that you have nutsedge. Looking down at a nutsedge plant, you can tell it's a little different from other plants right away because of two things. It has very shiny leaves, and there are three of them instead of just one or two leaves like grass plants would start with. We usually see yellow nutsedge in wetter parts of the fields. We find it in waterways, pothole type areas, and lower parts of fields. I've also seen yellow nutsedge in yards and other places you wouldn't normally expect this type of weed, but in most cases, it's just going to be in wetter soils.
So...when I looked in Botanary there was no definition of 'sedge'. In PlantFiles, there's a great definition of nutsedge from Hazel, but it appears that it's thought more of as a grass. My questions are - is there this 3rd class of weeds? and Is 'sedge' always a weed or is this just an overall term or classification?
And the discussion notes above are from a Farm website out in Iowa where they have problems in large fields of soybeans and corn.