I'm in the process of systematically removing all the weeds from my backyard -- starting with dandelions and thistle. All of a sudden, this thing sprouted in the middle of a bare patch. What on earth is it?
If it helps, I live in southern Ontario, near Toronto.
It's certainly teasel if it has a row of thorns along the underside spine of the leaf.
It's a dynamic accumulator, with a deep tap root. It pierces clay soils and brings up important micronutrients. The leaves are good for mulching. It collects water at the base of its leaves. It will mature with a distinctive spiky seed pod that crafters work into arrangements when dried. Goats love it.
As with most weeds, it's trying to tell you something. If it's not right where you want something else, I'd let it grow until before it goes to seed, then cut it down and spread the leaves out on your fall/winter beds for mulch. As a mulch, it will reduce erosion while putting into surface soil minerals and micro-nutrients that it probably lacks.
Either that, or feed it to someone's goat! As weeds go, it's a pretty nice one. :-)
This looks like some Teazel to me.
Update 2012-05-31: I made a photo of young Fuller's teasel plants growing near my home:
Now I am quite sure your plant is the same or other plant from the Dipsacaceae (teazel) genus. Notice the thorns on the lower side of the leaves. I can see them on your photo too.
Dipsacus fullonum, I'm pretty sure and I agree with Jacek.
In your picture the plant is still very young, it takes a couple of months to see the flowers. In Italy it is very common and it is called cardo dei lanaioli
I believe this is some kind of Dock/Sorrel - see http://en.wikipedia.org/wiki/Rumex
So, maybe not a weed, but an edible plant.
I think we will be more lucky in identifying the plant if you let it flower, and then upload a new picture.
