This perennial sunflower is about 7ft tall, growing in a big clump on the bank of a drainage ditch, and has just started to flower. I looked at the Jerusalem Artichoke pics in the database, and the leaves don't quite match, these have a more jagged edge. Then I looked at Giant Sunflower, and the flowers don't match the database pics. What could it be?

It's probably Sawtooth sunflower (Helianthus grosseserratus), although there are some other species that match as well

Sawtooth Sunflower (Helianthus grosseserratus)

Some additional information that might pin down the ID:

Stem is bristly, not smooth, and the color is pale green.

Leaves are opposite, except at the top of the stem where some are alternate.

Leaf feels flimsy, but very rough to the touch on upper surface, smooth and minutely-haired on under surface. A mature leaf is 7" long and 3" across at widest part. Where the leaf narrows towards the stalk, there are very slender strips of leaf continuing down the stalk to where it joins the stem.

I counted the petals on two flowers, one had 16 petals, the other 18.

Is there any other information that would help?

Try this key ..

http://michiganflora.net/genus...

It does seem to be Helianthus grosseserratus :

Leaf: opposite along the lower stem, centre and upper alternate; lance-like, usually with sharp teeth, upper surface slightly rough, lower surface with short whitish hairs, often folded upward along the midrib, stalk usually winged

http://wisplants.uwsp.edu/scri...

http://plants.usda.gov/core/pr...

Helianthus giganteus :

Leaf: opposite on the lower stem, upper alternate, flat, narrow, at least 3 times as long as wide, upper surface rough, lower with long hairs

http://wisplants.uwsp.edu/scri...

I should have clearer with what I meant by "probably". Your pictures are excellent, June, and provide more than enough information to make the ID. EXCEPT for the fact that most species of perennial sunflowers in North America exhibit a great deal of variability within species and a considerable amount of overlap between species. It's often all but impossible to identify them with 100% certainty just by looking at them.

That said, your plants look like Helianthus grosseserratus to me. That's probably what they are.

Many thanks, Kent and Janet. H. grosseserratus it is!

Kent and Janet are excellent sources for information