photo with scale.. and location if exotic and not personally sown seed ..

I wondered, do some species/variety/whatever bloom noticeably later than others?

Yes, they do bloom at different times - one of the charts Johnny's offers for growers is using the different days to maturity to extend the bloom availability for cut flowers. http://www.johnnyseeds.com/growers-library/flowers/sunflower-succession-planting-harvesting-program.html

Try Johnny Seeds. http://www.johnnyseeds.com/flowers/sunflowers/