David, do you know what kind they are? It really depends upon the variety of sunflower, as they vary in height from less than a meter to over 4m. tall at maturity. But if they are any type that make very large, heavy flowers that you will get seeds from even a large container will be blowing over at the slightest gust of wind unless you can tie them to a support - a fence or a large post of some kind. If you can find a spot, I'd advise putting them in the ground.
Here is my sunflower experiment from this spring.
This variety was supposed to max out at less than 2m. high but they grew to well over that (the metal bird feeder is over 2m. high). I pinched the growing tip out of the one in the second picture to try to make it branch and not grow so tall, but that didn't work, it kept growing and just made more, smaller flowers.
Now that the flower heads are getting heavy with seeds I can't keep them standing up even though they are planted in the ground.
