There's a thread in the Perennial forum re. "Your favourite" aggressive plants. One post referred to banishing the Helianthus you refer to due to its spreading nature.

Here is a link that might be useful: Favourite Aggressive Plants

Greengarderner, I discovered the very same thing 5 years ago when I found my H. maximilliani doing that too. I ended up ripping it out and moving it to less formal areas. I since discovered that it seems to be typical of Helianthus and it means doing a lot of culling in the spring to be rid of most of it. I have lots of natives that invade every year that do the same thing so it is not a big deal here.
A tip: cut down the H maximilliani in mid summer to about 2 feet or less. The plant will still flower but at a much lower height. I think the species was meant to be munched down by buffaloes in summer cuz it still flowers when you do that. It also keeps it from flopping over when fall rains hit it.

Maximillian needs an area to grow in. I did keep ONE planted in a city garden that I had. What is happening is every plant sends out short runners in a circle. That said, I found that if one kept just one, the healthiest one in the spring, that one would grow extremely tall and full. You will need to stake it. The runners only grow in the spring. Once the plants come up for the year, they will not make runners again. It is not an ongoing fight through the summer. Ifleft to their own, they do want to have an are and make a colony. The colony will not expand and expand exponentially.

I have a 4ft by 6 ft patch that has stayed about that size for several years, so I agree with the above that you don't have to worry about exponential growth.

I've tried cutting them down midsummer, but they still grow tall and flop. It's the only thing I don't like about these plants. Other than the floppiness, I love the yellow flowers that bring some sunny color in autumn.

~CA

Thanks for all the help.

But, it is now a moot point. I ripped out the plants and about 250 seedling volunteers (still pulling these out!) last weekend and threw them out.

This plant is a noxious weed!

In the case where you want one or two sunflowers on a small plot, how about planting in a large plastic container with the bottom cut out which is sunk in the ground (leave lip about an inch above ground level) and then, as dandy_line suggested, cut down in mid-summer to 2 feet or less? This would prevent runners from spreading and flopping of the tall stalks. . . . As an aside, I have learned to use a similar device when planting trees by installing a root barrier which forces a tree to send its roots farther down before traveling horizontally, thereby preventing disruption to sidewalks and other plants' root systems. I don't know how far down the sunflower's roots would travel which is a question yet to be answered for this to work.