I have a lavender starflower along my back fence for a privacy screen. They do grow fast once they get going. I trim mine quite frequently. I don't know about forming it into a tree, but the limbs can get quite thick. Mine got out of hand and I cut it quite severely. Some of the lower limbs were almost 2 inches in diameter.

The photo is when it was unruly. It is now nicely trimmed to about 7 feet tall.

Here is a link that might be useful: What's Growing On?

I had one at the old house that never did get dense, just long, arching branches. Perhaps I had a different cultivar than normal. I was always disappointed in my plant, but loved the flowers.

Thank you both. So nice to see a pic. It looks quite tall....over15 feet, perhaps. Worth putting on the list for much needed privacy screen.

I have 5 that are 2 yrs old and they die back from frost. I knew they would be frost tender but didn't expect almost all to be gone.
I put them in because they tolerate wind.
Since they are in the front of the house I might be replacing them.

I have heard that they are short-lived. I love the shape of the branches, especially when they cascade over a wall.