Sunflowers are said to be allelopathic to other broad leaved plants.
I know, big word. If you have never heard of this, Google "allellopathy of sunflowers".
In a nutshell it means that the sunflower produces some toxin that inhibits the growth of other broad leaved plants. Just how much or how serious this is, I don't know. Any way, it will be an interesting research project for you.
I have heard it is good to follow sunflowers with corn. The sunflowers supposedly inhibit broad leaved weeds, but have no affect on corn or grass.
Fact or fiction? There is some interesting reading on the internet about the subject.
Gardening at 5000 feet elevation, zone 4/5 Northern Utah, Frost free from May 25 to September 8 +/-