Will a giant sunflower self seed and spread in the rest of my garden?
only the seeds the birds don't eat and drop in you garden.
Sunflower readily self-seed, but the giant varieties don't self-seed often because their large seeds are easy targets for squirrels and birds. I've had ornamental sunflowers that have self-seeded in my mom's garden for at least 5 years. The seeds on ornamental varieties are much smaller, and are usually black or brown which make them more difficult for birds and squirrels to find. Sometimes squirrels bury the seeds or birds drop them and they end up in parts of the yard that you never planted them before.
Here is a link that might be useful: my giant sunflower website