I bought a package of giant zinnia seeds at Lowe's (Lowe's brand) this June and sprinkled them in an inch or two of dirt along one side of our patio. They sprouted right away and, according to the package, they were to grow to 2-3' in height.
They are now up to 5' tall with beautiful big blooms. The package reads "Zinnia Giant Cactus Flowered, Mixed Colors" and cost $1.25. I must have 50 plants and they have been the highlight of my first summer garden. The soil around my patio has alot of sand in it. Maybe that helped. I put MiracleGro on them twice.
I didn't know that hummingbirds would be attracted to zinnias, but they were and they loved them. The little yellow "mini-flowers" (?) inside the big flowers must have had good nectar.