They should germinate just fine. Around my birdfeeder I get a constant procession of seedlings. Black oil sunflowers are about 5 feet tall with a flower seedhead that is maybe 3-4 inches across (wider flower with the petals). The gray striped ones are the big guys that go up around 8 feet or more with a seed head that is a foot across. Last year I bought a few packs of sunflower seeds so I could plant specific varieties. There were some GIANTS that must have been over 12 feet tall with multiple big flower heads. They towered over a ten-foot fence. I planted some in several garden beds, so walking into the garden was like going into some kind of sunflower jungle. I'm doing it again this year.