So, I picked up a pouch of sunflowers seeds at my local Giant, (they said organic so i splurged and got them). When I got home, I looked at the pouch and saw a little 12 in the corner, and I was quick to jump to conclusions and thought it was 12 INCHES. Little did I know, it meant 12 feet, and after taking a second look, I realized they were Mammoth Sunflowers! I've planted them back around maybe early March, or late May? Only one survived (the rest fell victim to rabbits :( ) now it's planted, it already has its already growing it's 4th set of leaves. I have it in a milk jug cut in half, but I'd like to know how when to transplant it!
You can plant out your seedling, make sure you harden off the plant for a few days.
These links will help you.