yes, you can plant them all the way to the heads- they will grow more roots. seems like the 4/6 packs i buy to plant are often leggy and i do just that. my daughter brought one home much later last yr and i was amazed that they'd bothered; it was getting so warm. guess they don't take each areas' weather into account. we never turned hers in, though it did make it through the summer(it was on the semi shady cooler back deck). it got dinner plate size, but never headed up. so, maybe a little bigger than average, esp given we kept it in a 3 gallon pot, but not huge. not sure what to tell you to grow big ones other than keep them healthy- either spray with bt or cover with reemay to keep the loopers and other bugs off, and keep the watering as consistent as you can. good soil will help tremendously, too. i'm sure fertilizing would play a role, but i don't know how much is right for big guys. i've never been concerned with how big mine get, so much as not spraying with chemicals and them being yummy (so keeping the caterpillars & slugs off).