Hi Jeff, Either plant these seedlings this fall or put the pots in a cool corner of your garage but do not let them sit outside. Speaking from experience here, the freezing and thawing will kill them. You could also bury them under a thick blanket of hay mulch, but do not let them go through many freeze/thaw cycles. If you keep them in a garage or basement, make sure they do not dry out. If you decide to plant them this fall, make sure the soil drains fairly well. If not, the seedlings will be lifted by the frost. A good layer of mulch can help prevent this, but that also attracts evil voles that will girdle your seedlings. At least sycamore is not a tasty treat for deer.