When spring finally arrives put then outside in full sun. Keep them moist, stand them in shallow trays of water if you are not home enough to keep on top of watering. let them grow without pruning for 2 or more years. Let them get big. You want trunks thicker than 2 fingers (2 or more inches thick) before pruning again. Let willows grow out to 3, 4 or more, even 8 or 9 feet to fatten up the trunks, them cut them down to 4 inches. Them grow out again and repeat.