Combine a granular slow-release fertilizer with the potting soil. Use 2 1/2 tbsp of a 14-14-14 fertilizer blend for 5 gallons of soil, incorporating the fertilizer evenly into the soil.
Fill a 5-gallon tub, that is nearly as wide as it is tall, with the moistened potting mixture to within 3 inches of the rim. Use a tub with bottom drainage hole or drill four to six 1/4-inch holes in the bottom to supply drainage.
Sow the pumpkin seeds 1 inch deep in the soil mixture. Plant two or three seeds per container, planting them near the center of the container. Not all seeds germinate successfully.
Install a tomato cage in the center of the pot. Attach stakes to the cage legs and push the cage into the soil until the stakes hit the bottom of the pot. This anchors the cage so it doesn't blow over or topple under the weight of the developing pumpkins.
Thin the pumpkin seedlings after they reach a 6-inch height or produce their second set of leaves. Pluck out the weaker seedlings so only one remains in the pot.
Feel the soil daily and water when the top inch feels dry. Pumpkins my require twice daily watering in hot weather when the soil in the container dries out more quickly.
Train the growing pumpkin vines up the center of the tomato cage. Pull branches and vines through the cage opening as necessary to support the plant. Allow the vine to spill over the outside of the cage once it reaches the top.
Apply a soluble 15-30-15 fertilizer once the first pumpkins begin to form after flowering. Dilute the fertilizer in water and apply at the rate recommended on the package.
Harvest the pumpkins when they develop their full color and after the rind hardens. Cut the pumpkin from the vine with a sharp knife.