Answer: Giant pumpkins take quite a long time to develop and mature so you may have planted a little too late in the season. Ordinarily you'll need to start the seeds in April, keeping the plants and the soil warm with mulch material. Beginning early in the season with the right seeds, and if the weather cooperates, you stand a good chance of growing a giant pumpkin. There are a few tricks of the trade that you might try employing the next time you grow giant pumpkins: After the vines are established and you've got a few smallish pumpkins, cut back all the vines except one. Remove all but the one pumpkin that you want to grow to gigantic proportions so the plant will put all of its energy into ripening just that one fruit. Then water, water, water. Pumpkins are heavy feeders, so plant your potential prize-winners in rich organic soil amended with LOTS of aged manure. As the pumpkin grows, put a piece of plywood under it so it won't rot where it touches the earth. Just to hedge your bet, plant several pumpkin seeds a few yards apart and grow one pumpkin per plant. From this collection, you can choose the biggest to enter in the fair.
If you want to pursue this in earnest, there's a book on the subject - "How to Grow World-Class Giant Pumpkins", by Don Langevin (Annedawn Publishing, PO Box 247, Norton, MA 02766; ISBN# 0-9632793-4-3). Good luck!
Q&A Library Searching Tips