MaineRose,

A spot in full sun with well-drained rich soil would be great. I would foliar feed them every few days with a soluble nutrient like Miracle-Gro and let the runoff from the foliage feed the roots. As a secret weapon, add a little sugar to the foliar feed. Plants can absorb the sugar directly, which gives them a jump start on photosynthesis, and that can make a big difference in cloudy weather.

MM

Look for the book "How to Grow World Class Giant Pumpkins" by Don Langevin. Lots of great tips. Like anything worhtwhile, the more you put into it the better the results.
Start with the right variety: Dill's Atlantic Giant unless the rules allow for a hyrid squash type. Amend the soil, amend the soil, amend the soil. Do not crowd plants. Water steadily throughout the dry spells. Remove all but the one healthiest fruit on the plant. Protect the large plant and leaves from wind damage. There's much more to it, but those'r the basics.

MaineRose,

"We've planted seeds this week and will keep them under lights until after the last frost."

You may have "jumped the gun" in planting the seeds already. Sunflowers, and particularly pumpkins, are very fast growers and it will be definitely difficult to keep them happy under lights until your safe "no frost" date. Here in our area near Augusta that date is usually taken as Memorial Day.

I won't be planting pumpkins or sunflowers inside for another three or four weeks. And, to keep large seedlings "happy", I grow them in large pots made from the bottoms of 2-liter and 3-liter soft drink bottles. I feed them special soluble nutrients, including calcium nitrate, and I give them extra bright light by overdriving my fluorescent fixtures.

I don't grow pumpkins for competition, but for eating, so my varieties are much smaller than yours will be. And I will be growing several edible-seeded varieties with hull-less "naked" seeds. The naked seeds are yummy when roasted, and you don't have to spit hulls.

As you probably already know, pumpkins come up fast and quickly become large plants. The same for sunflowers. You might want to consider starting over a little later, particularly as your sunflowers and pumpkins start to get out of hand and spindly under limited indoor conditions.

That will give you a chance to follow Patrick's advice to grow Dill's Atlantic Giant pumpkins, to get the book he recommends, and to choose extra large varieties of sunflowers. Sunflowers have gotten a lot of attention from plant breeders in recent years. Some varieties grow huge.

MM

Ha, ha---I was afraid we were planting too early, Maineman, so I did save some seeds. I am also going to hunt for the pumpkin variety that was suggested---I think the ones we planted were Big Max or something like that. I have also ordered the book. My grandson is only 4, but is already quite a gardener. Unfortunately, I am a real pumpkin neophyte, but I know we'll have fun with this even if we don't win a thing!