CORVALLIS - For those of you trying to grow giant pumpkins this summer, here's some advice from Ross Penhallegon, an Oregon State University horticulturist, on how to get the biggest pumpkin possible.
If you get cooler summer nights, especially in the western regions of Oregon, give your pumpkin plants as much heat as possible by mulching the soil surface surrounding your pumpkin with black plastic. As the weather gets cool again in the early fall you can boost the heat by covering the growing vine with spun fiber row cover.
Only grow one plant per mound. Remove all vines except the strongest, most robust vine. When a pumpkin plant forms runners, it is time to give the plant more fertilizer. Ammonium sulfate or other source of nitrogen such as rotted manure, fish or blood meal is best. Rake it in lightly, then water thoroughly.
Remove moisture-robbing weeds.
Once three fruits set, pinch back the vines to encourage further development of the fruit. Any additional blooms should be picked. When the three baby pumpkins are about six inches in diameter, select the best-shaped one and pick the other two. All of the vine's food will now go to feed your potential giant.
For a perfectly shaped pumpkin, gently roll the pumpkin every week or so throughout the growing season. You may want to place the growing pumpkin on a large piece of cardboard or piece of wood or rigid packing foam to repel soil insects and prevent rot. Protect the pumpkin from aphids, mites, whiteflies, leafhoppers, and squash bugs. Since early pest detection and treatment result in better control, inspect your plants frequently.
Fertilize your single pumpkin every two weeks or so with decomposed manure, compost or other source of nitrogen. Always water it well.
Harvest before the first frost. Cut your orange monster off the vine with a sharp knife, when the stem becomes hard and woody, usually about three months after planting. In most places in the state, fall frosts can begin in September. If an early frost is predicted, use a row cover to protect the vine and pumpkin. When you cut it, leave two inches of stem on the pumpkin. A broken or missing stemmed pumpkin keeps poorly.