There are a couple of tricks to growing large glorious pumpkins in N.E. Washington. To get a jump on the growing season, plant pumpkins indoors two to three weeks ahead of the frost free date, and don’t transplant them until they are very well rooted. They do not like their roots disturbed. For this reason, some folks prefer planting in peat pots.
I only plant the 50 plus pound pumpkins earlier than that, starting them inside four to six weeks ahead of the frost free date. Start them in 4 inch pots and place them just a few inches below a full spectrum grow light. I fertilize only after the first true leaf has emerged. The true leaf is the round leaf that forms after the first two oval leaves. Use fish fertilizer at ½ strength to start. Save the full strength fertilizer a week after the plants are in the ground. This will ensure that their roots are actively growing and the plants will be able to utilize the nutrients fully. Choose a sunny location, and amend the soil with plenty of compost and ½ a cup of bone meal mixed in well per mound. They don’t need a ton of supplemental fertilizer, and I tend to quit fertilizing once the first flowers form. Too much fertilizer makes for soft skins and pumpkins that won’t hold well off the vine.
Pumpkins will languish in cold soil and may even rot away! They originated in Central America after all! Raised or mounded soil will heat faster, but think big when creating a mound. A proper mound for pumpkins should be six inches tall and 3 feet wide! You can also use clear or black plastic a few weeks before planting. Pull a plastic sheet tight over the top of a raised bed to eliminate the air beneath. This can raise soil temperatures as much as 15 degrees! The soil will retain the heat for several days after it is removed, helping those plants to really take.
Concerned about late frost? Cut the bottom off a clean plastic milk jug and place it over your plant. This acts like a mini greenhouse over the top of the baby pumpkin plants. To prevent the wind from blowing it away, simply stick a bamboo stick through the pour spout and into the soil below.
Tried and tested in my garden: Connecticut Field for large jack-o-lantern types, Wyatt’s Wonder Giant Pumpkins for the fair, Full Moon for large white pumpkins, Baby Boo for those mini’s, and lastly Sugar Pie for you guessed it… Starbucks lattes. I mean pie! LOL.
