Question: A client called the Plant Clinic this week. He and his son planted pumpkins this spring, hoping to culture a “Giant Pumpkin” for a local contest, but his pumpkins are not as large as he expected. What is the trick to growing giant pumpkins?

Answer: Pumpkins are a summer-growing annual of the genus Cucurbita, and the family Curcurbitaceae, which also includes squash and gourds. Growing giant pumpkins is a lot of fun and can win you the ooohs and ahhhs of neighbors, and the prize money of local festivals.

The production of giant pumpkins is a challenging hobby. Breaking into the world of growing a thousand-pound (or more) pumpkin involves complex genetic lineages, specialized care, some investment and a great deal of passion.

Large pumpkins need daily attention. The pumpkin has to be carefully tended through watering, fertilizing and pruning, so all the plant’s energy goes into just one or two pumpkin fruits. Here are some cultural practices you can do now to achieve your goal of a giant pumpkin, and some suggestions for next year.

First, make sure you have the right seed. Check seed catalogues and garden centers for cultivars known to produce large fruit. Suggested varieties are Atlantic Giant, Big Max, Prizewinner or Big Moon. It will take at least four months to get these pumpkin varieties to produce pumpkins in the 100+ pound range.

In the extreme case of large pumpkin growing, the seed is the result of careful crossing of different pumpkin plants, often obtained from a network of fellow giant pumpkin growers.

For example, a grower will get the seed from a pumpkin that weighed 1,500 pounds, plant it and, when it blossoms, cross with another pumpkin plant grown from a seed taken from a pumpkin that weighed 2,000 pounds! This insures the resulting pumpkin comes from winning stock. Seed prices in this arena of growers can be auctioned off at over $500 per seed! No kidding!

Start your plants in early April in a greenhouse or cold frame for transplanting in May. Keep only the strongest plants. Pumpkins love fertile soil and full sun. Before planting, spade the soil about a foot deep, and mix in some compost, manure and well-balanced fertilizer.

Mound up the soil in this area and plant your strongest-looking pumpkin in the middle. Keep the soil moist but not wet. Too much water will slow plant growth and encourage fungal diseases. Water deeply and regularly at the base of each plant, especially during hot weather, and once the fruits start to form.

Time your watering so the leaves have time to dry before dusk. This will reduce disease problems. Remove all young moisture and nutrient-robbing weeds by hand, or with a hoe, and use mulch around the plants to keep weeds from germinating. The goal is to maximize the amount of sunlight received by the leaves, as well as the amount of water and nutrients available to the roots, helping it to reach extreme weight and size.

Like other members of the squash family, pumpkins produce separate male and female flowers. Male flowers usually appear first. This is a normal growth habit and varies with cultivars. For a flower to develop into a fruit, pollen must be carried by insects from the male flower, on the same plant or different plants, to the female flower.

Poor fruit set is common during rainy weather when bees are inactive. When you are certain you have a pollinated plant, monitor the growth of the baby pumpkin fruits. When they are 6 to 8 inches in diameter, you should keep the largest and fastest growing fruit and remove the others.

Remove all new blossoms that appear. Place a piece of cardboard under your giant pumpkin to prevent soil rot. You may want to try rolling the pumpkin very gently into a new position each week to keep it from becoming lopsided.

When a plant forms runners, give them more fertilizer. Fertilize your plants with a water soluble fertilizer every week. Apply the liquid fertilizer to the soil around the base of the plant. The fertilizer should supply nitrogen, phosphorous and potassium. Alternatively, you can apply ½ cup of 10-10-10 fertilizer around the base of your plant every two weeks. But be careful! Over-fertilization can cause excessive vine growth and split your pumpkins.

After selecting your one or two fruits, pinch back the ends of your vines to direct the plant’s energy into the fruit. Some gardeners gently lift and move vines to run in one direction to make access easier. Trim the main vines and lateral vines as needed later in the season.

Harvest your pumpkin before the first frost. In our area, this is usually in late September or early October. The pumpkins should be fully colored, and the rind should be hard. Use a sharp knife or pruning shears to cut them from the vines, leaving 3 to 4 inches of stem attached to each fruit. Try to avoid cutting or bruising the fruit during harvest. Store your giant pumpkin on a pallet so that air can circulate underneath, and keep it in a protected and shaded location until you begin your giant pumpkin festival road trip.

Do you have a gardening question? Please e-mail, call or visit the Douglas County Master Gardener Plant Clinic at douglasmg@oregonstate.edu, 541-672-4461 or 1134 SE Douglas Ave., Roseburg.

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