Jennifer Schultz Nelson
Extension Educator, Horticulture
A few years ago I had an opportunity to visit my good friend Dr. Jeff Wong, a vegetable production professor at Cal Poly in San Louis Obispo, California. While his main work is research and teaching, occasionally local people contact him with questions. While I was visiting, he told me about a man named Paul Rys who came to him with questions about pumpkins. Giant pumpkins. Jeff decided to take me over to Paul's house to get his point across. He said I needed to see it to believe it.
It looked like any house on California's central coast–fairly dry, irrigation wherever you wanted plants to remain green during the hot summers. Its average appearance ended when we entered the back yard.
Paul's passion for pumpkins was evident from the start. Most people have an expanse of lawn with a vegetable garden tucked in one corner. This yard was an expanse of vegetable garden with a tidy square of grass to welcome you as you passed through the gate. I had never seen anything like it before, and I haven't seen anything like it since.
The vast majority of his vegetable garden was pumpkins. I was used to seeing pumpkins sprawling over compost heaps or taking over the back section of a garden, with little attention paid to them until the first fruits begin to ripen. These were the most meticulously cared for pumpkin vines I had ever seen.
To keep insects and other pests away from the crop, the pumpkin vines were covered in white row cover fabric, supported by arches constructed of PVC pipe. The outlines of vines and pumpkins were faintly visible beneath the fabric billowing across the yard like some strange animal.
The next thing I knew Paul was moving some rocks and cinder blocks from an edge of the row cover fabric, beckoning us to come inside to see the pumpkins up close.
Keep in mind that the row cover is not much taller than pumpkin vines normally grow–the maximum height was about three feet. To get up close and personal with the pumpkins we had to crawl under the row cover on our hands and knees. Some of the route was lined with old towels or carpet squares, but much of it was just good old garden soil.
Just when I was starting to wonder what in the world Jeff had gotten us into, an incredibly, ridiculously large pumpkin came into view. It lay on its side, resting on a large carpet square, wrapped carefully in an old blanket. Now I knew why Jeff insisted we make the trip to personally see the pumpkins–I would have thought he was just telling tales if he described this scene to me!
It turns out the carpet square and blanket were not an example of obsessive gardening. They really did serve a purpose. The carpet allowed for some air circulation under the pumpkin, to prevent it from rotting. The blanket helped protect the pumpkin from sunburn. All this hard work would hopefully produce a prize-winning pumpkin.
Jeff and I listened to Paul's description of the pumpkin that had produced the seed for that particular vine. The seed source was a prizewinner, and he had high hopes that the pumpkin we were looking at would attain the weight of a prizewinner too.
Undoubtedly, giant pumpkins owe much of their incredible size to genetics. The care given to the developing pumpkin also influences the weight it gains. An average pumpkin given the best of care might end up weighing twenty-five pounds instead of the usual fifteen pounds. But a pumpkin vine possessing the genes for giant size can achieve gargantuan proportions, many over one thousand pounds. The world record for pumpkin weight was broken this month by Ron Wallace of Greene, Rhode Island. His world record pumpkin weighed in at 1,502 pounds, earning it the title of world's heaviest pumpkin, as well as the first pumpkin to weigh over 1,500 pounds.
Most people that grow giant pumpkins hand pollinate the female flowers to insure they know the parentage of the seeds contained in the pumpkin that develops. You would think that by selecting the largest parents, producing even larger giant pumpkins should be fairly easy. But figuring out the genetics of pumpkins is proving to be a complicated endeavor. Pumpkins have a very diverse genetic background by themselves, plus they can cross with other types of squash, leading to a wide variety of genes in any one plant. Sometimes crosses between plants unite recessive genes, or otherwise activate genes that were suppressed in the parent plants.
For example, the gene for orange color is dominant, and the gene for green color is recessive. Each plant contains two copies of the color gene–one from the male parent, and one from the female parent. If one or two copies of the orange gene are in the plant, the pumpkin it produces will be orange. The only way to get a green pumpkin is if the plant has two copies of the green color gene.
Crossing two orange pumpkins will not necessarily result in seeds that produce orange pumpkins. After crossing the two orange pumpkin plants, the pumpkins produced will be orange like the female parent. The seeds inside each contain one gene from the female, and one from the male plant. Growing these seeds may produce some green pumpkins if each parent had one copy of the orange and one copy of the green color gene. Each seed produced from a cross of parents with one orange and one green gene has a 25% chance of producing green pumpkins.
This seems relatively complicated, but in the world of genetics it is actually a pretty straightforward one gene system. One factor that complicates breeding pumpkins (and any other plant or animal for that matter) is that most traits are controlled by more than one gene. Plus the environment usually effects how genes act in the organism.
Paul Rys is actively breeding giant pumpkins, developing what he calls the "Beautiful Giant Pumpkin". Many times in the quest for the heaviest giant pumpkin, traits like color are not selected for. Sometimes pumpkins are crossed with closely related giant squash to produce larger pumpkins. For these reasons, giant pumpkins often have a washed-out orange, sometimes with a green cast. Paul is selecting for vibrant orange color in his giant pumpkins, striving for overall quality as well as weight.
My friend Jeff has his own hypothesis on developing giant pumpkins. He thinks that rind and flesh thickness should be selected for directly, since they contribute the most to overall weight. His thinking is that a pumpkin with a bigger diameter that doesn't also have thicker rind and flesh will weigh more, but there is a point where structurally the pumpkin won't be able to support its own weight. He wonders if you could select for a pumpkin that is nearly solid on the inside. It's an interesting idea, one that if it worked could conceivably produce pumpkins weighing far more than 1,502 pounds.