Orginally published in the N.E.P.G.A. Newsletter - June, 1996
Pollination of Dill Atlantic Giant pumpkins can take place in three different ways; (1) Cross-Pollination, when a bee or a grower deposits pollen from a male flower of one plant onto a female flower of a different plant, (2) Self-Pollination is possible, since a pumpkin plant has both male and female flowers and can therefore pollinate itself, and (3) Open Pollination, which generally occurs in an open field where bees make numerous visits and supply pollen to the females from many different male flowers from the same or other plants.
What exactly is pollen? If you look inside a newly opened male flower, you will see a yellow dust-like substance inside the flower. Most of the pollen will be clinging to the anthers in the central part of the male flower. Pollen grains are tiny, measuring only 250 microns. As small as they are, pumpkin pollen grains are among the largest in the plant kingdom. There are two parts to a pollen grain; the tube nucleus and the reproductive nucleus, which we will talk more about later.
What actually happens when you pollinate a female flower? For newcomers to pumpkin growing, the female flower has a small pumpkin at the base of the flower, while the male has only a flower on a long stem. When the male pollen touches the stigma (center) of the female flower the pollen grain interacts with the hormones and enzymes on the surface of the female stigma. This reaction triggers the pollen grain to germinate much in the same way as a seed germinates. The tube nucleus inside the pollen grain begins to grow and actually forms a small tube, which travels down through the female stigma and connects to a single ovule. If you cut open a female flower you will see hundreds of tiny white ovules which are the immature seeds. The reproductive nucleus of the pollen grain can now travel down the pollen tube and fertilize the ovule. The genetic information from the male flower has now been transferred to the female ovule, or seed. Some growers hand pollinate specific male flowers to a female blossom to try to breed specific characteristics from one plant into another plant. If you are trying to do cross pollinations, you will not see the results of your experiment until you plant the resulting seeds the following year. It is easy to see that an open pollinated female flower may have pollen grains from dozens of different plants. The different pollen grains will mate up with their own ovules and produce seeds with many different genetic traits. Experienced growers will tell you that if you plant 10 seeds from an open pollinated pumpkin, you will see many different resulting shapes and traits the following year. It is possible to straighten out an open pollinated plant and make the seeds uniform genetically but it would take 5 or 6 years of hand pollinating and inbreeding, which would be quite a project.
Many growers hand pollinate and keep the bees away so they can apply one type of pollen that they like to a specific female flower. To do this you must find male and female flowers the night before they open and cover them with cheesecloth. This keeps the bees out of your blossoms so they don't contaminate the flowers with unwanted strains of pollen. No matter how early you get up in the morning the bees usually are already out and pollinating. The best time to pollinate is early in the morning when the flowers first open. Uncover the male flower and transfer the pollen onto a soft bristled artist's brush. Next, uncover the female flower and gently brush the male pollen onto the stigma (center) of the female flower. After you are finished, cover the female flower with the cheesecloth to keep the bees out. Within two days the male pollen will have mated with the ovules in the female flower and the cheesecloth can be removed. Covering of the blossoms is only necessary if you are hand pollinating.
You can see the different parts that make up the male and female flowers. The center of the female flower (stigma) has a certain number of sections, or segments. These are called carpels. I have seen blossoms with from 3 to 7 carpels. Each carpel will correspond to a seed cluster inside the fruit. You would think the more carpels the better, but I am not sure. In 1989 I had a 7 carpel female weigh 425 pounds and a 5 carpel female pumpkin weighed over 600 pounds. Reports from other growers seem to indicate large pumpkins are likely from 4, 5 and 6 carpel females. I have not heard of any large pumpkins from a 3 carpel female flower. Most growers like to pollinate flowers with 5 carpels.
By the time you get this letter it will be almost time to pollinate. Some early birds may have pollinated already, but pollination in the first week or two of July leaves you enough time to develop a big pumpkin. The exception to this would be in the cool, slow growing areas like Canada and the Pacific Northwest. Growers there like to pollinate in late June.
Some people wonder what part of the plant is best to set fruit on. Some growers set fruit 5 feet, 10 feet or 15 feet out on the main vine, conducting several breeding experiments. Many people hand pollinate and let the bees get in also for extra pollinating.
You will find that quite often you will get a female that appears to have developed and then withers and falls off the vine. This is not unusual so just keep pollinating your females and eventually you will have success. A female will wither because the pollination did not take place properly. There are several genetic and environmental factors that determine the success of pollination. The chromosomes (20 male and 20 female) must be compatible, proper hormone levels must exist in the plant, viable ovules must be present in the female and viable pollen grains must be present in the male. If the temperature is too cool (below 70) the pollen tube grows too slowly in the female and never makes it to the ovules in time. Temperatures above 90 to 95 degrees are very bad for pollinating pumpkins. High temperatures can kill the pollen grains before they can complete their job. A few years back I did over 100 hand pollinations in a large field during a time of 90 to 100 degree weather. I pollinated the different plants with different male flowers and all of the pollinated females aborted. When the weather cooled to 80 to 85 degrees, almost every plant set fruit. If you have a female flower open on a very hot day you can expect the embryonic pumpkin to abort.
Some people set fruit on side vines. We have seen pumpkins over 600 pounds set virtually anywhere on the plant, so pollinate everything that shows up and make your determination later as to what pumpkin will remain on the plant. Don't pick off all the female blossoms until you are sure you have one or two good pumpkins going. Pumpkins can get as large as a beach ball and still abort. If the young pumpkin has a shiny appearance and is growing rapidly each day it is usually on for the long run. If you are planning to leave 2 pumpkins on the same plant try to set them on separate vines. If you have two pumpkins within a few feet of each other they will be competing with each other. The pumpkin growing fraternity is divided on this. Some growers allow only one fruit to develop per plant others allow two. There has been more than one case where two pumpkins on the same plant have weighed more than 800 pounds each so a good plant can apparently support more than one pumpkin as long as they are not close together. If you are pruning fruit off, let them develop to beach ball size in order to get a good look at them. Then thin out your fruit to the best looking contender. Good Luck!