© Paula Trites
A Pumpkin is a culinary term for some varieties of winter squash. A pumpkin can be grown for its flesh (for use in pies, livestock feed, etc), for its seeds, or for its ornamental looks.
Which types of winter squash a pumpkin can be isn't determined by anything as cut and dry as the various botanical categories that squash is broken down into (e.g. Curcubita pepo, Curcubita moschata, etc.) Instead, it depends on certain physical characteristics: coarser flesh, and rinds somewhere between summer squash and winter squash in hardness.
Consequently, what we call a pumpkin is very arbitrary and subjective.  There's no actual corresponding scientific term or way of botanically classing what is and isn't a pumpkin. In fact, many languages don't even have a separate word for pumpkin -- they just use the word "squash."
Even in English, in one region, a squash may be called a squash, but in another area, people might think of that particular squash as a pumpkin.  And to make matters worse, many people feel that the best pumpkin pie isn't actually made with a pumpkin, that it's made with a squash such as Hubbard Squash.
One definitive difference between the types of squash called "squash" and those labelled "pumpkin" is that pumpkins will not store as long or as well as (winter) squashes such as acorn squash, etc.
Jack-O-Lantern type Pumpkins tend to be from the subfamily of pumpkins called "Cucurbita pepo." Giant ones tend to be from the subfamily of pumpkins called "Cucurbita maxima." Processing ones, good for pies and eating, tend to be from the subfamily of pumpkins called "Cucurbita moschata."
Pumpkins require bees for pollination. Farmers growing large fields of them often deliberately plan for colonies of bees as well.
Pumpkins do better in longer-day growing regions. The further north you go, the longer the day you get, and that is why gigantic competition ones are grown in the northern US and up in Canada, and in the UK, as opposed to Florida, Mexico or Spain.
When choosing a pumpkin that you either want to last a long time on display as an un-carved ornamental, or that you want to store, or an eating pumpkin that you want to cook with, choose one whose stem isn't missing or cut too low. A missing stem or a low-cut one will allow the pumpkin to deteriorate more quickly, and may mean that it has already started to inside.
"Hallowe'en" PumpkinsThe large "Hallowe'en" Pumpkins are perfectly fine for cooking. Some of God's creatures grow big, and some grow small, and these ones just happen to grow BIG. Only thing is, because they're so big, they hold a lot of water, and their flavour is spread out across all that bigness and diluted by all that water. And, they have been bred to not have as much flesh inside as other Pumpkins whose primary market would be cooking. When you cook with these Pumpkins, ideally you want recipes that let you cook some of the water out, concentrating the flavour. Most chefs today say not to cook with these Pumpkins at all. But many a person's grandmother made her Pumpkin pies with them, steaming the Pumpkin pulp, and then whipping it into pies that were always amazing. Maybe these grandmothers just knew how to coax performance out of food in a way that these chefs don't? Or the chefs don't have something that the grandmothers did -- grandchildren who wouldn't have been satisfied if they couldn't have picked out the BIGGEST Pumpkins at the store.
Sugar PumpkinsThese are also called pie Pumpkins. They only grow about 8 inches (20 cm) wide. Many people prefer them for pies as they are sweeter than the large Pumpkins, and the flavour is more Pumpkiny. You can get away with using these if you don't have kids -- kids are going to be mightily unimpressed if you hand them these Pumpkin shells to carve.
Jack-be-littlesThese are even smaller than Sugar Pumpkins, barely bigger than an orange, about 3 to 4 inches (7.5 to 10 cm) wide. Cut the top off, clean inside, and bake. If you want, you can stuff them with something and put the lid back on. To eat, scoop out the inside with a fork or spoon. Don't try to do anything with these that requires peeling, as the skin is very hard to peel.
And for what it's worth, 1 medium Pumpkin weighs 5 pounds (2.25 kg). What's considered a medium Pumpkin, though, varies considerably based on what you -- or your children -- think is worthy of being called a large one!
Once you've cut into it, use it at once, of course.
Pumpkin is a low acid food, and so is hard to preserve safely using traditional preserving methods. Some home canning recipes added sugar or lemon juice to help preserve the pumpkin and keep it safe, but even using the same recipe on different varieties of pumpkin, they found too much variation, some of which would not have necessarily inhibit the development of pathogens. Studies in the 1970s at the University of Minnesota (Zottola et. al, 1978) showed there was too much water variation in puréed pumpkin to allow them to calculate reliably.
The United States Department of Agriculture (USDA) used to give procedures for home canning of Pumpkin Purée, but since 1989 they have withdrawn those guidelines and now recommend against home canning. They do give guidelines for canning cubed pumpkin (or squash) using pressure-canning, but are careful to add": "Caution: Do not mash or puree." They did not say why unmashed -- e.g. cubed pumpkin -- was still considered safe, even though it would presumably have the same (unreleased) water content in it that the mashed would.
Eventually, health authorities decided that they just couldn't say what was safe, so now they say "freeze it."
Freezing: Wash. Don't peel. Cut into, remove seeds. Chop into small pieces. Boil, steam or bake until soft. Let cool. Scrape flesh from rind. Package and freeze.
Literature & Lore
 "…on the one hand the family is extremely polymorphic for size, shape, and colour of the fruits; on the other, the fruits of some cultigens of one species can exhibit great similarity to the fruit of a different species. Often, the result has been different names for the same species and the same name for different species" -- Paris, H.S. Summer Squash. In: Prohens-Tomás, J., Nuez, F. (Eds)., Vegetables I Asteraceae, Brassicaceae, Chenopodicaceae, and Cucurbitaceae (Handbbok of Plant Breeding). New York: Springer. 2007. P. 354.
Andress, Elizabeth L. Canning Pumpkin Butter and Mashed or Pureed Squashes. Athens, GA: University of Georgia, Cooperative Extension Service. 1997.
Pendleton, Scott. Where Pumpkin Pies Come From. The Christian Science Monitor. 31 October 1990.
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