First you should think about what variety of pumpkin (or winter squash) you are planning to grow. You can do so by deciding for what purpose you will be growing, for carving, size, decoration, for pies, etc. The different types of pumpkins have different harvest times and so knowing what you are planting will help you decide when to plant pumpkin seeds so that you will harvest the fruit at the right time.
At what time are you going to need your pumpkins? If you plant pumpkin seeds early you could conceivably harvest pumpkins by mid-summer but if you waited you could harvest in September or October. Planting early is a good method to limiting insect and disease damage that will intensify later in the season. The only issue with planting early is that you may be storing your pumpkin for a few weeks or even months.
Your region also plays an important role in deciding for you when to plant pumpkin seeds. Your pumpkin plants should never be exposed to early or late season frosts that can happen with planting too early or too late, respectively. The only exception to this is if your plants are in a greenhouse or kept warm by some other means. The fact is pumpkin and winter squash plants will not tolerate any kind of frost, if they experience one they will die. Shorter growing seasons are typical in the Northern regions of North America and Europe as well as higher elevation (mountainous) locations (the growing season refers to the interval between last and first season frosts). More temperate regions such as the Southern United States, Central America and Southern Europe have an extended length growing season, but pumpkin plants in these regions must withstand in some cases very intense heat. Very humid environments can also increase the propagation of disease pathogens and their exposure to the plants. Know well the region you are growing in.
If you are growing pumpkins for carving, you may choose the Connecticut field pumpkin or the Howden field pumpkin. These as well as other Jack o Lantern type varieties are members of the C. Pepo species and will harvest (from seed to fruit maturity) in about 100 days. You will probably want to have these pumpkins by September or October, so planting in early to mid June would be about right. You might even choose to plant in late April or early May (if your region allows, most commonly in more temperate, warmer climates) but you would harvest sometime in early August, this would mean you would need to store the pumpkins for several weeks, perhaps even a couple of months. Normally, this shouldn’t be a problem unless the fruit was harvested immaturely or the plants underwent severe disease and insect damage.
If you plan on growing giant pumpkins for size and weight, you might select the Atlantic Giant, Big Moon, or Big Max variety pumpkins. These are all members of the C. Maxima species and will harvest from seed in about 120 to 150 days. Growing giant pumpkins that are perhaps 200 to 300 lbs can be harvested closer to the 120 day range (of course they may not yet be fully matured). Much larger giant pumpkins will require 150 days or more to harvest (healthier plants will typically grow longer producing larger fruit). If you’re planning to enter a giant pumpkin weigh off you would want atleast 5 months of growing (the bigger the pumpkin the longer the growth). Plan to grow large and start early. If say for example, the weigh off is September 25, start your seeds around April 25 would be a good time frame to begin.
If you want to grow for Fall decoration, you have plenty of pumpkin varieties to choose from that offer different colors and shapes. You may select the Lumina (white color), Jarrahdale (Bluish color), or Rouge Vif d’Etampes aka Cinderella (bright red color) pumpkins. These pumpkins (sometimes referred to as winter squashes) are more flattened or rounded in shape and bring such a classic, beautiful old world look to any Fall display. You can expect them to harvest in about 110 days give or take a few days depending on size. These pumpkin varieties belong to the species C. Maxima and are also very good for baking.
If you are planning to cook your pumpkins, you have many really good choices to choose from. The Musque de Provence aka Fairytale, Old Timey Field aka Indian River pie, Butternut Squash or Long Island Cheese pumpkin varieties have exceptionally sweet flesh and are members of the C. Moschata species. These pumpkins (or winter squashes) grow a little bit longer for their size and will harvest in 115 to 130 days. The varieties of this species tend to keep well in storage for a very long period and so you can plan to plant very early in the season to minimize insect and disease damage and still have fresh pumpkin to cook with in November in time for Thanksgiving.
The New England Sugar pie and other small Jack o Lantern type baking pumpkins also have a tasty flesh great for baking and can harvest in as little as 80 to 90 days. These pumpkins will commonly grow to around 6 lbs and are members of the C. Pepo species.
With these considerations in mind you will be able to make the right decision as to when to plant to so that you can harvest pumpkins at just the right time.