Transferring pumpkin plants
I found sprouts inside my pumpkins in November and decided to plant them to see if I can grow them further as an experiment. As a result I have 5 giant plants covering my windows, three of which are carrying growing pumpkins already. The weather is warm enough outside, but I'm not sure if my plants will survive the sudden transfer to outdoors. The temperature inside is always between 17°C and 21°C, while outside it's around 20°C and cools down to around 8°C. Should I wait a while longer? I'd just leave them indoors but my space is getting cramped.
Outside United States
Thank you for contacting Extension, and congratulations on growing pumpkins indoors. That is quite an accomplishment. You did not mention where you are, but I understand it is a warm climate.
There are two considerations in hardening-off plants to move outdoors. Temperature, as you mentioned, is one. The other is the intensity of sunlight. If the day-night temperature differentials are minor, you can probably move your plants outdoors. However, it would be best to give them some light shade protection, such as a summer-weight rowcover (sometimes called Remay) cloth. Use the covering during the mid-day hours, gradually exposing the plants to more sun over the course of a week or two.
Plants have difficulty adjusting to differences in light intensity. If the change is too great, they often lose their leaves and grow new ones because the cells cannot rearrange themselves. This is especially problematic with an annual plant and I am not sure how your plant will react; but since you need to move them outdoors at some point, do so gradually. Row-cover will also help mitigate small temperature differences (perhaps a couple of degrees).