This is an easiest way to make pumpkin vase, all you need to do is cut the top part of the pumpkin and remove the seeds, then place a bowl or other container you need inside before arranging flowers (Image Source: proflowers).
You may have the doubt as I did that how long will the pumpkin last? Will it get mold? Well, you may have some trick if you have experience for your pumpkin lattern. Or you can try this add bleach inside to attempt to keep the mold away and keep the planter from rotting right away (Souce: baptismalgownsplus)
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Or make or carve a pumpkin face for Halloween~ Having fun there~
You can get the perfect pumpkin centerpiece tutorial from meaniegreene: