Answer: It's never a good idea to leave plant debris in the garden over the winter. The practice seems like a lazy-man's approach to composting. Rotting pumpkins will supply the soil with some nutrients, but will also provide a place for overwintering insects and disease pathogens. Plus, the seeds from the pumpkins will certainly sprout in the spring. This may not be a problem in a commercial field because the debris will be tilled into the soil before the field is planted again. I expect disease problems and errant sprouting seeds are dealt with chemically. But for the home gardener, the stuff will have to be picked up and thrown in a compost pile before the site can be used again. I'd rather handle the plants and pumpkins before they turn into a slimy mess! For the healthiest garden site, compost your end-of-season plant debris and add the compost to the soil before planting in the spring.
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