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Watermelons require a longer

Watermelons require a longer growing season with constant warm temperatures--which is why the plant favors the southern regions and can be a challenge to grow in northern regions. At this point, you can try to cut back any leaves that block the sun and stop providing water to speed up the ripening process. However, the day length is shortening as we near the autumnal equinox and nature is taking its course.
If you try again next year, you've gained some learning. Consider using transplants and selecting early varieties with short growing seasons. Mulch with black plastic film to warm the soil. Use floating row covers to protect from cooler temps.