Sometime this summer, I hosted a birthday party for a young'un. We had watermelon on the deck and of course, spit seeds everywhere. On my deck, I have a small 6" transition between the concrete and the wood, and I filled that space with lava rocks, like you'd put in your outdoor grill.
Come the middle of August, and I notice one small plant working its way up between the rocks. I mention that I need to get out the Round-Up and Milady says it's a watermelon vine. Leave it alone, so I left it alone.
The darn thing has taken over the whole area.