My husband and I just started a new garden. Previously the area was covered in annual grass and assorted other weeds (purslane, spotted spurge, bindweed). We planted sunflowers, carrots and marigolds. This interesting plant came up in the line of marigolds. I've never seen anything like it before, and I don't just want to kill it. The leaves are similarly shaped with watermelons. It has lots of spines- along the stems, under the leaves, and on top of the leaves. Anybody know what it is?