Last year I had some tomatillos planted in the center of a tomato/pepper patch, and I planted them so close together I could not get to the tomatillos. I didn't harvest any from them, and I'm guessing several dozen or more fruits dropped and rotted on the ground. This year, I have in that one area only, hundreds of little plants growing. I thought at first they were weeds, but they only grow in this one area, and now that they are getting bigger, they look very suspicisously like baby tomatillos. Here is a pic:
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is a real tomatillo that I planted:
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I don't recognize a weed that looks like that - I've started to think that they are indeed tomatillos from last year. And there are literally hundreds and hundreds of them!