Well, since I have already told the story of the Grasp vine, I might as well tell that of the hellweed.
This story begins with my sorting through of senna seed. Amongst the things I would find frequently were small brown pointed seeds, which I took for some sort of grass. Like always I saved them and planted them. In fact gave them their own pot when I planted them, as I had so many. In due course they sprouted and I wound up with the pot of...
things.
The first thing I quickly noticed is that whatever this stuff is, it is most definitely NOT a grass (by my best guess, it belongs in the Amaranth family). It is 9-12' in height and unbranched. The leaves are wide and vaguely fuzzy. In fact the best description I can come up with of the plants is what you would get if you took a bunch of long straight dogwood twigs and stuck them upright in the ground. At least until they flower.
Flower is a relative term her. The plants produce a spike of small bud like thins that are probably flowers but eventually become the seeds two. These things can actually be kind of attractive when they are young and green, since about half of the plants make them with hot pink edges.
The problem comes about once they mature and the plant dries down. At that point, those little seeds capsules become hard and woody, and the tips become RAZOR sharp. In fact the reason I call the stuff hellweed, is that, of all the plant I have grown, this is the only one that has left me with actual physical damage. One of those seeds pricked me and left me with a small but permanent scar on my right ring finger (though to be fair to it, the scar probably has more to do with the bad infection I got than the plant itself.
I still find the stuff in the senna all the time and still sort it out. Only now I'm sorting it out to KEEP from planting it again (either by throwing it out with leftover into the lawn or mulch pile, or by it leaking out of a trash bag. Any hellweed seed I see is thrown out in containers, so I can be sure it goes nowhere except the sanitation departments incinerator.