Intermediate status:
I said I would go for a year. In the meantime here is what I did. I have two multi tree pots, one elm and one pine. The trees all get pretty uniform water, sun, and fertilizer, so I sprayed the trees (and weeds) on the left side of the two pots. A great experiment with decent controls.
After three weeks, the weeds and grass growing on the left are doing very nicely, much better than the grass on the right. The elms may be doing a little better, with maybe a little more leaves and a little healthier appearance.
The pine tree on the left is dropping needles, turning brown and not looking happy. The other two are growing new needles. I am going to stop spraying the pine trees because it seems to be killing them (which sounds like what Clicio said). I am also going to stop spraying the grass and the weeds. I don't really want them to be all that strong.
I also sprayed the tomatoes. They are doing great.
Experimenter's required statement: no animals were harmed in the conduct of this experiment.
