the link suggests it's invasive in some areas ... might want to research that... before you go encouraging it ... a good hint as to whether something is invasive.. is the fact that you found the volunteer in the first place ... not everything that spreads around is all that bad... but you need to check it out ... ken Here is a link that might be useful: link

thith-ul ... if you lithp ... named after Thith-ul B. DeMille, i think ... the fact that you did not plant it under the rock.. is a good indication that it is a weed.. which is defined as ANYTHING you dont want in your yard ... let me guess.. how far from the bird feeder??? it will slice you open like there is no tomorrow .. its one of the mean ones .. or really hurt kids feet if stepped on .. but if that can be avoided... it is a truly marvelous show in fall.. when the finches go nuts over it.. hanging upside down to get at it all ... your choice.. ken

I agree that it looks like black locust, based on the compound leaves, the leaflet shape, and the root suckering. I would guess that perhaps one of your neighbors has black locusts and the roots have moved into your garden. I am really sorry! It sounds like it will be something of an ongoing battle. I think maybe your "disturbing the soil by planting, then watering has stimulated plant growth" or even that the neighbor's trees have just gotten large enough for interference in your garden. I did a search for black locust control. There are a lot of really nasty chemical controls, but happily it sounds like glysophate (generic roundup) will keep the ones in your garden under control if you keep after it. As Missingtheobvious said, "Cut whatever you can cut, and immediately paint the fresh wound with Roundup concentrate. That should do for it! [Roundup doesn't migrate along the roots to unrelated plants.]" http://wric.ucdavis.edu/information/natural%20areas/wr_R/Robinia.pdf http://mdc.mo.gov/your-property/problem-plants-and-animals/nuisance-native-plants/black-locust-control Maybe in the long term you can join with the neighbor with the tree(s) to get them taken down and paint every sprout with Glysophate. It sounds like otherwise you will have an ongoing battle. Maybe your treating the roots in your garden will eventually do in the trees.

THANK YOU ALL. Yes, the flower of Erigeron (I think this is Annual rather than Daisy Fleabane) looks very familiar to me and I think I've seen it in the yard before. I found a good photo of the leaf on the web and it looks just like what I have. Interesting that it's a native species. Thanks very much.