They will re-sprout. Cut them near the ground. Use a spade bit (a.k.a. paddle bit) to drill several large holes in the trunk. Fill the holes with Round-up BRUSH killer. Cover with a pot to keep the sun from evaporating the Roundup, to keep birds from drinking and to keep rain from diluting it. Do this about 3 times over the next 3 months. It won't come back and will kill the roots

Thank you. I'll be sure to use that forum when I need ID assistance again. The area where this flower is will eventually be cleared out, and I'll be putting gravel there. I've decided to try to salvage the random nice flowers that pop up there from time to time-particularly hummingbird friendly plants. A couple months ago I found some nice garden phlox and put it in a flowerbed in our yard.

As you stated, there indeed are a number of plants that have a similar appearance. A picture of one going to flower would allow a more sure ID, but, chances are that products that treat dandelions would work on this one. hortster

I think E. corollata is probably it, there is a lot of this plant growing in this area and I have photographed it often but in apparent later stages of growth; I think the unusual appearance of the flower and the overall shape of the plant is what threw me off but looking at some of my other E. corollata photos at the base of the plant, the leaf shape and habit seem to match this. I may try to confirm this later this spring and see if I can find some more plants in the same stage of growth. Thanks all.