It appears that, by removing the ivy, you uncovered numerous kinds of seeds that needed light in order to sprout and grow.
Thank you for including an image with your inquiry. The plant is tansy ragwort, an invasive species against which the Oregon Department of Agriculture has released 3 different natural insect enemies. The most easily recognized of those is the cinnabar moth and its banded black and orange caterpillars. None of the three will completely kill all the tansy but they will moderate its spread. Some years more tansy will be relatively common, other years less so. In other words, //
As is important with all weeds, don’t allow the plants to set seed.
Resources for you:
- Plant profile: Tansy ragwort (Senecio jacobaea): http://www.oregon.gov/ODA/PLANT/weeds/Pages/profile_tansyragwort.aspx
You can obtain first-hand assistance with additional plant identifications from the Multnomah County Master Gardeners. You take in plant samples; if so, take a good sized sample, or even an entire plant; include flowers whenever possible. The office is in Montgomery Park, 2701 NW Vaughn St., Suite 450, Portland, OR 97210. Hours: 10 AM – 2 PM, Monday – Friday, except holidays. Phone: 503–445–4608. Email: email@example.com/