There are several closely related species of these lovely relatives of parsley, dill and Queen Anne's Lace. We planted a few in various spots on the trail and they have spread into many places, like into the knotweed patches that you see here. Golden alexanders are what is called a short-lived perennial, but they will seed readily into areas where there is open soil. We have lots of areas like that, and we welcome this native plant wherever it occurs.
These plants are beautiful all year; the early golden flowers give way to attractive starburst fruiting structures, and in the fall the leaves turn a lovely bronzy color.
Golden alexander leaves feed larvae of swallowtail butterflies, and the flowers are an important early source of nectar and pollen for small bees.
If you want this in your garden, you will probably need to get a plant or start some seeds. Once you have it, you can watch for seedlings to come up; the leaves are pretty distinctive. Just be careful not to weed it out or suffocate it with mulch. It's easy to transplant if it comes up in a place where you don't want it. While it does spread by seed, it really isn't considered weedy like fleabane or spiderwort might be. Golden alexanders often bloom at the same time as Amsonia hubrictii (bluestar) and they look great planted together.
Zizia aurea is the species we have on the trail; it likes plenty of moisture and sun but seems pretty adaptable to partial shade. A related species, Zizia aptera, is also available. It seems to be less common and more suited to dryer areas.