Possible hemlock growth?
In reading a post on FB by the Master Gardeners, I think i may recognize the spotted water hemlock as the invasive plant that is taking over sections of my garden. I have repeatedly pulled this plant, even attempting to dig down to get the spreading root system but it comes back with a vengeance. The plant is about 2-3 feet tall with a hollow stem. The stem is solid green but does have some purple tint close to the base. The leaves do not resemble carrots, which is what the pictures of the regular hemlock look like.
Harford County Maryland
Hi - This looks like Bishop's goutweed, (Aegopodium podagraria). It is used in ornamental gardens but can get out of control and it become difficult to manage. You are correct that poison hemlock has foliage that looks similar to carrot foliage (it's in the carrot family). This page has good photos of what poison hemlock looks like, https://www.marylandbiodiversity.com/view/2342
As you've discovered, trying to hand pull or dig this out bishop's weed can be futile. We recommend removing any surrounding plants you want to save, shaking off all the soil and being extremely careful not to include any bishop's weed roots. Pot the plants or relocate them. It may be a year or so before you can plant anything in the bishop's weed beds. Then apply the systemic herbicide glyphosate. Glyphosate is a total vegetation killer, so shield nearby ornamental plants with a cardboard or a plastic shield.
Early fall is a good time to spray glyphosate because plants are then getting ready for winter and translocating chemicals down to their roots better than at other times of the year. But you could also work on it now -- and definitely remove the flowers to avoid spread by seeds. If one spray does not brown all the foliage, wait 3-4 weeks and spray again. Many tough invasive plants require more than one treatment of herbicide.
Timing is crucial when tackling aggressive invasive plants like bishop's weed. Read through the following fact sheet very carefully: https://www.invasive.org/weedcd/pdfs/wgw/goutweed.pdf
You will see that cutting, smothering, and spraying are more effective at particular times of the year.