I have a rather large plant in my garden that I don’t remember planting. I posted pictures on an Ontario gardener Facebook page but I am getting lots of different suggestions. One was poison hemlock. It does look similar from pictures I see online but would like to know for sure before I dispose of it. Thanks for your help
Your photo shows a pretty impressive plant with attractive foliage. It certainly has the appearance of Hemlock. Research on Hemlock (Conium maculatum), shows it belongs to the carrot or parsley family and there appears to be several species. Many are indeed HIGHLY poisonous. The roots being the most poisonous part of these plants. Hemlock originated from Eurasia and is now widely naturalized as a weed in waste places and on roadsides, usually in damp places, in parts of the U.S. and southern Canada. Not sure why this plant has appeared in your “urban” garden. If your plant is one of the poisonous varieties it would have spotted or streaked stems and an unpleasant smell.
Having looked closely at the leaves of your plant it appears to more closely resemble Water Parsnip (Sium suave), which is from the same family. The literature on this specific plant states that it is not poisonous, but since it is so similar in appearance to poisonous Water Hemlock (Cicuta maculata), you may not want to take a chance by keeping it.
It is really hard to say definitively that your plant is one of the poisonous varieties but erring on the side of caution I would dig it up.
For further reading on Water Hemlock and Water Parsnip visit:
https://ontariowildflowers.com/main/species.php?id=148
https://ontariowildflowers.com/main/species.php?id=117
https://www.omafra.gov.on.ca/english/crops/facts/ontweeds/poison_hemlock.htm
https://www.omafra.gov.on.ca/english/crops/facts/ontweeds/water_parsnip.htm