Is this tall plant a weed
Hi - This is garlic mustard (Alliaria petiolata), a very invasive non-native plant that alters soil chemistry and outcompetes native plants. It spreads very effectively by seeds and the seeds can remain viable in the soil for ~5-10 years. If you have had these plants in your area for awhile, it may take several years to get them under control because of the seed bank in the soil.
Refer to this fact sheet for more specific guidance on mechanical removal, https://www.invasive.org/weedcd/pdfs/wgw/garlicmustard.pdf
Also, here is our video about garlic mustard, https://youtu.be/WvJMab8b7PQ.
Thank you so much! Will dig it up today and dispose of it in a separate bag and clean my gloves and boots to prevent spreading any seeds. Will mulch the spot and keep my eye open for any further spreading. Fortunately (knock on wood) this is the only site in the garden bed where this has appeared.