wild parsley, Pastinaca sativa, perhaps; causes phyto-photo-dermatitis. Hogweed or cow parsnip, Heracleum sphondylium, also causes phyto-photo-dermatitis. Giant hogweed, Heracleum mantegazzianum, also causes phyto-photo-dermatitis. Hemlock, Conium maculatum, also causes phyto-photo-dermatitis. All of these are in the “carrot” (apiaceae) family. All of these contain furanocoumarins which causes the reactions. the burns can cause loss of skin, permanent scarring, blindness. Take phyto-photo-dermatitis seriously. https://emedicine.medscape.com/article/1119566-overview has this to say..... “Phytophotodermatitis (PPD) is a cutaneous phototoxic inflammatory eruption resulting from contact with light-sensitizing botanical substances, also known as furanocoumarins, and long-wave ultraviolet A (UV-A, 320-380 nm) radiation. Psoralen is the particle that is active in furanocoumarins. [1] Furanocoumarins, present in some plants (eg, parsley, celery, carrots, limes), react using their parent compound, psoralen, with the UV radiation, which forms the eruptions on the skin. [2] The eruption usually begins approximately 24 hours after exposure and peaks at 48-72 hours. [3]” https://www.first-nature.com/flowers/~apiaceae.php this site has photos of the plants and descriptions. Very helpful for identification.

See if the identification guide, linked below, helps. The article was written for Australian weeds, but seems to apply to plants here, too. The "plants of similar appearance" section, toward the bottom of the article, may be of particular interest. If Poison Hemlock produces hybrids, it isn't well documented on the Internet. Conium maculatum

Yes, I think it is Torilis arvensis (hedge parsley/tall sock-destroyer), as its flower petals seem wider and larger away from the center than those of Conium maculatum (Poison Hemlock) - which seems to have uniformly narrower petals in even 5-star patterns. Hedge Parsley is what's actually responsible for those notorious Velcro wedge sock burrs!

Many thanks for posting this. I have seen this plant before and not known what it was. Cheers, Werner