POISON IVY VINE  all parts & the smoke from burning them
 

http://en.wikipedia.org/wiki/Toxicodendron_radicans  Urushiol-induced contact dermatitis is the allergic reaction caused by poison ivy. In extreme cases, a reaction can progress to anaphylaxis. Around 15% to 30% of people have no allergic reaction to urushiol, but most people will have a greater reaction with repeated or more concentrated exposure.

 

WHITE HELLEBORE / FALSE HELLEBORE – looks like the edible hosta

https://backyardsafari.wordpress.com/2010/04/26/lessons-from-an-edible-plant/   The plant contains calcium oxalate crystals as raphides in all parts, and because of this consumption of the raw plant material results in a powerful burning sensation. It can cause irritation of the mouth and digestive system, and on rare occasions the swelling of the mouth and throat may be severe enough to affect breathing.If the plant is properly dried or cooked it can be eaten as a root vegetable. A preparation of the root was reported to have been used by Native Americans as a treatment for sore eyes. Preparations were also made to treat rheumatism, bronchitis, and snakebites, as well as to induce sterility.
 

NIGHTSHADE / HORSE NETTLE  - all parts toxic
 

http://en.wikipedia.org/wiki/Solanum_carolinense While ingesting any part of the plant can cause fever, headache, scratchy throat, nausea, vomiting, and diarrhea, ingesting the fruit can cause abdominal pain, circulatory and respiratory depression, or even death.

BITTER NIGHTSHADE – avoid all parts, berry toxic

CANADIAN MOONSEED – all parts toxic, no tendrils as with edible grapevines, half-moon seeds, unlike round grape seeds

http://en.wikipedia.org/wiki/Menispermum_canadense   All parts of these plants are known to be poisonous. The principal toxin is the alkaloid dauricine. The fruit of Canada Moonseed are poisonous and can be fatal.

DOGBANE – all parts toxic, except perhaps seeds (see below), looks like milkweed, but woody & multi-stemmed
 

 http://en.wikipedia.org/wiki/Apocynum_cannabinum All parts of the plant are poisonous and can cause cardiac arrest if ingested.

GREATER CELENDINE  - all parts toxic, orange-yellow latex sap

http://en.wikipedia.org/wiki/Chelidonium  The whole plant is toxic in moderate doses as it contains a range of isoquinoline alkaloids but there are numerous therapeutic uses when used at the correct dosage.

 http://www.botanical-online.com/english/celandine_poisonous.htm Yes, celandine is a toxic plant. It contains a corrosive caustic juice, and it is rich in toxic alkaloids. Its medicinal use must be carefully controlled . It is Not recommended for internal use.

YEW BUSH / TREE – http://www.wildfoodies.org/Yew.htm - Most parts toxic, except red sweet flesh that surrounds its highly toxic black seed

 

http://en.wikipedia.org/wiki/Taxus_baccata Symptoms of yew poisoning include an accelerated heart rate, muscle tremors, convulsions, collapse, difficulty breathing, circulation impairment and eventually heart failure. However, there may be no symptoms, and if poisoning remains undetected death may occur within hours. Fatal poisoning in humans is very rare, usually occurring after consuming yew foliage. The leaves are more toxic than the seed.

 

GIANT HOGWEED, GIANT COW PARSNIP – highly toxic
 

 

http://en.wikipedia.org/wiki/Heracleum_mantegazzianum - The sap of giant hogweed causes phytophotodermatitis in humans, resulting in blisters, long-lasting scars, and—if it comes in contact with eyes—blindness. These serious reactions are due to the furocoumarin derivatives in the leaves, roots, stems, flowers, and seeds of the plant.

MISTLETOE – all parts toxic, grows in trees, visible in winter

http://en.wikipedia.org/wiki/Phoradendron_leucarpum Ingesting the berries can cause "stomach and intestinal irritation with diarrhea, lowered blood pressure, and slow pulse".

 http://www.hiltonpond.org/thisweek051208.html
https://www.allianceforthebay.org/2017/12/mistletoe-a-merry-parasite