Giant hogweed (Heracleum mantegazzianum)
Giant hogweed has phototoxic sap, which means that touching it in daylight can bring about large painful blisters. DO NOT TOUCH. It has leaves that are large, rounder in shape and with sharp points on its leaflets. At full height it will be well over 2m with very large flowering stems and flower heads. The dead, dried stems persist into the following spring.
Hemlock (Conium maculatum)
This extremely tall (often 2m) and slender member of the carrot family is often seen growing alongside motorways and dual carriageways but also waste ground and damp places. It has small, delicate pure white flowers and finely divided pinnate leaves, like a cross between carrot leaves and parsley. The stem often has purple splashes on it like someone has flicked purple paint on it. It has an unpleasant mousey smell. First growth appears in early spring and flowers in June/July.Hemlock is extremely poisonous! It contains a toxin called coiine, which affects the central nervous system causing respiratory failure.
Hemlock water dropwort (Oenanthe croata)
Usually found growing on riverbanks, ditches, marshes and wet areas, this umbellifer is EXTREMELY POISONOUS. Around 50-150cm tall, the first shoots appear in February and is in full flower by June/July. The leaves are smooth and hairless, pinnate with blunt tips to the leaflets - scarily similar to flat leaved parsley. The stems are green and also hairless; the base of the young leaves wrap around the stem like a sheath. Hemlock water dropwort is the most poisonous plant in the UK with all parts of the plant being toxic but most especially the roots. The plant contains a toxin called oenanthotoxin, which causes increased heart rate, seizures, abdominal pain, vomiting and, sadly, death.