Poison hemlock

Conium maculatum is a herb growing up to nine feet high that resembles a carrot plant.  The leaves are large (may reach up to 4 feet) and lacy, with alternate distribution.  The small and white flowers are borne in flat clusters like an umbrella ("umbelliferae").  The stem is typically hollow and has purple spots.  The  tuberous root is white.  The fruit is small, 3 to 4  millimeters long and flattened on its sides.  When crushed, the leaves and flowers produce an offensive "mousy" odour.