Famously known as the poison which killed Socrates, hemlock is alive and living in New Zealand, and is still capable of causing death or at least birth defects in pregnant animals which eat the foliage. Regarded as a noxious weed in many parts of NZ, if it appears on or around your land, it would pay to check its status with your local regional council.
When young, its sub-divided leaflets make it look much like a fern, with leaves which are green on top and greyish underneath. A biennial, it usually flowers (September to January) in its second year, and then the plant dies. The flowering stem can grow to 2m with small flowers carried in flat white umbels at the top. The stems are hairless, and either purple tinged or dotted with purple spots. It is sometimes confused with wild carrot (which has similar leaves), but the carrot has hairy green stems and is not poisonous.
Hemlock is common throughout the North Island and parts of the SI, especially Otago and Southland. Often seen on roadsides, river banks, under hedges, in orchards and in damp places, it generally grows just in patches, although if not killed it can overrun paddocks. The seeds are tiny dark brown barrel-shaped capsules, with light brown wavy ridges. Seeds can be carried by water, on machinery, clothing or animal coats.
Hemlock’s foul smell acts as a warning to stock, who avoid it unless hard pressed for feed. However, it remains poisonous after treatment until necrotic or withered to nothing. Like callies, clumps of hemlock will severely reduce the area actually grazed by animals.
All parts of hemlock are poisonous, containing toxic alkaloids that cause respiratory failure both in humans and other animals. Poisoning signs include (in progression): frothing at the mouth, uneasiness, dilated pupils, convulsions, and death. If the animal eats enough hemlock, death can occur within 15 minutes, so with pastured animals, which aren’t observed continuously, the first sign of hemlock poisoning is often a dead animal.
It can be grubbed, spot sprayed (eg Tordon 50 Gold) or sprinkled with weedkiller granules. For bigger infestations glyphosphate can be used with a rotary weed wiper. Be aware however, that in a wilted state it can appeal to stock (in the same manner as cut and wilted callies), so dying plants must be either removed or stock kept off the pasture until plants have rotted or withered away.
Like ragwort, hemlock produces copious amounts of seed, so attack at the ferny stage, or at latest the flowering period, is the best form of defence.