AgPest » Hemlock

Tall, branching, foul-smelling, ferny-leaved annual or biennial with purple spotted stems arising from a woody taproot; all parts of the plant are poisonous
Small white flowers in umbels on stems up to 2 m tall
Plants form a low-growing rosette of ferny leaves in their first year and the tall flowering stems usually emerge late in the same year or in the second year
Leaves in the rosette form can be confused with ferns or with the superficially similar wild carrot (Daucus carota), but the leaf stalks of hemlock are hairless and often have a purplish tinge
Sometimes also confused with fennel (Foeniculum vulgare). However, fennel has yellow flowers, the leaves are more finely divided and smell of aniseed. By contrast hemlock leaves have an unpleasant strong musty odour.
Has been known to poison children who have used the hollow, hairless stems as ‘pea-shooters’.

Native to Europe, Asia and North Africa, hemlock has now spread to many other parts of the world
Probably introduced to New Zealand by herbalists in the mid-1800s but some introductions may have been accidental, with seeds arriving in hay, straw or bedding
Added to the Noxious Weeds Act in 1902 and currently listed as a Pest Plant on several Regional Pest Management Strategies – see your local Regional Council website for details
Now very common throughout New Zealand in waste places, riverbeds and damp areas, although sometimes also found in drier locations
Very poisonous in all its parts, slightly less so when dried. Reputedly hemlock was the poison the Greek philosopher Socrates was made to drink after he had been found guilty of corrupting the minds of the youth of Athens, amongst other charges.

In its first year the plant forms a large rosette and in the second year a tall flowering stem emerges. The plant dies after it has flowered
Seeds are quite large and unlikely to be blown far from the parent plant. However, they may float and be carried along waterways
Most seeds emerge as soon as conditions are suitable for germination, although a few may survive in the soil for up to 3 years.

Hemlock was probably first introduced for herbal purposes and used medicinally as a sedative and antispasmodic. However, it is rarely used today as an overdose can have disastrous effect on the patient.

Hemlock has been responsible for non-fatal poisonings of children in New Zealand. Children may mistake the leaves for edible garden plants, chew the seeds (the most toxic part), or use the hollow stems as pea-shooters or whistles
Can be a problem in reserves and other walkways accessed by the public.

Individual plants are usually avoided by livestock, and take up significant pasture space, and can therefore displace more valuable fodder species
A problem weed in maize crops in Manawatu and some other locations
A very common weed in paddocks grazed solely by horses, particularly if overgrazed; its presence severely limits grass growth.

Fortunately livestock normally avoid hemlock so do not suffer from poisoning
Tired and hungry animals, such as those driven or mob-stocked, are at greatest danger when exposed to large infestations of the plant
Plants become more palatable to livestock, but no less poisonous, after being sprayed or cut, so stock must not be allowed access to a sprayed infestation
Dairy cattle are the most publicised casualties of poisoning from hemlock, although in 1950 a circus elephant died of hemlock poisoning at Lower Hutt.

Do not allow livestock access to hemlock after it has been treated with herbicides, after it has been cut, or when the animals are tired and very hungry
Dense, vigorous pasture will prevent hemlock seedlings from becoming established.

Mowing of flowering plants gives temporary control, but livestock must not be allowed access to cut and wilted plant material.

Young hemlock plants can be treated with 2,4-D or flumetsulam without harming most other pasture components
Larger rosettes or flowering plants are best controlled by spot treatment with aminopyralid, clopyralid or dicamba, or treated with a rotary weed wiper using a herbicide such as glyphosate. All of these herbicides damage clovers, some of them for weeks or months, although glyphosate is the least damaging
Best controlled with post-emergence herbicides in crops such as maize
Do not allow livestock access to paddocks after hemlock has been sprayed until after the plants have lost their green colour
In orchards and waste areas, glyphosate gives effective control.

Popay I, Champion P, James T 2010. An illustrated guide to common weeds of New Zealand. New Zealand Plant Protection Society, Christchurch, New Zealand. 416 p.