Hemlock

Other names: 

common hemlock, poison hemlock, poison parsley

Latin names: 

Conium maculatum L

Weed Type: 
Occurrence: 

An erect foetid annual, biennial or monocarpic perennial, native in damp ground, ditches, roadsides, hedgerows and waste ground. Hemlock is common throughout most of Britain. It is not recorded above 1,000 ft. It may be a pasture weed and also infests cereals, vegetable crops and orchards in many countries. In southern England in 1985, it was found in 3% of winter oilseed rape crops surveyed. Plants establish readily on disturbed sites and are highly competitive with other vegetation.

The entire plant is poisonous. The active principles are several alkaloids, all of which are poisonous but the most important is coniine. Hemlock is toxic to cattle, sheep, goats, pigs and poultry, with cattle being most sensitive. Growing conditions can affect the level of toxicity but it is not as toxic as hemlock water-dropwort (Oenanthe crocata). Poisoning of livestock occurs mainly in spring when the new leaves emerge and the pasture grasses are short, but the young growth is less poisonous than the mature plant. The odd animal may get a taste for it but most are put off by the strong mousey odour of the fresh plant. Animals are more readily poisoned through eating contaminated hay or silage, although, it is said that the alkaloid content is gradually lost by slow drying. Hemlock is more palatable when wilted after cutting but the plant tissue remains toxic. Roots left laying on the soil surface after ditch clearance may be eaten by animals with fatal results.

Hemlock is also toxic to man but many medicinal and therapeutic uses are listed for the plant.

Biology: 

Hemlock flowers from June to July. A plant may produce 38,000 seeds that fall around the parent plant or are dispersed by water, rodents and birds. Some seeds remain clinging to the parent plant and dispersal takes place over several months. The embryos are not fully developed in ripe seeds and the seeds have morphological dormancy when shed. The embryos therefore require a short period of maturation prior to germination. The seeds can sometimes develop physiological dormancy when the embryos are fully grown.

In a 5-year study, seeds sown in the field and stirred periodically emerged mainly from January to April and July to September. Most seedlings emerged in the first 12 months after sowing with odd seedling emerging until year 5 when a few viable seeds still remained in the soil. The plant develops a long white taproot.

Persistence and Spread: 

Seeds may remain viable in soil for several years. Seed recovered from excavations and dated at over 50 years old has been reported to germinate. Seed in dry storage has persisted for 5 years.

Management: 

Hand pulling of hemlock may be effective prior to seed set. Mowing in spring kills mature plants, a second mowing in late summer kills emerged seedlings and any regrowth. Constant cutting of the leaves and grubbing up of the roots will eradicate hemlock. The plant debris must be disposed of safely.

Hemlock is not eaten by rabbits.

Updated October 2007.

Fully referenced review: