Hemlock Water Dropwort

Control of Hemlock Water Dropwort (Oenanthe crocata) with Roundup ProBio and ProVantage

Hemlock Water Dropwort

Background

Hemlock Water Dropwort, (Oenanthe crocata), is the most toxic plant growing in Britain to both humans and animals. It contains a powerful neurotoxin called oenanthetoxin, which triggers spasmodic convulsions, usually followed by sudden death.

Whilst the most toxic part of the plant are the tuberous roots which give rise to the alternative name ‘Dead Man’s Fingers’, all parts of the plant are poisonous and only a small amount of plant material can be fatal.

If forage is in short supply as happens in dry summers like 2006 or when stocking rates are high, stock may be tempted to eat the foliage. The risk is also high after ditching operations have been carried out, when the roots are exposed.

Hemlock Water Dropwort is a member of the Umbellifera family but is distinctive because of its broader leaves and its narrow flowering period during June/July. (Compare it with Cow Parsley, flowering from April to June, Hedge Parsley or another common wetland umbellifera, Wild Angelica flowering from July to September).  It is a perennial growing to over a metre high in shallow water like streams, ditches, rivers and lakes and is particularly prevalent in western Britain.

Apart from ditch clearing, animals and people are far more likely to come in contact with the leaves than the roots. The leaves are dark green and look very similar to Italian flat-leaved parsley with a distinctive celery or parsley smell.

Control

Mechanical

Digging up and handling the roots presents a hazard to the operator and plant material must be carefully disposed off, ideally by burning.

Chemical

Use of Roundup to spot treat individual plants is an excellent way to control the weed, remember that use of any pesticide in or near water requires Environment Agency approval.

Apply Roundup just as the plant comes into flower, but before seed set or senescence in June or July. Glyphosate also kills grass, so spot treat with care to minimise any grass dieback.

An alternative method is to use a Hand-held weedwiper.

Treatment with Roundup whilst in flower will prevent the production of viable seed.

Monitor the area each year and spray any new plants arising from seed already in the soil.

Caution

  • All parts of the plant remain poisonous after spraying and so stock should be excluded from the treated area until all the foliage has gone brown and died back completely.
  • Wear protective gloves and coveralls when handling the plant whether alive or dead.

Rates, Timings and Water Volumes

MethodDose Rate of Roundup ProBioDose Rate of Roundup ProVantageApplication Advice
Spot Treatment 5l/ha, 1:40 dilution in a standard knapsack sprayer 3.75l/ha, 1:57 dilution in a standard knapsack sprayer Spray just as the plant comes into flower, but before seed set or senescence in June or July
Weed Wiper 1 part Roundup Pro Biactive to 2 parts water 1 part Roundup ProVantage to 3 parts water Useful method to minimise grass die-back