Horehound

(Marrubium vulgare)

Flowering Horehound

What is horehound?

  • Horehound is a weed of crops and pasture. Horehound is also a significant environmental weed of disturbed bushland.
  • Horehound is a declared weed under the Tasmanian Weed Management Act 1999. The importation, sale and distribution of horehound are prohibited in Tasmania.

How to identify horehound

  • Horehound is a branching, perennial (long-lived) plant growing to a height of about 80 cm.
  • The stems and lower surface of the leaves are covered with white woolly hairs, giving the plant a silvery appearance. The leaves have a "crinkly" appearance and the leaf margins have rounded teeth. The white flower clusters are densely packed, forming balls of flowers that surround the upper stems at each leaf node.
  • Most seed germinates after autumn rains but some germination also occurs through winter into spring. Established plants flower over several months during summer and autumn and new growth is produced each year in autumn and spring.
  • For help in identifying horehound, search the Dennis Morris Weeds and Endemic Flora Databse. If you are still in doubt about the weed you are dealing with, contact Biosecurity Tasmania on 03 6165 3777 for help.

Paddock infested with horehound
Image top: horehound in flower, © Tasmanian Institute of Agricultural Research.
Image above: horehound infested paddock, © Tasmanian Institute of Agricultural Research.

Horehound in Tasmania

  • Horehound is found throughout Tasmania, and is most troublesome in the Midlands grazing areas. Infestations of horehound occur on roadsides, waste areas, stockyards, dry banks, near farm buildings and on the site of old homesteads.
  • Horehound can contaminate sheep and goat fleeces by its dry fruits (or burrs), leading to significant losses in fleece value due to matting.
  • Horehound is also an important environmental weed due to its ability to invade disturbed native vegetation.

What is the legal status of horehound in your area?

The legal responsibilities of landholders and other stakeholders in dealing with horehound are laid out in the Horehound Statutory Weed Management Plan.

Use \Table 1 (Zone A municipalities) and Table 2 (Zone B municipalities) in the Horehound Statutory Weed Management Plan to find out whether your area falls in an eradication or containment zone.

Detailed management and control guidelines for horehound can be found in the Horehound Control Guide. Refer also to Herbicides for Horehound Control. For further information see DPIPWE's Weed Links and Resources.


See also
Herbicides for Horehound Control
Horehound Statutory Weed Management Plan
Weed Links and Resources

Other useful links
Pest Genie
APVMA

Horehound Control Guide

Do

  • Plan your control program, this will save time and money in the long-run;
  • Consider the impact of your control methods on off-target species, especially if herbicides are used;
  • Ensure machinery and equipment is washed down between sites or prior to contractors leaving site;
  • Get in early - for new infestations, eradicate before the plants reach the flowering stage: once plants begin seeding, control becomes more difficult and expensive; and
  • Carefully time your use of herbicide for best results (see Herbicides for Horehound Control for more information);
  • Coordinate your control program with neighbouring landholders where your weed problem crosses property boundaries;
  • Revisit and regularly inspect the site and ensure follow-up is undertaken;
  • Use a combination of different control methods; and
  • Establish vigorous pasture (or native species) after removal to reduce re-infestation.

Don't

  • Don't introduce horehound to horehound-free areas (e.g. by failing to wash down machinery and equipment between sites);
  • Don't start your control program without first planning your approach;
  • Don't allow horehound to flower and set seed before treatment;
  • Don't rely on one attempt at removal - follow-up is essential; and
  • Don't rely on just one control method.

Spread of horehound

  • Horehound is spread by seed. Seed is spread by stock, as the fruit or burr readily attaches to wool and fur. Seed can also be spread in horse faeces, and in water.

Physical removal

  • Small infestations can be controlled by grubbing plants.
Cultivation
  • For dense infestations, the area should be burnt (to stimulate seed germination) and then ploughed to bury the plants. Alternatively un-rooted plants can be removed completely as partially buried plants can continue to grow.
  • Summer cultivation is preferred because the disturbed plants are readily killed by the heat of the sun.
  • Repeated cultivation is necessary to up-root any new horehound growth, followed by sowing to crop or pasture.
  • Use a strongly competitive grass/clover mixture. In areas subject to drought or severe attack by pasture insects, either phalaris or cocksfoot should be included.
  • Spotspray any surviving horehound in the new crop or pasture.
  • New pasture should not be grazed in its first year to give maximum competition with horehound seedlings. Rabbits should also be controlled on the treated areas.
  • Non-arable areas, such as stony ridges or sheep camps can be hand sown or sown through a fertiliser spreader after herbicide application.
  • Planting trees is another option where it is difficult to establish pasture.

Burning

  • Burn dense infestations to stimulate seed germination before cultivation.

Grazing

  • Horehound infestations in pasture respond differently to different grazing pressures.
  • Heavy sheep grazing (block grazing) can eliminate horehound seedlings.
  • Less intense grazing pressures can favour horehound by allowing stock to graze desirable species and avoid the less palatable weed.

Biological control

  • Biological control is the use of a living species, usually an insect, mite or disease, to control a weed;
  • Biological control will not eradicate horehound, but can be used in conjunction with other control methods;
  • Biological control agents that have been released in Tasmania include the horehound plume moth.
  • For more information on biological control programs in Tasmania contact the Tasmanian Institute of Agriculture

Chemical control

  • A number of herbicides are registered for use on horehounds in Tasmania. See Herbicides for Horehound Control for more information.
  • For best results, apply herbicides when the plants are actively growing (usually spring and autumn). Poor results will result if horehound plants are suffering moisture stress at the time of application.
  • Spring spraying will reduce the formation of flowers and seed, reducing the soil seed bank and fleece contamination.
  • Horehound shows herbicide symptoms relatively slowly; it can take between 6 and 20 weeks for symptoms to develop.
  • All herbicides require the addition of a surfactant as the hairy leaf of the plant is difficult to wet.

Herbicides for Horehound Control

Herbicides for Horehound Control


Important Disclaimer
To the extent permitted by law, the Tasmanian Department of Primary Industries, Parks, Water and Environment (including its employees and consultants) excludes all liability to any person for any consequences, including but not limited to all losses, damages, costs, expenses and any other compensation, arising directly or indirectly from using information or material (in part or in whole) contained on this website.

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