Control of Whiteweed

Do's and don'ts of whiteweed control

Flowering whiteweed, image: Saint Mary's College of California 

Do's

  • Plan your control program, this will save time and money in the long-run;
  • Ensure cultivation, harvesting and road-grading machinery used in an infested area is washed down to remove whiteweed seed and root fragments;
  • Use sheep grazing to keep whiteweed down in pasture - but remember! sheep will not remove the weed from the soil;
  • Remember that when grazed pasture infested with whiteweed is cultivated for pasture or cropping, whiteweed numbers may increase dramatically from root fragments;
  • For cereals or forage-grass cultivation in whiteweed infested areas, use the herbicide chlorsulfuron to control whiteweed; and
  • For cultivation of other crops in whiteweed infested areas, consider one or more initial cereal crop cycles with chlorsulfuron treatment before planting non-cereal crops.

Don'ts

  • Don't introduce whiteweed to whiteweed 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 expect sheep grazing to remove whiteweed from an infested area - whiteweed roots will remain in the soil and regenerate when sheep are removed;and
  • Don't cultivate herbaceous perennial crops (e.g. lucerne, essential oils) in areas heavily infested with whiteweed.


Spread of whiteweed

  • Whiteweed is a prolific seeder. However whiteweed seeds do not readily germinate under dense whiteweed infestations and seed is not usually important in the regeneration of established infestations. However, seed is an important method of spread of whiteweed into new areas.
  • Whiteweed has an extensive and persistent root system, reaching depths of over 1.5 meters. Regeneration of plants from root fragments following cultivation is rapid, and fragments are the principal method of spread of whiteweed within and between paddocks.

Avoid the introduction of whiteweed

  • Preventing the introduction of whiteweed to whiteweed free areas is the best means of control. Good hygiene practices are vital.
  • Machinery can carry both seeds and root fragments of whiteweed. Thorough cleaning of cultivation, harvesting and road-grading machinery which has been working in infested areas will greatly reduce the risk of spread into other areas
  • Hay and crop seeds can contain whiteweed seed if they have been sourced from infested paddocks.
  • Feed out hay in restricted areas on the farm and check for whiteweed seedlings.
  • Check cropping paddocks at an early stage of crop growth and remove any whiteweed before they become well established.

Cultivation

  • When infested pastures are cultivated for resowing to pasture or for cropping, whiteweed numbers may increase dramatically. This results from both the fragmentation of the root system during cultivation and the removal of grazing pressure.
  • Cultivation alone will not provide long-term control of whiteweed. Continuous cultivation will kill whiteweed roots in the topsoil, however new shoots can continue to emerge from roots below the depth of cultivation.

Grazing

  • Whiteweed is readily grazed by sheep and may not be noticed in heavily grazed sheep pastures. However, grazing only removes the shoots of the plant and whiteweed will persist even under very heavy grazing.
  • Cattle and horses usually avoid grazing on whiteweed.
  • If a landholder resows to pasture or commences cropping in infested paddocks without being aware that whiteweed is present (due to heavy grazing by sheep), whiteweed may appear in large numbers from regeneration from root fragments.
  • Whiteweed may taint the meat of grazing stock, and sheep should not be used for control of the weed immediately before being sent for slaughter.

Chemical control

  • The choice of selective herbicides for whiteweed control in pasture and crops is relatively limited. See Herbicides for Whiteweed Control for more information.
  • No currently available herbicide can eliminate an established whiteweed infestation in one application. Follow up treatment is essential.
  • Selective chemical control of whiteweed is only available for cereal crops, forage grasses, pastures and some fruit crops. For other crops, whiteweed infestations should be reduced prior to crop sowing.

Integrated management of whiteweed

Control in permanent pasture

  • Heavy grazing by sheep is the best method of controlling whiteweed in permanent pasture. Stock that have grazed mature whiteweed plants can spread seed to clean areas, so grazing should be timed to avoid seed production.
  • Where sheep grazing is not possible, slashing of the flowering and seeding stems in spring may be used to reduce competition and to open up the infested area for grazing by cattle or horses. However, slashing will have little long-term effect on the whiteweed infestation, even if seed production is totally prevented.

Control in cropping systems

  • In pasture or fallow, treat whiteweed with herbicide at the bud stage in spring prior to cropping. Where crop sowing is to be undertaken immediately after herbicide application, glyphosate or glyphosate/dicamba mixtures can be used. Where suitable crops (cereals or forage grasses) are to be sown in the following autumn, the herbicide chlorsulfuron can be used. Consult an agronomist. Where herbicides are used adhere to product labels.
  • Where crops other than cereals or forage grasses are to be sown, no further herbicide treatment is available. Wherever possible, a cereal or grass crop should be sown as the initial crop in such areas. These crops can be treated with chlorsulfuron as above.
  • Where the whiteweed infestation is very dense, a cereal cropping program may need to be undertaken for two seasons to gain control of the weed.
  • After one or more seasons of non-cereal cropping, the whiteweed may again increase to significant levels. In such situations, rotation back to a cereal crop and in-crop treatment with chlorsulfuron may be necessary to reduce the infestation.
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