Square-Stemmed St John's Wort Control Guide

Square stemmed St Johns wort, photo: John Crellin 


  • 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;
  • 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 introduce square-stemmed St John's wort to square-stemmed St John's wort to-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 square-stemmed St John's wort to to flower and set seed before treatment;
  • Don't rely on one attempt at removal - follow-up is essential;
  • Don't rely on just one control method.

Spread of square-stemmed St John's wort

  • Square-stemmed St John's wort spreads by seed and by vegetative means (crown and rhizomes).
  • Seed is the main means of spread. Seed is spread in water and soil, and as a contaminant of agricultural produce.
  • Spread within patches occurs from rhizome growth and stems which develop roots near the base. Cultivation, earthworks and roadside machinery can spread fragments of rhizomes to clean areas where the fragments produce new plants.

Avoid the introduction of square-stemmed St John's wort

  • Preventing the introduction of square-stemmed St John's wort to clean areas is the best means of control.
  • Avoid spreading seed or fragments of rhizome in mud when using machinery and vehicles used in infested areas.
  • Small isolated infestations should be removed as soon as possible, preventing further spread.
  • See the Washdown Guidelines for Weed and Disease Control for detailed information on how to wash-down equipment and personnel to reduce the chance of spreading square-stemmed St John's wort.

Physical removal

  • Isolated plants and small infestations can be removed by hand, preferably before seeding.
  • Remove as much of the root system as possible as square-stemmed St John's wort can sucker from roots left in the ground.
  • Follow-up will be required to deal with any further germination or suckering.


  • Square-stemmed St John's wort can be set back by cultivation which exposes and dries out the roots, then sowing to pasture.
  • The developing pasture should be left un-grazed for the first year to allow the subterranean clover maximum chance to smother the St John's wort.
  • Over-grazing will favour the weed as it reduces the competition from the pasture plants.

Chemical control

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