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St. John's Wort Flowers
  • 30 to 90 centimetres tall

  • Contains a toxin that causes sun sensitivity in light-coloured livestock

  • Introduced from Eurasia by gardeners

St. John's Wort Flowers

St. John's wort
(Hypericum perforatum)

Quick Identification

  • Bright yellow five-petaled flowers

  • Oblong leaves with transparent spots when held to the light

  • Plants turns a rusty red colour at maturity

Contains a toxin that causes grazing animals to become sensitive to sunlight resulting in intense skin irritation. St. John's wort invades pastures, ranges, and forests, competing with native species and reducing forage fore wildlife and livestock.

St. John's wort is controlled by bio-agents. Effectiveness varies as the insect population goes through natural cycles. Selective herbicides are effective.

Five-petaled yellow flowers grow in clusters

Leaves & Stems
The woody stem branches in pairs, going from yellow-green in spring to rusty red at maturity. The leaves are oblong and show transparent spots when held to the light.

Preferred Habitat
Grows best in course soil with acidic to neutral pH but can succeed under many different habitat conditions.

St. John's Wort

A stout taproot with many branching lateral roots.

Reproduction & Dispersal
Spreads by seeds carried by wind, water, humans, and animals. New plants grown from root crowns and lateral roots.


One plant can produce 15,000 to 34,000 small, dark, capsule-like seeds. They can remain dormant in the soil for up to 10 years.

Interesting Facts
St. John's wort has been used for centuries medicinally for a number of ailments, including depression. It should not be used without consulting a medical professional as it can interfere with other medication and cause photo sensitivity and a number of other unpleasant side effects.

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