The flax-leaved St John's Wort is now spreading at Craig y Dre in Tremadog, according to Natural Resources Wales (NRW).
BBC: St John's Wort boosted by Gwynedd cliff scrub clearance
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"Thanks to the clearance the plant is now present over a large area of the cliff above Tremadog Square, " said Doug Oliver, NRW national nature reserve warden in the area.
BBC: St John's Wort boosted by Gwynedd cliff scrub clearance
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Last week part of a long-awaited bypass to the Gwynedd town was opened temporarily to help another bicentenary celebration event in the nearby town of Tremadog, which Madocks also built.
BBC: Porthmadog marks the 200th anniversary of its 'Cob'