Common garden weed, spur valerian, wreaking havoc on native vegetation in Lyttelton Harbour

Spur valerian. Photo / Supplied

A common garden plant is wreaking havoc on rare natives in Lyttelton Harbour, near Christchurch.

Spur valerian, a cottage garden plant with pink, magenta or white flowers, is spreading across rocky outcrops and coastal cliffs around Banks Peninsula and the harbour.

"Don't let that pretty splash of pink fool you – spur valerian has no place in our natural environment," said Christchurch City Council park ranger Di Carter.

"Spur valerian, originally from the Mediterranean, is an aggressive species and a serious threat to the rare native plants that call Whakaraupō home."

The rocky volcanic bluffs and coastal cliffs of Whakaraupō Harbour and Banks Peninsula are home to many rare and fragile native species found nowhere else in the world.

Many of these plants are threatened with extinction from weed invasion.

Spur valerian seeds have blown from gardens around Lyttelton/Whakaraupō Harbour.

Once in the rocks, it grows into thickets and outcompetes small plants, including one of the most threatened species in New Zealand, the Banks Peninsula forget-me-not, which is found naturally around Mount Evans, and the Banks Peninsula sun hebe.

Both are endemic to Banks Peninsula and found nowhere else in the world.

Christchurch City Council and Environment Canterbury have provided resources to help control spur valerian on private land.

The council's park rangers are working closely with landowners to prevent spread beyond Lyttelton and to protect the special plants within the Lyttelton basin.

The control work covers 18 sites and will run twice a year in spring and autumn, for the next six years.

"Flowering occurs in two main bursts, so control is done in spring and autumn before any flowering plants set seed," Di says.

"We need to carry out ongoing survey and control to make sure no new plants are spreading beyond the containment zone."

The council is urging landowners to plant alternatives such as lavenders for the cottage look.