Gardeners can play an important role in helping stamp out a potentially serious weed from the country’s damp spots and waterways

The Department of Conservation (DOC), Ngai Tahu, Environment Canterbury and the Christchurch City Council are working together to try to eradicate purple loosestrife (Lythrum salicaria) from Canterbury. Purple loosestrife  thrives in damp places, particularly river or lake margins, and can clog drains and irrigation ditches. It also crowds out native plants, and changes habitat for wetland birds and fish.

It is one of the worst agricultural and environmental weeds in North America, invading large areas and displacing other plants and potentially it could do the same here.

Fortunately there are very few places in New Zealand so far where purple loosestrife is growing in the wild.  However, if no action is taken, this species may spread out of control. Purple loosestrife has the potential to be as serious a problem as weeds like old man's beard, gorse and broom.

All the small purple loosestrife sites found so far in Canterbury have been successfully controlled.  There is an ongoing control programme for the larger sites, such as Cockayne Reserve (a Christchurch City Council reserve near the Avon River/Ōtākaro).  The control on these larger sites has been even better than expected, confirming that it should be possible to beat this weed.

Purple loosestrife was until recently sold as a garden plant. However, it is now declared an unwanted organism in New Zealand and consequently is banned from sale, propagation or division.

The plant can grow to 3 metres high with up to 50 stems per plant (usually square in cross-section). It flowers from December to February with showy spikes of purple flowers at the end of the stems.  The leaves and stems die off in winter, to re-sprout in spring. Purple loosestrife can produce over 2 million seeds per plant per year and most seeds last at least 3 years.  Seeds are dispersed by water, but may also be spread by wind and birds and on machinery.  Because it has so many seeds, once established, purple loosestrife can quickly form a dense stand that excludes most other vegetation.

What can Canterbury gardeners do?

If you suspect you have purple loosestrife or think you have seen it, please call the potential pest line on (03) 363 9380 and leave a message with your name, contact details and location of sighting.  A good identification tip is to roll the stem between your finger and thumb, purple loosestrife stems are distinctively angled rather than round.