Invasive weeds are adapting to climate change, presenting new problems. By Elaine Fisher.

Changing behaviours of some pest plants, probably triggered by milder-than-usual winters, has the Tasman District Council’s biosecurity team nervous about the future and reviewing its management practices in case the trend is ongoing.

“We have observed some pest plant species that have not been responding as usual to the onset of winter. In several cases, this seasonal trigger is what allows us to identify some of our pest weed species. The warm, wet winter we’ve had here hasn’t provided enough of an impact to trigger the usual key changes that enable us to identify and control them,” Tasman District biosecurity officer Briar Cook says.

One example is climbing spindleberry, which is only identifiable in May when the leaves yellow and droop briefly before dropping off. Briar says this is a very narrow window of opportunity to check known sites and look for new ones. This season, known sites were visited in May as planned, and no yellowing was visible at several locations.

“We usually use the local vineyards as a good indicator of when exactly things are starting to change. This year we noticed the vineyards were also late to change. We have had to go back two to three times to some of the sites to check for the yellowing. I happened across a new site in early July that I only saw because it had the yellow drooping leaves – this is two months late.”

Variegated thistle is another pest plant the team has observed, in both 2021 and this year, as germinating early at some sites. It is an agricultural pest, forming dense stands in pasture and wasteland. It will suppress desirable pasture and its spines can be toxic and cause injury to animals. It has the potential to have a significant impact on pastoral and crop production.

“This change is quite concerning as we have recorded mid-winter germination, when typically they don’t germinate until September/October. This plant is a fast-growing annual or biennial, with a seed life in the ground of around 20 years.

“If we miss a seeding event from even a single seed head that puts us right back to square one at that site, and so it is vital to get to the plants before this occurs.

“This extending germination period means we will need to start checking our known sites significantly earlier in the season – and we have already done this for a couple of the sites this year. It’s made our team very nervous.”

Old man’s beard (OMB) is another species which is not behaving as usual.

“We have observed on several occasions, including early August, that vines still had green leaves on them. OMB is deciduous, dropping leaves after flowering in May. Some have dropped their leaves; others are still green and a lot we have seen are somewhere in between.

“This is not a species we target as biosecurity officers in Tasman (with the exception of Golden Bay and Upper Buller) however I can appreciate due to its widespread distribution that it is likely an issue for farmers and growers.”

OMB is a highly effective smothering vine, and the winter leaf drop is often the only opportunity that smothered trees get to see the light of day. “Unfortunately, the warm winter has further limited this already brief reprieve.”

The Tasman District biosecurity team keeps farmers and orchardists up to date about large variety of pest plants including climbers, woody weeds, aquatic weeds and ground cover plants and the best way to control them, through a range of pamphlets on the council website and from its service centres.

“We also use social media and the council newsletter to pass information to the public about particularly concerning pest weeds or pertinent updates and directly respond to enquiries on how to control certain weeds.

“More specifically, and particularly on farmland where biosecurity threats can impact productivity, we are always in touch with the landowners before carrying out surveillance, and often carry out the control on the spot if we identify a target plant. Landowners with pest plant sites are kept up to date following the surveillance as to what we find, any sudden changes in our knowledge and what to keep an eye out for.”

Briar says what the team have observed is “an early days’ trend”.

“However, it wouldn’t surprise me if the trend becomes more of a feature in the future, given the ominous loom of climate change.

“It’s definitely making us think more about the way we manage our top priority pest plants and what we may need to put in place to account for the extending seasons we are experiencing. It has also put us on the alert for other upcoming annual summer weeds in our calendar, so we will be keeping a close eye on that to make sure there are no more surprises.”