In a session on invasive plants, Professor Philip Hulme from Lincoln University in New Zealand told delegates that "ornamentals are a major source of alien plants that impact the environment" and "this is partly due to the selection of plant traits that facilitate cultivation and also assist subsequent invasion".
The session was subtitled: "Are current polices and knowledge fit for purpose in combating globally invasive plant species?" Hulme explained that popular species are widely planted, increasing their likelihood of escape and invasion, citing the lupin in New Zealand, which shades out native flora and fauna in the wild.
He added: "The horticulture industry is aware of these risks and has been proactive in tackling this problem and banning high-risk species."
The EU has banned the sale of water primrose, floating pennywort and parrot's feather. In addition, the industry has campaigned to raise awareness of the damage they cause.
But Hulme said: "Such initiatives help but are not a panacea. Ornamental plant invasions continue to occur and a more effective approach to risk assessment of new and existing ornamental stock is needed. The rapid change in horticultural fashion and globalisation of formal and informal plant trade pose significant future challenges."
He pointed out that because the UK does not screen, people can bring in banned species. He advocated weed risk assessments but added that they have drawbacks for the trade because 15 per cent of non-invasives are classed as invasive under the system - and 15 per cent of invasive plants manage to slip through.
Hulme also advocated "whitelisting" of acceptable plants rather than blacklisting damaging ones, as happens in the EU, saying the process is more expensive in the short term but not in the long-term. He said ornamentals make up the majority of naturalised and invasive introductions, with Award of Garden Merit winners succeeding particularly well - 239 of them are non-invasive and 314 are invasive.
He said being easy to grow and popular goes "hand in hand" with invasiveness, and garden escapes are the "worst offender".
Hulme's methodology is to use E Weber's Invasive Plant Species of the World. Of 450 plants listed in the book, ornamental introductions made up 60 per cent of invasive plants.
In botanic gardens, fewer than 10 per cent have an invasives policy, only 15 per cent monitor and 15-30 per cent research the issue. Hulme concluded that voluntary codes of conduct need to be better enforced and campaigns are needed to make the public change their behaviour and plant non-invasives. Plant bans are costly, unpopular with the industry and cost a lot to police, he added.