Prof. Philip Hulme, Lincoln University, New Zealand said the "biggest problem is internet trade", for bringing in invasive species.
He said as the UK did not screen, people can bring in banned species. He advocated weed risk assessments but added that they had drawbacks for the trade as 15 per cent of non-invasives are classed as invasive under the system.
Prof. Yvonne Buckley, Trinity College Dublin, Ireland said "leaky borders" give "economic and social benefits" that you "weigh up against the negative affects of invasive plants. She said to stop invasive plants "you shut the borders and manage the mail coming in but you'd miss out on the whole new horticulture going to be coming up".
Dr. Gerda A. van Uffelen, Hortus botanicus Leiden, Netherlands, said freedom needed to be given back to nations because "it appears Europe is too large and has too many habitats and climate zones to make common trading possible".
Buckley and Hulme advocated 'whitelisting' of acceptable plants rather than blacklisting damaging ones as happens in the EU, with Hulme saying the process was more expensive in the short-term but not in the long-term.
Van Uffelen was concerned whether her botanic garden could display plants such as gunnera if it got on the EU banned list.
Hulme said ornamentals were the majority of naturalised and invasive introductions, with Award of Garden Merit winners succeeding particularly well - 239 were non-invasive and 314 invasive.
He said being easy to grow and popular and invasiveness "go hand in hand" but the industry is "pretty good at mitigation", though voluntary codes of conduct "need to be better enforced".
Kew arboretum head Tony Kirkham said "our biggest problem is plant movement", adding that the industry goes "to Europe to buy trees and this is how some of these plant diseases are likely to be spread".
See more in HW next issue.