Plant pests and diseases were hot topics at the RHS Chelsea Flower Show, which featured displays that aimed to raise awareness about the UK's biosecurity and susceptibility to imported problems.
The Food & Environment Research Agency (FERA) exhibit showed the major pathways that bring in pests and diseases to the UK, while the Corporation of London had a "Fresh" garden warning of the dangers of oak processionary moth (OPM).
Designer Helen Elks-Smith said the oaks in the Corporation of London's Chelsea garden were from Deepdale Trees and were sourced from Germany seven years ago. Deepdale was the only nursery allowed to bring in oaks to the show - oak and ash exhibits were banned.
"I don't have a problem necessarily with big trees coming in," said Elks-Smith. "We need to be mindful of making sure quarantine periods are completed and making sure inspections are done at both ends. The public and professionals have to take on the issues of biosecurity."
The FERA exhibit showed the three major pathways through which pests and diseases could inadvertently be brought to the UK. They are from ornamental plants from Asia, specimen trees from continental Europe and private imports.
The Forestry Commission has set up the OPM London Advisory Group, which includes the City of London Corporation, the capital's boroughs, the Royal Parks, the London Tree Officers Association and contractors who spray the tree pests.
They have developed a control programme chaired by City of London Corporation open spaces director Sue Ireland. This has led to a 50 per cent reduction in numbers since 2012. But Ireland warned that if the group does not sustain the momentum in 2014, OPM will spread further again.
The group's next aim is to seek "full containment" so OPM no longer spreads, followed by full eradication. But that will need greater resources and more in-depth research gained through engaging policymakers, decision-makers and researchers on anti-OPM strategy as well as improved OPM surveillance, communication and control across London.
In 2013, FERA had a "Stop the Spread" garden at Chelsea highlighting pests and diseases. Ireland said: "OPM is still spreading and until we've spent several years tackling it together it will carry on.
"The Forestry's Commission's current view is that we will do well to contain it but our advisory group is of the opinion we should eradicate it. The commission still needs encouraging. We're beginning to show that eradication is possible but because of the potential health impact we need to challenge the commission more."
Defra gave £2m to the advisory group in 2013 and £1.5m in 2014. But Ireland said because of the mild winter and early spring, cutting OPM numbers will be "harder this year". Funding needs to continue at the same level for a decade, she added.
Rather than imposing blanket bans on tree imports, landscapers need to be "confident about where trees are coming from", said Ireland. "British is better" and the Chelsea garden "will get designers talking about it", she added. "The industry needs to understand the implications of buying non-British trees and they need to work out how they can deliver pest-free trees."
She said the RHS will have carried out risk assessments and welcomed a ban on oak movement in affected areas.
Availability Deciding issue for tree imports
"If the trees were available in this country then you wouldn't have to import them," said Former Association of Professional Landscapers chief executive officer and House of Fraser Chelsea garden designer Jason Lock.
"We're forced to because there's not the stock available - and if you do buy from this country they've probably been grown abroad anyway. Unless you are looking for something specific you head to Europe.
"Bruns is so easy - it's a one-hour flight and they pick you up and take you to look at everything. It's a 700ha area so you are going to find something. There's not the space in England for that sort of nursery."
Crocus uses German grower Bruns and Lock uses Deepdale.