De Vries, who is also a board member of Anthos, the Royal Trade Association for nursery stock and flower bulbs, said recruiting migrant workers would be an issue for horticulture because they are chosing cities over rural areas and the UK "won't have it easy".
The UK "can't do without Europe" for plant imports, he told the HTA Contact conference for nursery stock growers, adding a "new form of free trade [agreement] will certainly come about", not least because 25 per cent of Dutch production is going to the UK. But he said trade agreements "will be focused on plant health", which he suggested could be protectionist.
De Vries warned inflation caused by Brexit should be a concern to the UK industry and that Prime Minister Theresa May's statement that controlling immigration was more important than trade meant "omens for this are not good".
He added that losing EU funds for spatial environment projects would damage the UK unless UK government took up the mantle. De Vries also worried about other countries leaving the EU, leaving a Western Europe rump.
He added that chemicals may not be registered by companies for the UK's specific needs.
Also speaking at the event was Defra chief plant health officer Nicola Spence who presented progress on pre-border activities to reduce the risk of pests and diseases arriving here from overseas, including work with countries beyond the EU to drive up standards, activities at the border to reduce the risk of pests and diseases entering the EU and the UK, and action inland to step up surveillance and improve preparedness.
This includes developing a publicly-available risk register, novel approaches to detection and diagnosis of pests and pathogens and contingency planning.
She discussed overall approaches to plant health in the UK and the challenges and opportunities for the future.
Spence said the UK Plant Health Risk Register, developed in response to ash dieback three years ago, had added 100 new risks in the last year.
Insects are the biggest issue, she said, with 527 on the register. Red palm weevil is the latest problem to strike, in Essex in late 2016, but Spence said that had been tackled.
Spence said sweet chestnuts were also under threat from blight and gall wasp, with ash and oak also "under pressure". Sirococcus tsugae is another new issue for cedar and hemlock.
On the horizon are myzus mumeola on prunus in Italy, another "high risk genus"; Illinoia liriodendra on tulip tree in the Czech republic; emerald ash borer in three new US States and Asian Longhorn Beetle in the US.
She said pine processionary moth, red palm weevil and elm yellow protected zones were due to be implemented in the UK this year.
She said the new EU Plant Health Regime regulations will still be implemented in the UK post-Brexit.
Questioned on whether having a high level of interceptions of problem plants in the UK solved the problem when "virtual nurseries and traders" can import direct to UK sites, Spence said the new EU regulations would require all plant traders to register and "we will track them down" if they don't.
HTA horticulture head Raoul Curtis-Machin said more than 80 per cent of plant trade was done through "respectable nurseries". He added that a "suite" of assurance schemes such as the Boningale scheme, BOPP and others could reassure purchasers that plants were free from pests and diseases and otherwise legitimate, and could help solve the issue of "rogue traders". But he said he was aware of the dangers of "audit fatigue" on growers.
Delegates to the conference also heard HTA consultant David Brown consider some of the reasons behind the volumes of Quercus being imported from other EU Member States.
He said imports of oak were not slowing post-Brexit with 155,799 trees since June 2016, following 1.1m in the three years before that. He said one consignment was for 12,000 seedlings. He added that planting grant tree sales had fallen from 61m in 2014 to 40m last year, with one nursery throwing away 2.5m trees, worth £400,000.
Brown called for the Government to source home grown oak to protect plant health, help the economy, help import substitution, and to create wealth and jobs in the UK.
Nicola Spence said this was an "exciting initiative" with a "good evidence basis", which she could take to government departments to talk about opportunities with "biosecurity benefits".
* Brown also said the Dutch were proposing a "tailor-made" approach to any xylella outbreak at a European Nurserystock Association meeting at IPM Essen next week, with fears of an exclusion zone around Boskoop thought to be behind the idea.