Speaking to Horticulture Week Dan Cook said the institute’s members were concerned about such a scenario for three main reasons: business and trade, accreditation and the skills shortage.
His warnings echo others from senior built environment figures last week, including RIBA chief executive Alan Vallance.
Cook said the landscape industry was "really reliant" on the EU supply chain and a no-deal Brexit would cause serious problems.
"It would take many years for British nurseries to grow all the different sorts of trees we get from European nurseries and for UK quarries to be reopened. If there were delays on goods it would make it much more difficult to deliver projects," he said.
The institute is concerned about signs of a slowdown in construction, delays in major construction projects and reports that investors are wary of committing while Brexit uncertainty continues.
"Anything that delays investment definitely has an impact on all the professional services and all the natural environment," Cook said.
A worsening of the existing landscape skills crisis is also a problem. When the institute did a talent survey in 2017, 40% of leaders in the sector reported skills shortages and 8% were EU nationals, not including those with dual citizenship from an EU country and the UK.
"We don’t to want to lose them," Cook said. "That’s one of our major concerns. They are a crucial part of the workforce."
Cook doesn’t believe a post-Brexit brain drain of professionals will be any more severe than that which currently occurs and many UK landscape architects will continue to choose move to work abroad.
However if changes to immigration make it harder for talent to come the other way that will cause landscape architecture practices a severe problem.
In addition the growing profession relies on graduates coming out of courses in the UK. Some 30% of landscape architecture students are international and 7% of these are from the EU.
Another concern is whether UK professionals will be able to practice or bid for contracts in Europe. Currently, along with other European counterparts the qualifications it offers for members, up to full chartership are recognised in Europe.
"As we understand it if there was a deal these professional qualifications would continue to be recognised. If there is an absence of a deal they might not." Members in Northern Ireland would be particularly affected, Cook said. The LI has more than 100 in the province and many operate as a service hub to countries across Europe. In markets where only certain qualifications are recognised, this could seriously impede both landscape professionals and their practices operations outside the UK.
