Kew arboretum head Tony Kirkham said at the Asociation of Professional Landscapers (APL)/Society of Garden Designers seminar day that the new Treetop Walkway’s final cost was £4m, which was £600,000 over budget including £300,000 contingency.
The walkway’s elevator, which has yet to work, is due to open next week.
Kirkham is now raising funds for phase two of the development.
A Kew representative said phase two will involve “putting in more interpretation in the Rhizotron and on the walkway.
"This will be funded by the public donations being put into the donation tube at the top of the Walkway. It is planned for the Rhizotron to feature more technical information about roots and for additional spring information about trees to feature on the walkway.”