Immediate past-president of the Landscape Institute Nigel Thorne urged colleagues to create designs using the National Plant Specification (NPS) to ensure the final scheme closely resembles the plans.
"When compiling documents it would be unthinkable not to specify all the particulars of the hard landscaping," explained Thorne during Coles Nurseries' Unearthed event on 11 September.
"But for some inexplicable reason, the designer constantly fails to specify the full details of the plants, allowing the contractor or developer full carte blanche on what they plant."
Thorne added that if landscape architects make a detailed plan of the type and size of plant required in the design, it would mean unscrupulous contractors could not return much cheaper tenders by using a smaller or different plant in order to win work.
Speaking at Unearthed - which aimed to provide a guide to specifying UK trees and shrubs for landscaping - Thorne said that by using the NPS, available online through website www.gohelios.co.uk, landscape architects could help achieve a fair tendering process.
"It allows you to specify the size, species and variety accurately," Thorne said. "A contractor can then be sure the tender bids have all been submitted on the same basis.
"If you are not prepared to follow that through then why specify in the first place?"
Speakers from the HTA, garden charity Perennial and Nottingham Trent University's Trees Project also spoke at Unearthed.