Palmstead Nurseries and Deepdale Trees are growing the plants and trees for the Bridge, which came under fire for not achieving value for money for the taxpaper in a report by Dame Margaret Hodge this month.
London Mayor Sadiq Khan has yet to decide whether he will guarantee the project so building and planting can start by the end of this year.
A Garden Bridge Trust spokesperson said: "Our contractors including Dan Pearson, Willerby and Palmstead are awaiting a decision from the mayor to sign a guarantee underwriting the annual maintenance of the bridge. We have to have that guarantee." It added that Hodge's report was "one-sided".
The Trust says it had a "robust business plan" and has a meeting with Khan's office this week to present details, and "expects a decision in from the mayor the next few weeks" on whether Khan will sign the guarantee so the bridge can go ahead.
If the bridge is not built the Trust said "there would be cancellation costs" paid to contractors including the growers involved.
Khan has said he will not spend any more taxpayers' money on the project, which has cost taxpaers £37.4m so far.
The Trust says: "We need certainty to be able to get on and deliver the project. It's now running late and the contractors have been on stand by. We have to start building by the end of this year because planning permission will run out."
Shrubs and perennial plants have been propagated from seed and cuttings by Palmstead Nurseries in Kent. Planting is due to begin in November 2017, with opening to the public in the summer of 2018.
Trees and shrubs were being collated at Deepdale Trees, in Bedfordshire from late December 2015.