Kew is proposing 30 three-year career development research fellowships as part of the restructuring of its science department which has been in consultation.
Prospect negotiator Julie Flanagan said existing staff will apply for research roles.
Kew said the 30 fellowships will have "permanent core funding", adding: "As they are fixed-term posts the scientists will move on at the end of their contracts and new talent will fill these fixed-term positions. This will give Kew the flexibility to grow and train the next generation of scientists.
"The introduction of these posts is genuinely driven by the need to provide better career development opportunities for promising scientists and for Kew to invest in their development as researchers."
Flanagan said Kew wants applicants to have a PhD but many Kew scientists' experience outweighs qualifications. "It would be a sin for Kew to lose these experienced and globally respected staff because they didn't get a PhD 20 years ago."
She wants the consultation extended "because it feels rushed". Prospect has proposed "alternative scenarios" and written to Kew trustees. It said 37 per cent of current permanent science posts will go within three years. The new structure goes live on 1 December.
Kew's consultation on science redundancies, which Prospect said will number 42, or 21 per cent of 200 science staff, ended on 3 October. Kew said it has "contingency plans in place" should workers go on strike.
Proposed structure - Loss of full-time posts
A representative said Kew could not comment until the consultation is over but added: "The proposed structure has 170 full-time equivalent posts. This equates to a loss of 30 (15 per cent) full-time equivalent posts."
After their three years, Kew will advise fellows on setting up and running a research group, how to prepare research grant proposals, managing projects and budgets, how to write reports and publications, managing people and partnerships and managing their own career. "This will equip them to work in some of the world's leading universities and scientific institutions."