Speaking at the RSPB State of Nature conference on Wednesday Clegg said the one-off payment would help the world-famous botanic garden and its Sussex country estate Wakehurst Place in the current financial year.
Kew staff have welcomed the move – but a representative said that it will not "solve our longer-term need to move to a new staffing level and structure that allows us to become financially sustainable" prompted by a £5m shortfall in its budget.
Director Richard Deverell said: "We welcome this Government recognition of the Royal Botanic Gardens, Kew’s importance and the value of our work. In recent months we have been actively articulating this.
"We are committed to continuing to grow our self-generated income and managing our cost base so that we can put Kew on a firm financial footing and ensure that our science and education programmes will continue to make an impact in the areas where they are needed most."
Kew unions said the only solution was to restore Kew funding to 2009/10 levels, in line with the recommendation from the last independent review of Kew.
Prospect and PCS delivered a petition with more than 100,000 signatures to Downing Street in July. It called on the Department for Environment, Food and Rural Affairs to "urgently reverse existing, proposed and further cuts to Kew’s annual operating grant in aid".
Prospect’s parliamentary officer Parmjit Dhanda said: "We are pleased at this temporary reprieve which shows the importance of the widely backed campaign. Of course, increased funding and longer-term security beyond the election would have been even better. But Nick Clegg’s statement is a bit like bedding plants – a temporary show."
Prospect pointed out that Kew has had to use up most of its reserves to survive four preceding years of government cuts. Kew has already had to lay off more than 50 staff who have left on a voluntary basis and major cuts in the number of science staff are still on the table. Many decades of skill and experience have been lost and more staff are expected to go.
Dhanda added: "Kew needs long-term certainty about its funding to be able to plan and manage its limited resources efficiently and effectively. Kew unions are calling for an independent review to examine this and move to a funding cycle that is longer than three months."
Clegg also announced the creation of a national green space map and extra funding for the Coastal Path project at the conference.