Horticultural consultant and London in Bloom executive trustee Peter Holman sought figures on bedding planting from all London's 33 boroughs and received information from 22.
The London Borough of Bexley planted 8,000 bedding plants last year, down from 300,000 two years ago. But Islington, a Britain in Bloom competitor this year, has maintained its floral displays while adding perennial material too.
Holman said: "There have been some dramatic declines. There are two sides to the argument. There is a view that bedding is not sustainable. I don't quite agree with that if you take sustainability in its truest sense.
"A lot of London boroughs are using bedding to cut budgets and be seen as being sustainable. Perennial material is sustainable but doesn't achieve the same thing. I want to see a balance rather than a knee-jerk reaction.
"A lot of council bedding has been reduced for reasons of sustainability when the real reason is budget. That's a fact that few would find difficult to challenge. Whichever party wins the election I can see green spaces being hit very hard afterwards."
Holman added that north of Birmingham traditional bedding was faring better, with Harrogate, Nottingham, Aberdeen, Edinburgh and Solihull maintaining schemes, often connected with the In Bloom competition. But RHS Britain in Bloom changed its judging criteria in 2009 to give less emphasis to bedding.
Growers have hit out at the cuts. Midlothian's Pentland Plants partner David Spray said: "Some local authorities aren't ordering at all, while others have cut maybe 10 per cent. It's the soft option when budgets are tight. It's purely economic - nothing to do with sustainability. They have also lost the staff and the skills."
Baginton Nurseries managing director Will Lamb added: "There are spending cuts left, right and centre and (parks departments) are responding to that. Our trade has definitely reduced."
London Borough of Bromley assistant director of parks and green space Patrick Phillips said: "We need to move more towards sustainable planting. We have done some - maybe five per cent of the borough. We give away some old bedding plants to friends groups or hospices, but people may say: 'That's my money paying for that.'"
- See page 22.