Britons’ love of gardening, from growing their own plants, trees and vegetables in their backyard to visiting public parks and gardens, is important for the economy, provides thousands of jobs and helps health, the report said.
But the size and provision of private gardens are in decline, councils are seeing park budgets slashed and the Government does not value the sector, the industry claims.
Ornamental horticulture is directly worth £12.6 billion - more than the aerospace industry - including sales of plants and equipment, spending on gardeners and landscape designers, and tourist revenue from public gardens.
The sector generates 370,300 jobs from landscaping and retail to manufacturing and tree care experts, the report by Oxford Economics for the Ornamental Horticulture Roundtable Group said.
When indirect impacts including the supply chain and spending of wages earned in the sector are taken into account, the report estimates the total economic footprint is £24.2 billion and provides 568,700 jobs, the study said.
Households spent around £7.5 billion on garden goods last year, from cut flowers and bulbs, plants and seeds to power tools, sheds and greenhouses.
Half of UK adults undertake gardening activities every week.
But experts warn that the number of British adults with a garden is in decline, private outdoor spaces are getting smaller and a quarter (26%) of new homes do not have a garden, up from 18% in 1996.
Although ornamental horticulture generated £5.4 billion in tax revenue for the Government in 2017, the industry said it gets little direct support or incentives, and the sector is hardly referenced in the national curriculum or promoted as a career path.
Despite the value of green spaces in generating tourism revenue, providing services such as soaking up pollution and boosting physical and mental health, councils across the UK have seen parks budgets cut by 40% on average.
Mr Titchmarsh said: “We must urgently stop the demise of the UK’s gardens and landscapes for the sake of the economy and, equally importantly, for the sake of our health and well-being and that of the environment and wildlife.
“The health-giving properties of green spaces are well documented and their provision helps to relieve pressure on our troubled National Health Service.”
The TV gardening expert said around £1 in every £100 of household spending was going on horticultural products and services.
But he said: “If horticulture continues to be undervalued and overlooked, and we carry on building houses without gardens, it will have a devastating impact not only on our national economy but also on the environment and our living conditions as a whole.”
He added: “It’s time to sit up, take note and act to support this invaluable and immense industry for a better future for all.”
Mr Titchmarsh is speaking on behalf of the industry in Parliament next week, calling on MPs to better recognise and invest in the sector.
The Ornamental Horticulture Roundtable Group is made up of a number of organisations including the Royal Horticultural Society, Horticultural Trades Association and the National Farmers’ Union.