Titchmarsh, speaking at a RHS Chelsea Flower Show Greening Grey Britain 'Front Garden Summit' said Britain is becoming "a nation where beauty has been sacrificed at the altar of utilitarianism and economy".
RHS figures show five million front gardens have been paved over, often to park cars, causing potential flooding and climate change issues and worsening biodiversity.
A survey by the charity has found that one in three front gardens have no plants growing in them and three times as many have been paved over compared with ten years ago.
Figures show the proportion of households in the private rented sector nearly doubled from 10 per cent to 19 per cent in the decade to 2013.
Titchmarsh said at the event: "When the Prime Minister returns from the next conference on climate change, I suggest that he invests in a pair of statuesque urns – handsome in shape, classy in design, capacious in stature, and positions them either side of the doorstep in Number Ten Downing Street – the greyest frontage known to man – even the bricks are painted black.
"They will act as a reminder to the nation, and to every cabinet minister on his or her way in for yet another fraught meeting about Europe, that plants and flowers really do make a difference to our way of life. You may laugh. It may seem a frivolous suggestion. But I’m deadly serious. These urns, well planted, their occupants changed to suit the seasons, will be a daily reminder of our responsibility to the natural world around us. They will be seen on camera across the globe; a gesture from the British – supposedly a nation of gardeners – that nudges us in the direction of making a difference, rather than simply talking about it.
He added: "There will be a fly in the ointment, of course. The Treasury will find it difficult to sanction such expenditure. Don’t worry Prime Minister. I’ll put my money where my mouth is. I’ll buy them. I’ll even come and plant them up, every season if need be. And that’s a promise."
Titchmarsh said: "No-one could deny that cars and wheely-bins are useful, but they have shouldered out the beauty; pushed flowers and plants that lift our spirits and enrich our lives into oblivion, especially with this ‘lost generation’ who were not so avidly encouraged to care for the natural world as are children today."
Planning minister Greg Clark MP suggested at the event that "voluntary action" was the way to alleviate the problem, particularly in relation to flooding caused by paving over gardens: "People therefore need to be inspired, not dictated to."
He pledged to "ask my officials to review the rules [on permeable paving] we have in place, how well they’re working and if we need to improve them"
He sauid: "Any parish council or Neighbourhood Forum now has the right to develop its own Neighbourhood Plan – giving residents a direct say over the development of their community.
"Decisions about how, what and where we build are best made locally – and that includes the incorporation of gardens and green space."
Clark said 15 per cent the Community Infrastructure Levy charge on new development that goes to the council to pay for infrastructure improvements goes to immediate localties of building and where there’s a Neighbourhood Plan in place the share goes up to 25 per cent.
The RHS last year launched its Greening Grey Britain campaign with the launch of Europe’s biggest community gardening campaign and a three year target to transform 6,000 grey spaces into thriving gardens.