Britain is suffering from a “lost generation of gardeners” in their twenties, thirties and forties because their baby-boomer parents did not bother to teach them, according to the Royal Horticultural Society.

Gardens owned by people in this age group were likely either to be neglected or treated as “outdoor living rooms”, with decking, patios and barbecues but few plants.

Sue Biggs, director-general of the RHS, also blamed the buy-to-let boom for the neglect of gardens. Tenants lacked the incentive to invest in gardens, she said, which had fuelled the rise in poorly maintained outside spaces.

“Tenants think, ‘I’m not allowed