The latest incident took place last Sunday when thieves methodically stole the entire national collection of the plant group Pseudopanax, a rare New Zealand genus, from the Ventnor Botanic Garden on the Isle of Wight.
It took Simon Goodenough, the curator, six years to build up the unique collection, and less than an hour for the thieves - probably two men in a Transit van - to remove it. They ignored other, less important plants and filled in the holes to cover up their crime.
The rarity of the plants almost certainly means they are destined for the private garden of an obsessive collector who will keep them hidden from public view.
'People seem to want to have plants at any cost,' Mr Goodenough said. 'Very often it is people who are well-heeled with sizeable gardens. There appears to be a black market in specialised plants. It's happening nearly all the time.'
Tony Lowe, the general secretary of the National Council for the Conservation of Plants and Gardens, said: 'There is a general feeling in the horticultural world that there is a specialised team nicking plants to order.'
He said there had always been a 'light-fingered brigade' who took cuttings or potted plants on impulse. 'But the more worrying side is theft to order on a grand scale. There's a stamp-collecting mania in the gardening world.'
John Sales, the chief gardens adviser for the National Trust, said there were thieves who stole common plants to sell to garden centres and those who took rare plants for private collections.
At the trust's Trelissick Gardens in Cornwall, thieves recently stole a range of rare rhododendrons, he said. 'One assumes they are collectors. It seems they go for the valuable things.'
Apart from tightening security where it can, the trust now tries to hide its rarer plants. 'We tend to leave the labels off young plants so as not to draw attention to them,' Mr Sales said.