Sharpe said there is pressure to go 50/50 on the split between vegetables and flowers in garden centre displays.
"However, T&M trades on the depth of its range and there has to be a point of difference between going to the garden centre to buy our wide range and going to Tesco for their small range," he said.
"If you eliminate choice you close down the market."
He said 10 years ago seed ranges were 60/40 in favour of flowers but now has flipped to 40/60 in favour of fruit and vegetables.
But he warned: "Flower sales are still growing so centres should not neglect them."
T&M vegetable seed sales are 20 per cent up this year.
Meanwhile, Sharpe said the RHS's decision to display seeds by variety rather than by seed firm was "totally the wrong thing to do".
RHS commercial director Gordon Seabright has had bespoke stands made in order to display seeds from A-Z so customers could find seeds faster, stemming from an idea taken from his book-retailing background.
But Sharpe said: "Each seed company's packets are made to work with each other.
"If you mix brands it's a recipe for disaster. There's too much choice and you've not got the continuity there.
"A big wall of seeds is too daunting and will confuse customers, who often have seed brand loyalty."