The garden centre will be the same donut-shaped footprint and design as the £9m Le Friquet centre in Guernsey, which opened in 2009.
Roper added that Blue Diamond, which has 12 centres and saw 2009 sales increase by 15.6 per cent to £44.6m, was looking at more opportunities.
"We're keen to talk to any family-owned business that don't want to sell but want rental income. They will be surprised with the rent they could receive."
Blue Diamond's latest centre, Redfields Garden Centre in Hampshire, should turnover £10m when it opens in two years, said Roper, who will take over in 2012 when Redfields is rebuilt. The long-established centre is being leased to Blue Diamond because the owners, the Goaters, had withdrawn from retailing.
Roper said 70 per cent of Redfields customers were high-earning professionals with 40 per cent of these wealthy achievers living within 20 minutes.
"That's the hook," he explained. "They build the walls and roof and we will fit out." He said the Goaters could not sell because of tax and family trust complications so Blue Diamond took a 35-year lease. Gilbert Evans was agent.
Redfields is 3,000sq m and will have a 3,000sq m building put up alongside. Then the original building will be replaced by a linked structure. A builder has yet to be appointed, with the contract currently out to tender. Work should begin in a year and is expected to take 12 months to complete.
Roper said spring was "so far so good". He added that 2009 was 14 per cent up on 2008 and that the group was eight per cent up in May 2010. He added that grow-your-own had "no sign" of slowing down.
"It's something that can be maintained. But you have to cultivate and work and make sure the offer is right and it won't wane soon."
He pointed out that there had been "big innovation" this year with vegetable tables at Glee that, when well presented, made sales take off.
Roper added that grow-your-own is a "nice area to work with and promote because it looks good in the centre and enhances your credibility in the garden centre". The more suppliers innovate, the better sales would get, he insisted.
Glee is a "changing animal - much reduced", said Roper, who also visits Frankfurt in September and other shows. He also plans to attend the National Plant Show on 29-30 June. "It's a good idea to get all the plant suppliers under one roof - that never happens," he explained. "I can compare growers' quality and prices."
He said he would visit Solex too but had some reservations about shows getting earlier and earlier. But he stressed that the season's sales were fresh in the mind.