The BPOA group, which came from as far afield as Jersey and Cumbria, saw new glass from Kees Greeve covering 40,000sqm.
The turnkey project is due for completion by the end of 2015.
They bought Neame Lea, which was set up by the late Malcolm Barney 35 years ago, in 2010.
David Ball became managing director in 2012.
He has driven expansion, use of automisation to combat wage rises, and general efficiency at the nursery. Neame Lea supplies Asda via IPL, Waitrose and Tesco with bedding plants.
Latest Defra figures showed glasshouse space in the UK had fallen by 8.6 per cent last year and that gave "a huge amount of opportunity", said Ball.
The company has moved from £1m turnover in 2012 to £18m in 2015 and from three acres of glass to 42 acres, having spent up to £10m on developments.
Ball said he wants to move back into herbs and to start growing cut flowers such as tulips and lilies in the future to give the business more flexibility. Neame Lea sold its shares in JVG Herbs business in 2014.
The BPOA also outlined its Home Grown scheme, which now covers 20 per cent of membership.
See more on the BPOA visit to Neame Lea in Horticulture Week 30 October.