Garden Organic will close off parts of its four hectare Ryton Gardens near Coventry after a redesign.
Designer David Stevens is overseeing the design and students from Sparsholt College are working on the Warwickshire organic showpiece.
Stevens said: "It’s a long-term project. It’s a big site and maintenance has become something of a problem at the moment.
"We’re looking at the general flow around the site and we’re still working on the plan with students at Sparsholt.
"Parts will be shut to the public and will be more for research than day-to-day and will be maintained for that."
"Like any organisation, the Rose Society and all sorts of membership organisations, everything has become quite different over the last 10 years, so this is about making site the place remains viable.
"It’s going to be a place for the future with more contemporary designs, as there’s not too much of that at the moment. The place has grown like topsy over the years and they want to consolidate and get an overall plan with visitor flow and contemporary design to underline the importance of organic gardening.
"There’s a big area in the middle with a lot of grass that takes a lot of maintenance and it’s this area they are looking at reducing but the plan is evolving and there’s no finished design at the moment."
He said fears that parts of the garden will be lost are an "internal matter" but "with change, there are always concerns".
A source says there are plans for three new gardens in the low maintenance border while areas such as the orchard, veg garden, rose garden, allotment garden, cooks garden, all shrub borders, schools orchard, event lawn, small organic, memorial garden and biodynamic garden could be closed to the public.
The source said: "They are talking about reducing the gardens by 80 per cent but they have covered it all by saying it is a design awards that they put out to local designers. But they always had ideas to reduce them. They have a budget of £15,000 so I believe. I am dismayed. This is a place that children need to come and learn about organic practices."
The gardens launched in 1986. The redesign is due start in autumn 2016 and will be completed in time for Garden Organic’s Diamond Jubilee in 2018.
Garden Organic chief executive James Campbell said: "We’re really looking forward to the opportunity to redevelop the site as a whole, as part of our Diamond Jubilee celebrations in 2018. Part of the redesign work will allow us to open areas of the site not previously accessible to the public, including an intended expansion to the heritage seed library growing area and some of the other ‘behind the scenes’ growing areas."
In February, when the plan was launched, he said: "Just as organic horticultural knowledge and practice has evolved, we felt it was time that the overall design of the gardens evolved too. As the original pioneers of organic horticulture, it is important to ensure our gardens continue to demonstrate the best organic practice, to inspire and educate the growers of today and the future."
The Society of Garden Designers coordinated a competition to enter designs. Garden Organic decided that a partnership between several designers "would offer the best solution for both the gardens and the designers".
In November 2014, the charity considered the options for Ryton Organic Gardens, "which have seen declining visitor numbers (and income) for many years, principally due to the much wider availability of a good range of organic demonstration gardens across the country (for example from Edinburgh to Eden)".
Marc Hammond is now head gardener under contractor Newman Landscapes.
Former head gardener Andi Strachan was made redundant in 2014.
Meanwhile, Garden Organic is recruiting a head of organic horticulture, responsible for the development and implementation of an organic horticultural strategy.
Garden Organic is holding its AGM and members day on 12 May at The Green Britain Centre, Turbine Way, Swaffham, Norfolk, PE37 7HT rather than at Ryton this year.
Bob Flowerdew is keynote speaker.