He may even need a calming bowl or two of the briar pipe he keeps in his jacket pocket.

Two years after construction began, the garden’s £16m John Hope Gateway building finally opens today and Mr Bennell will be on hand to watch as the Scottish public inspects the architects’ handiwork.

Despite the nerves, he is confident of a thumbs-up for this newest addition to one of the capital’s most venerable institutions. Although not, perhaps, from everyone.

“I live in trepidation of a visit from our royal patron, Prince Charles,” he said as his team put the finishing touches to the building. “But I do believe the best judge will be the people and I think they’re going to adore it.

“They will see it as a new footprint into the garden and that at last we have a welcome fit for the 20th century and for one of Scotland’s premier international showcases.”

The building is named after Professor John Hope, an 18th century Regius Keeper of the Gardens and leading Enlightenment figure. The construction involved the removal of the shop and a lodge house, and the closure of the West Gate entrance into the gardens.

Dwarfing thosee reopened gates today is an imposing two-storey building constructed from Douglas fir and Caithness stone and designed by London-based Edward Cullinan Architects.

It contains a temperate garden, two exhibition spaces, a shop, science studio, 200-seat restaurant and a wooden spiral staircase which bounces as you walk.

Sustainability drove much of the gateway’s design. The roof is covered with a grass product, sedum, which acts as an insulator as well as attracting insects. Toilets use recycled “grey” water collected in a funnel. Solar panels and a wind turbine offset electricity useage, and heating boilers are fed with locally sourced woodchip.

With the official title of Scotland’s National Bio-diversity Interpretation Centre, the gateway’s educational role is clear. But the building must also function as a revenue-generator and on that front the Royal Botanic Garden has not had its troubles to seek.

The Icelandic banking collapse swallowed £1m which had been deposited there and more than half of that could be permanently lost. Some trustees mooted charging an admission fee of £4, although current Regius Keeper Professor Stephen Blackmore has said he does not expect that to happen.

Still, while the future may not be rosy, it is hardly weed-strewn either. Mr Bennell’s ambition is boundless. “At the moment we get 600,000 visitors per year. My aim is to get one million.”

He will now turn his attention to the East Gate and a modernising programme next spring. He also plans to entice a few passing theatre companies into the gateway building. An outdoor auditorium on the south side of the wide terrace could be perfect for A Midsummer’s Night’s Dream and, if sculptor Anthony Gormley would like to make a work for the impressive terrace area, that would be fine too.

Finally, Mr Bennell plans to introduce some sort of transport system to ferry people around the 70-acre site. He had considered a horse and cart, but is now thinking of an electric vehicle: “It’ll be very green, very tranquil, very environmentally friendly,” he said. “We don’t have it yet but we’re going to. Watch this space.”