The Polly Hill Arboretum is full of special trees, but if there is one which is more special than all the rest, according to executive director Tim Boland, it is a magnolia Mrs. Hill planted more than 40 years ago.

What makes it extra special is not its beauty or it rarity, although it is both a gorgeous tree and one which really should not grow in these climes at all; it is the fact that it saved the arboretum.

Indirectly, but nonetheless it did.

The story goes like this: A bit over a decade ago, Polly Hill was seriously thinking of selling the place. Money was running short. Island conservationists were desperately searching for some white knight with lots of money.

Brendan O’Neill, director of the Vineyard Conservation Society, identified a prospective savior: David Smith, a Harvard microbiologist who had made a substantial fortune from his discovery of a vaccine against spinal meningitis.

Mr. Smith was not really interested, but he agreed to meet with Polly.

“And they were going around and got to this tree,” Mr. Boland recounts. “It’s called a Julian Hill magnolia, a southern species which really had no right to grow this far north.

But Polly had succeeded, and she started to rattle off all these incredible facts — about how she grew it from a seed planted in 1967, how it took nine years to flower, and so on.

“And he just looked at her and asked how she had all this in her head. And she goes, ‘Well, I computerized all my plant records.’ She was one of the first to do that, and she had a database established.

“Well, right then, David said ‘Wow, you’re scientist just like I am.’ And he arranged the purchase of this property from the Hill family and then established our board.

“Unfortunately he died of melanoma shortly after we became established as a public garden. So David is considered a real co-founder of the arboretum.”

Now, as the arboretum celebrates the 10th anniversary of its public opening and the 50th anniversary of the first plantings in Polly Hill’s grand botanical experiment on the abandoned sheep farm in West Tisbury her family purchased in 1927, it is again looking to donors to continue and expand their work.

The situation now, though, is nothing like the dire circumstance of a decade ago; now things are looking, pardon the expression, pretty rosy.

And on Saturday, from 10 a.m. to noon, the arboretum will host an open house to celebrate the latest step in its progress as a scientific endeavor and a public resource, the renovation of the cow barn.

Actually, it is 75 years since the building last served as a barn. In 1933, Polly Hill’s mother, Margaret Butcher converted it to a summer residence. But over the past winter it was remodeled again.

“Polly’s family had a life use policy with it, and after Polly died in April 2007 it came over into our hands. We wanted to make it a useful building for practical horticultural knowledge,” Mr. Boland said.

From the outside the building looks the same, but the interior has been transformed into a gardening library, a member resource center, additional office space and a small apartment for visiting speakers and researchers.

“The year before Polly’s death I went and got all of her archives, which include 10,000 colored slides, all of her letters to famous horticulturists, and a goodly number of books that she donated,” he said.

“Our very first director, Stephen Spongberg, also donated a wonderful collection of books, and by Saturday we’ll have a plaque up there honoring their friendship. Those two rooms upstairs will have all of our archival slides and everything.”

But the cow barn renovation is only one part of the changes happening at the arboretum.

Also planned are a new maintenance building and a new herbarium to preserve specimens, essentially an archive of what has been found.

The herbarium also will include a small classroom area, which Mr. Boland smilingly decribed as “a propaganda thing,” where they will begin educating kids throughout their first five years of schooling in the ways of botany and conservation.

And there will be a new, fenced, one-acre collections expansion garden.

Mr. Boland smiles again as he recounts how people ask when the grand opening of that one will be. As if it will be one of those trophy home gardens that are brought in and just plonked down, fully formed.

“Polly didn’t think of instant gratification. She just planted things. That’s what we’ll do. The new garden will be 10 to 15 years out.”

The fact is, though things have changed a bit from the way she did it. While a lot of seeds are just sown, to either grow or die outdoors, there is now also a greenhouse and shade house to give young plants — some from cuttings, not seed — an easier start in life.

And there is now a new emphasis on Island native plants.

People tend to associate Polly Hill most strongly with her experiments in planting exotic things here. In fact she always went for about a 50-50 mix of them with natives.

But the arboretum now is pushing beyond that. They are devoting huge energies to locating, mapping, propagating and selling Island genotypes.

“We’ll also start to produce more of our own publications on what we call Vineyard appropriate plants — plants that will grow well and are not invasive,” he said.

As Mr. Boland explains, the biggest threat to native vegetation is fragmentation. If people can be encouraged to make native plantings, it will help provide corridors between the preserved parts of the natural habitat.

It’s not a complete shift; there are still planned collecting trips to Asia — the source of many of the arboretum’s exotics. They have the national collection of Stewartias, the biggest in the world. A paper on it was recently published by the Royal Horticultural Society, he said.

It’s technologically slick, too. Using a system like GPS, they can record, on a digital map a site to within 15 feet of where a population is.

He calls it conservation geography.

“We record not only what we collect, but also associated plants.”

Plus, they are logging invasive species on the Island and using the information to encourage towns in eradication campaigns.

“So,” said Mr. Boland, “there’s a lot going on behind the scene. When people ask what are we doing besides gardening, we can say we’re doing science and research and following up on the legacy of our founders.

“We now have a huge database, all of our plants are mapped.”

And to fund all this, the arboretum is now in the middle of a $6 million fund-raising drive — $4.5 million to go towards its endowment, and another $1.5 million for capital projects.

The good news is, about half the money is already promised.

All are invited to the arboretum open house tomorrow morning from 10 a.m. to noon.