Renovations at a private home fronting the Edgartown harbor have put a spotlight on a historic shade tree that graces South Water street and is likely the largest of its kind in the state.

The majestic pagoda tree, brought to the Vineyard as a seedling in a flowerpot by a sea captain more than 175 years ago, is a popular tourist landmark. The tree is a public shade tree whose root system spreads beneath private property on the street of closely built homes.

Now the new owners of 29 South Water street, who are conducting major renovations to their property, want to build a garage above the tree’s root system. The Edgartown conservation commission last week issued an amended order of conditions that would allow the owners to build a cantilevered garage with an elaborate plan to protect the tree’s roots.

“It’s a national champion, so you have to dot every i and cross every t to make sure that it’s protected,” said Stuart Fuller, the Edgartown tree warden, speaking of the tree.

Majestic tree depicted on old post card (undated) from Gazette archives.

The Edgartown selectmen are not so sure about the plan and have put the matter on their agenda for discussion at their regular meeting next Monday.

Thomas and Mary Folliard bought the property, which abuts the Harborside Inn, a year ago for $7.9 million. Renovation work has included rebuilding the residence and the installation of a pool. But construction of the planned garage has been stalled out of concern for the pagoda tree.

According to a plaque fixed at its base, the tree was brought to Edgartown from China by Capt. Thomas Milton in 1837.

Over the years, the tree has grown to impressive scale, measuring 77 feet tall at the last measurement.

“It is quite notable in terms of its size and girth and the fact that it’s done so well and continued to do so well in an urban environment,” said Tim Boland, executive director of the Polly Hill Arboretum in West Tisbury. This week the sidewalk around the base of the pagoda was littered with the tree’s pointy, green leaflets.

Though the plaque proclaims it to be the oldest pagoda in the country, a 90-foot-tall tree in Essex, Conn. holds that record, according to North American Landscape Trees, a tree encyclopedia.

Still, it is the largest Sophora japonica in the commonwealth — only the giant pagoda in the Boston Public Garden rivals this one for size, Mr. Boland said. The pagoda’s root system is likely very extensive — typically a tree’s roots reach out as far as the branches above them, he said.

The roots are thought to extend beneath a part of the Folliard property, which until recently was the site of a lawn and parking area. As the “only space without a road or a street in the immediate vicinity and favorable for root growth, it is more likely to contain a high root population,” wrote David Hawkins, a consulting arborist for the town, in a report this year.

This is the area where the owners plan to install the garage, a project Mr. Hawkins called a “risky endeavor with many unknowns that could adversely affect the tree.”

Specifically, he outlined concerns about root damage or loss that might result during construction, and the problem of how water and nutrients will reach the roots once a garage is built above them.

An order of conditions issued in January by the conservation commission authorized work at the site, but directed the builders to take precautions with regard to the tree. “Prior to any work on this site, the applicant shall provide a protocol of specific measures that will be taken to protect the historic pagoda tree from any damage during and after construction,” the order reads in part.

The amended order approved last week allows for the construction of a cantilevered garage. The structure would be supported partly by the foundation of a previous garage and partly by two support piers, which would be installed on the north side to reduce interference with the delivery of water to the roots.

Hand digging at the site will allow the workers to avoid roots when choosing a spot for the support piers, Mr. Fuller said.

The proposal also includes a plan to provide water and nutrients to the tree. “The proposal that they have I think is very sensitive and innovative and other than doing nothing, I think this minimizes any potential impacts,” he said.

Mr. Hawkins, the arborist, agreed, writing in a letter in October that the “cultivation/aeration process and fertilizer application will help improve the soil and the tree’s ability to counteract any negative effects of the construction and encourage root growth in that area.”

This is not the first time that the tree has been threatened by a home expansion. In 2003 the town commissioned an appraisal of the pagoda and also a nearby linden tree when an abutting property owner was involved in an expansion project and requesting removal of the tree. The appraisal was requested “for the town’s records and for public awareness of the value of shade trees,” a report said at the time. “A tree of this size and stature is irreplaceable and priceless. No amount of money could ever justify its removal,” the report said.

The appraised landscape value of the tree was $98,600.