Travelers down busy U.S. 1 toward the bright exotic flowers of Longwood Gardens, built by the late DuPont Co./General Motors chief Pierre S. du Pont II and led by his heirs, might not notice Potts Meadow, a grassy field by the bridge over Brandywine Creek.

But a campaign 50 years ago to keep these 40 acres open sparked land-use and tax arrangements that help conserve a green and privileged way of life for a much larger area.

In 1967, young du Pont heirs Francis I. du Pont III and George "Frolic" Weymouth and du Pont in-law William Prickett faced big changes in their familiar Brandywine countryside: A more recent industrial clan, the Bowerses, wanted to expand their protective-gear factory, Fibre-Metal Products, from Chester to Potts Meadow.

It was one thing for successful Wilmington- and Philadelphia-area residents to plan new homes on valley roads. But a hard-hat plant, they feared, could erupt in their leafy neighborhood like a loud drunk at a prayer meeting: The meadow lay downhill from the 1777 Brandywine Battlefield, across Harvey's Run from the rustic Wyeth family homes and studios, up the creek from Weymouth's 1700s estate, Big Bend, and next to ruined Hoffman's Mill, a tall relic of more picturesque technology.

"A lot of people understood that would forever change the culture of Chadds Ford," burying it in "the encroaching megalopolis," recounted Virginia Logan, executive director of what is now the Brandywine Conservancy and Museum of Art, the group the trio started to shape Brandywine country in Chester, Delaware, and New Castle Counties by making it less costly to hold on to private land.

The conservancy has helped owners of 62,000 acres — nearly 100 square miles — on 458 properties arrange conservation easements, trading away development rights and entitling them to lower school, county, and property taxes. Hoffman's Mill is now the group's museum, featuring Brandywine School pictures.

Besides lower property taxes, conservation agreements enable landowners to cut federal tax liabilities – arrangements so valuable they have, in places, been syndicated and sold to investors, sparking an IRS review.

The conservancy's success sparked imitators. Open-space and agricultural easements now cover 20 percent of Chester County — in some townships, more than half (not counting government land). In Chadds Ford, across the creek in Delaware County, Brandywine owns 241 acres directly and has easements on 890 more privately owned acres, totaling 21 percent of the land.

Led by chairman Morris Stroud II, the conservancy marked 50 years April 29 by planting three sycamores, starting a Founder's Grove to honor the original trio, and opening a trail built with township and state aid.

Prickett, a corporate litigator who headed the Delaware Bar Association, died in 2014. Weymouth died last year; his son, Mac, a prep-school teacher, joined the tree-planting. So did Francis I. du Pont III, who at 91 wielded a ceremonial gold-colored shovel. Guests snacked on watercress and cucumber sandwiches, under a white tent.

U.S. Rep. Patrick Meehan (R., Pa.)  strode across the meadow (he lives in Chadds Ford, too) "to express my deep appreciation." He said he worked with Democrats on national laws to help "preserve open space that will be handed down."

The conservancy prospered as the big firms that built du Pont fortunes faded. Francis I. du Pont & Co., founded by Francis III's chemist namesake, in the late 1960s rivaled Merrill Lynch among the largest U.S. brokerages but collapsed in 1974. The shrunken DuPont Co. will merge into Dow Chemical Co. this summer, after more cost cuts. Heirs long ago diversified their holdings.

Fibre-Metal took its plant to Concordville. The family sold in 2008; it closed soon after. The government offered workers import-displacement benefits. Honeywell Inc. now owns the brand; a medical clinic occupies the site.

When some pay less, others can end up paying more. In Philadelphia, where my colleague Harold Brubaker found that 27 percent of assessed property is owned by tax-free nonprofits or government agencies, leaving private owners with a growing tax burden, critics want institutions to pay more for public services.

In Chadds Ford, with a population of just 3,200, the conservancy hasn't blown the township budget: its tax levy, at less than $1 for every $1,000 of assessed property, is the second-lowest in Delaware County — even with a special tax for land acquisition, which voters approved.

Public schools are more affected: The Unionville-Chadds Ford district's property tax rate is $26 for every $1 charged by the township.

Township officials told me they'd have to analyze tax records to measure the cost of conservation. But the benefits "are priceless," township and sewer authority manager Amanda Serock assured me: "Not only does it protect the beautiful bucolic feel of our community that is being stressed by regional development, but it helps to maintain our home values, lessen traffic congestion," improve air, assure "better quality of life," and prevent "overcrowding of schools," which, she noted, would lead to higher property taxes.

More land: Last week, the Brandywine Conservancy, the Conservation Fund, and the Delaware-based Mount Cuba Center, which is based at another du Pont family estate, announced they were buying 270 acres in Concord Township, bordering Chadds Ford, from developers who planned 160 houses there. They plan to attach it to adjoining First State National Historical Park.

Donors raised $8 million — not counting the largest single gift, from Mount Cuba, Blaine Phillips, the Conservation Fund's Mid-Atlantic director, told me. The price is a lot higher, per acre, than the $20.8 million Mount Cuba spent three years ago to buy more than 1,000 next-door acres to create the national park: This time, the ground had been all set for development.