Two strategic tracts of natural land in southern Washington County will be protected permanently against possible development under a $3.5 million agreement reached last week.

The 85 acres owned by the Carpenter St. Croix Valley Nature Center, in Denmark Township, will be placed in conservation easement to safeguard them from housing construction or any other future changes.

“We just want to make sure we maintain the Carpenters’ wish that the property be protected for habitat preservation, and this locks that in place,” said Vickie Batroot, who chairs the Carpenter Foundation board.

Last week’s 5-0 vote of approval by the Washington County Board also ensures that a significant portion of the envisioned St. Croix Valley Regional Trail will be built on Carpenter property — and that a connecting trail someday will bring walkers and cyclists to the front door of the nature center.

“The trail adds a real gem. It’s a big leap forward for the county,” said senior planner June Mathiowetz, who negotiated the easement.

A conservation easement is a purchase of the development rights of a property to forestall legal challenges and forever preserve the land as open space. The Carpenter Foundation will continue to own the land, but the easement ties closely with the county’s interest in protecting natural areas and water quality.

About $2.3 million of the cost of the easement will come from the county’s voter-approved Land and Water Legacy program, and the rest from Met Council and state outdoor heritage grants.

The Carpenters, Thomas and Edna, died many years ago. They began buying land along the St. Croix River in the late 1930s, lived on the property for 30 years, and added a plantation of red pine and an apple orchard, among other amenities.

More than 20,000 people a year — about 7,000 of them schoolchildren — visit the nature center. Natural land is used for hiking, nature observation, environmental education and forestry habitat restoration. Under the agreement, those uses will continue, Batroot said.

Land secured in the easement includes a northern segment, adjacent to the nature center, and a small southern segment that will link with the county’s new Point Douglas Regional Trail at the confluence of the St. Croix and Mississippi rivers.

Last week’s purchase also includes an easement for the planned St. Croix Valley trail along an abandoned Chicago, Milwaukee, St. Paul and Pacific Railroad route. The “connector trail,” as decided under the agreement, would follow County Road 21 and link to the St. Croix Valley trail at the north and south ends of the Carpenter property.

Another feature of the agreement allows the county to build a trailhead and parking lot near the entrance to the nature center on County Road 21.

“The idea of a loop trail system is highly valued by the public,” said Commissioner Lisa Weik. “People have said that one of the things they value the most is the location and beauty of Washington County. We can’t necessarily improve on what God gave to this part of the state of Minnesota, but we can invest in these outdoor amenities so the public has better access and can really enjoy the area.”

Batroot said that the Carpenter Foundation’s interest in the conservation easement didn’t result from any immediate development threats to the land. Instead, she said, “this guarantees that no board or individual in the future will be able to do anything else. Development is not something we want in that area at all.”

Preservation of Carpenter land has been under negotiation in Washington County for at least 10 years.

“It’s such an asset to our county,” said Commissioner Karla Bigham, whose district encompasses the Carpenter land.