The U.S. Fish and Wildlife Service has formally designated nearly 6,300 acres as critical habitat for the Holmgren milkvetch and the Shivwitz milkvetch.

It marks only the second and third time the federal agency has created critical habitat for an endangered plant species in Utah. Under the designation, geographic areas containing features essential for the survival are managed for the protection of the species.

"What has occurred in this instance is quite exceptional for Utah," Tony Frates, conservation co-chair of the Utah Native Plant Society, said Tuesday. "It's a long and complicated ruling, but the local field office did a real thorough job. And the [critical habitat] designation will be very helpful in keeping these two species from becoming extinct."

Prompting the listing and new habitat designation has been Washington County's explosive growth and development.

Specifically, the two plant species are in the path of a planned freeway interchange and roadway in St. George's south corridor that will link the city with its future airport and a planned community.

That development now will occur around three main areas, and five smaller parcels that will be managed to protect the Holmgren milkvetch.

The largest unit is located on the Utah-Arizona border and comprises 5,546 acres. Included is a 1,146-acre tract in an area east of Interstate 15 called White Dome that is managed by the Utah School and Institutional Trust Lands Administration.

Fish and Wildlife Service officials were not available for comment Tuesday. Federal offices were closed for the national day of mourning following the death of former President Ford.

But School and Institutional Trust Lands Director Kevin Carter said Tuesday that his agency is cooperating with the critical habitat listing.

The Endangered Species Act listing does not provide protection off federal land for plant species.

"We have voluntarily protected some of the habitat and are working with the Nature Conservancy and the Utah Department of Transportation on further mitigation," said Carter, noting that the state lands also include another threatened plant, the Bearclaw poppy.

The Trust Lands agency is in the process, he added, of selling 100 to 200 acres to the Nature Conservancy for habitat protection.

Carter says it also is rerouting a planned road and putting up fencing around poppy habitat.

Critical habitat for the Shivwitz milkvetch has been divided into four areas, the largest of which is a 1,201-acre parcel in Zion National Park.

Not included as critical habitat for the plant are 240 acres of tribal lands near Ivins managed by the Shivwits Band of Paiutes.

But the Fish and Wildlife Service and and the tribe have created a management plan that will provide greater protection for the species than through the critical habitat designation, the agency said.

jbaird@sltrib.com