Extra protection could be coming for the San Diego ambrosia, an imperiled plant often associated with rare vernal pools.

The Arizona-based Center for Biological Diversity, an environmental group, reached an agreement Friday with the U.S. Fish and Wildlife Service to determine by August 2009 whether "critical habitat" should be designated for the plant.

If the service concludes such a designation is necessary, the agency must set aside habitat for the plant by Aug. 19, 2010, under the settlement approved by a federal district court in San Diego.

The designation would not establish reserves, but it would require landowners with ambrosia plants on their properties to take measures to protect them when they develop.

"Critical habitat is one of the most important safety nets for species listed under the Endangered Species Act," said Andrew Orahoske, conservation advocate at the Center for Biological Diversity.

The San Diego ambrosia is a perennial blue-gray herb with clusters of yellow flowers that bloom in summer and fall. The plant grows in grasslands, upper terraces of stream beds and next to vernal pools in San Diego and Riverside counties.