Mrs. Obama gave the Agriculture Department a seedling from a magnolia tree on the White House lawn. President Andrew Jackson planted the tree in honor of his late wife, who died before he moved to 1600 Pennsylvania Ave.
"I hope that this seedling brings years of joy and beauty to the garden that will be planted here, in the same way it has brought beauty to the White House for 180 years," Mrs. Obama said.
The seedling will be part of the agency's community garden initiative, which aims to create gardens at the department's facilities worldwide. It will go into a garden at the department's headquarters that faces the National Mall.
The first lady's stop at the Agriculture Department was her latest in a series of visits to federal agencies. She has already talked to workers at the departments of Education, Interior, and Housing and Urban Development.
Agriculture Secretary Tom Vilsack introduced her to an audience of agency employees, who greeted her with a standing ovation and snapped photos.
Vilsack said the first lady probably has "the most difficult job of all," because the president needs someone to lean on during difficult times.
"It benefits all of us," Vilsack said, "because it allows (the president) to do his job for all of us as well as he possibly can."
Mrs. Obama has said she wants to learn about the agencies and meet government employees on her listening tour through the federal bureaucracy.
She said her purpose for Thursday's visit was simple. "It's to say thank you," she said.
The first lady appeared on stage with about 20 department employees, who had each served the agency between 30 and 50 years.
"This nation is built on the backs of you," she told the crowd, "workers who have dedicated their lifetimes to working on behalf of the issues that are so important to this nation."