Volunteers and landscapers will be planting 15,000 seedlings at the Flight 93 National Memorial in southwestern Pennsylvania.
The workers will plant the trees Friday and Saturday and again April 27-28 as the first step in reforesting the site where a United Airlines flight crashed as passengers struggled with four hijackers during the Sept. 11, 2001 terrorist attacks.
The trees being planted this month are the first of about 150,000 planned for the site about 60 miles southeast of Pittsburgh. It will take about three to five years to plant all the trees.
The first phase of the national park commemorating the crash site was dedicated last year.
The trees being planted will form a windbreak to protect other trees planted in nearby memorial groves in honor of the plane's 40 passengers and crew.