Baez, 65, who gave voice to civil rights and anti-war campaigners in the 1960s, joined Julia "Butterfly" Hill, an anti-logging activist, in taking up residence in the tree in the 14-acre (5.7-hectare) fruit and vegetable garden in gritty south Los Angeles.
Baez will take shifts occupying the tree with Hill, who spent two years in the late 1990s sitting in a northern California redwood to highlight the plight of ancient forests.
The threatened Los Angeles garden is tended by some 350 farmers, many of them immigrants, who have been growing fruits and vegetables there since 1992.
"It's an extraordinary community of people and creativity in this industrial part of the city and it literally gives life every way," Baez told reporters, after singing a verse of "We Shall Overcome" in Spanish.
The farmers are threatened with eviction after a court battle over ownership of the land between the city of Los Angeles and a developer who wants to build a warehouse there. The developer has offered to sell the land for $16 million but no one has yet come up with the money.
Actress Daryl Hannah, a keen environmentalist, joined a small group of supporters who have raised some of the money.
"We've come up with $6 million, which is unbelievable. If everyone in the city just gave one dollar, this place could be saved," she said.