The rain lashing the Washington area Sunday was perfect for the 42nd annual Earth Day — if you were a tree, shrub or flower.

For people hoping to celebrate the planet outdoors, the weather, which followed a week of gorgeous sunny days, was less thrilling. But it didn’t stop the die-hards.

“I’m a hippie, so I’m all about Earth Day,” said Matt Lateulere, 18, of Marbury, one of a trickle of spectators huddled under umbrellas before a large stage on the Mall. “I just want to see people respect the Earth and treat it good.”

Lateulere, a guitar technician, was also there because his boss had told him to help out one of the bands scheduled to play. Organized by the Washington-based Earth Day Network, the day-long roster of performers and speakers included the Rev. Jesse L. Jackson, D.C. Mayor Vincent C. Gray (D) and the band Cheap Trick.

It was one of countless gatherings across the world held to mark Earth Day, which began in 1970 as an environmental teach-in after an oil spill in California.

The day is observed by about a billion people around the world, according to organizers. Communities typically participate through activities such as cleaning polluted rivers, participating in rallies or planting gardens.

On Sunday, the organization announced it had reached its two-year-long goal of getting 1 billion “acts of green” pledged from individuals, businesses, civic organizations and governments around the world. These included the provincial government of Iraqi Kurdestan, promising to plant a million trees, and one man — location unknown — who e-mailed to say he had broken up with his girlfriend because she wouldn’t recycle.

Melissa Alsbergas, 52, of Fredericksburg said she braved the rain to come to the Mall with her son because she was worried about the effects of global warming and pollution on her grandchildren’s future.

“They’re not going to have the lifestyle we’re used to if we continue the way we’re going with this attitude that ‘It doesn’t affect us,’ ” she said.

Celebrations planned around the Washington area included cleanups of the Potomac and Anacostia watersheds and a crafts and education day at the National Zoo. On Saturday, a couple of thousand people gathered at Ben Brenman Park in Alexandria, taking advantage of summerlike weather.

But on Sunday, as the storm clouds gathered and the thermometer readings plummeted, some organizers had to rethink their plans.

At Brookside Gardens in Wheaton, a scavenger hunt was moved indoors, and plants were brought inside for a discussion. Organizers said the 800-person turnout was about one-third what they usually see.

“The gardening side of me is very happy that we’re getting the rain,” said Stephanie Oberle, the gardens’ director. “But the events side of me just wishes it could have held off till tomorrow.”

As rain drenched the historic grounds of Tudor Place in Georgetown, a garden tour turned into a tour of the 200-year-old house, which once belonged to Martha Washington’s granddaughter.

“This is a Franklin stove,” Talia Mosconi, director of education, told a group of Brownie and Daisy troops gathered in a stately bedroom. “Who do you think invented it?”

One girl took a guess: “Frankenstein?”

As part of Tudor Place’s Earth Day activities, Brooke Burton, 6, of Upper Marlboro made a bird feeder from seeds and planted wheatgrass in a pot. Her mother, Doretea, noted that 200 years ago, the owners of the house probably left a lighter environmental footprint than people do today.

“Do you think they used a lawn mower to mess up the environment?” she said, and Brooke shook her head. “So maybe we could get a cow at home?”

Her daughter shrugged and dismissed the idea as unnecessary: “We already have milk,” she said.