National Park Service worker Dennis Townsend cleans up a pile of trash on the National Mall near 14th Street and Jefferson Drive on Sunday. (Craig Hudson/For The Washington Post)

The National Park Service, responding to criticism of its cleanup work at the Cherry Blossom Festival, said a confluence of factors — gorgeous weather, peak blossom blooms and not being ready enough for mammoth crowds — led to the mounds of trash on the Mall this past weekend.

As many as 30 tons of trash, on par with July 4 tonnage, piled up, Carol Johnson, a spokeswoman for the Park Service, said Monday.

Johnson said it was clear that “in retrospect, we wish we had brought on more staff.” Robert Vogel, superintendent of the National Mall and Memorial Parks, conceded Sunday evening that “we did drop the ball a little bit.”

Blossom fans encountered mounds of trash more than waist-high in some cases — full of plastic foam containers, bottles, wrappers, bags and uneaten food, along with what appeared to be commercial packaging from food trucks and other vendors that lined nearby streets to soak up cash.

Many who visited early Sunday morning arrived to find that the trash was already piled high, apparently from the day before. Johnson said the Park Service does not have overnight staffing.

One reader sent an e-mail to The Washington Post about trash observed at the Tidal Basin even earlier, on Saturday about 6 a.m.: “We noticed all of the trash cans around the Tidal Basin were overflowing with trash THEN and there were piles of trash next to many of them.”

Crews never caught up. Park Service officials said trash trucks and cleanup crews were unable to maneuver through and around the big crowds.

Cleanup crews were able to get most of the trash by 8 a.m. Monday.

Officials did not have an exact count of the number of Mall visitors this weekend.

However, Metro officials reported “especially high ridership.” On Saturday, there were 638,474 trips on the system, the highest on a Saturday since the Jon Stewart and Stephen Colbert rally in October 2010.

Sunday brought 405,154 trips — 83 percent more than average. It was the busiest day for Metro since July 4, 2010.

Crowds at the Smithsonian station were especially large, with complaints and critiques that piled up on social networks. “The Metro during cherry blossom season,” one tweeter wrote, “is almost certainly purgatory.”