It hopes the extra weeks will give the vegetation “time to bed in and grow”, while other “teething problems” can be fixed.
Those who have already paid the £4.50 to £6.50 entry price for this week and next can still choose to walk up the 130 steps, or apply for a refund and a free visit later in the year.
A council spokesman said: “We’re sorry for the delay and look forward to welcoming visitors when they’re ready to enjoy all the mound has to offer.”
The West End‘s newest tourist attraction was built in a bid to entice people back into central London and Oxford Street.
Westminster council hopes to recoup the £2million it spent constructing the mound through ticket sales and sponsorship before it is dismantled in January 2022.
However, when it opened to the public on Monday the hollow hill - designed by architects MVRDV - received less than enthusiastic reviews.
Visitors complained they did not see the “soaring views across central London and Hyde Park” they were promised.
Instead they were treated to sights of rubble, building works and scaffolding from the viewing platform.
Sharing photos from her visit, Emma Wright said: “Marble Arch Mound is the worst thing I’ve ever done in London.”
Another unimpressed observer said: “What a load of rubbish! The new ‘viewing platform’ at Marble Arch that’s basically a mound of rubble, that you have to pay to access.”
Others mocked the attraction for its resemblance to the Teletubbies house and block-like nineties video game scenery.