Royal Parks workers in central London struck on Tuesday of this week against bosses’ plans to change the way they are paid.
Workers also demand the Living Wage.
The GMB union members are employed by outsourcing giant OCS.
It won a seven-year contract last August after the service was privatised.
Bosses are trying to withhold two weeks’ pay that workers have already earned.
It is part a move from weekly to monthly pay packets.
The workers maintain Hyde Park, Kensington Gardens, St James’s Park, Green Park, Grosvenor Square and Brompton Cemetery.
They also maintain Downing Street gardens and held a protest outside.
The walkout took place one day before the queen’s official opening of parliament.