Currently, a major sewer runs through the Chelsea site and work to build structures over the top of to protect it from the weight of show gardens has delayed the start of garden construction for this May 23-28's show.
Thames Water said: "We just assess the impact their plans may have on our assets. If mitigation work has to be done then the RHS are responsible for it."
Sam Ovens’ Cloudy Bay’s garden, which The Outdoor Room is building, has been delayed this year, while Dan Pearson had to redesign his Chatsworth garden, also on Chelsea’s triangle site, around the sewer in 2015.
Helical Systems has installed "structural reinforcements", Thames Water. Thames added: "They contacted Thames Water to ask for permission to carry out work above our sewers on the flower show site."
The work involves 7m deep holes.
Thames Water said: "This happened last year as well, to ensure the sewer is not damaged basically.
"Any disruption to the sewer could have massive disruption you wouldn’t want at a flower show. It’s precautionary."
The RHS said: "In order for the trees in one of the gardens to sit securely on the plot, without damaging the sewer below, we are working on constructing a temporary layer of steel above the sewer to reinforce it. As with any major venture on the scale of the RHS Chelsea Flower Show it is not uncommon to have to deal with operational challenges like this which also requires that we liaise with our engineers and Thames Water."