Some plants are predicted to not last until next Saturday when the show reaches it finale.
BBC TV star James Wong's Malaysia-themed garden has been hit by scorch, while Philippa Pearson's Victorian Aviary Garden has been thrown into disarray because turf has gone brown and tulips have wilted.
Edible Drugs star Wong said: "It's been hot for a week but the damage was done in the first 24 hours. It's not the end of the world but the heat makes things more difficult. We have had to change the planting plan to create 'huddles' to shade plants."
Wong is launching Tacca integrifolia, which likes "the darkest of the darks-total shade". There are only 24 outside Malaysia but Wong is worried the 30°C heat and intense light will affect the rare plant.
Exhibitor Mark Gregory said: "James Wong has tropical plants and the light intensity and heat seen in the outdoor gardens on the top storey is causing him problems. It's a high-risk strategy doing a tropical garden anyway but I don't think he expected it to be this hot.
"Plants throughout the show won't last as long in flower. For those trying to get plants into flower it's perfect but for those suffering it's not."
Chelsea designer Philippa Pearson said: "The turf arrived on Friday stressed and we had to replace it in a hurry on Sunday. We were telling the TV crew to go away and film later.
"I'm worried about the tulips. They need to hang on for judging but we're being judged last on Monday. I've got spares but if we aren't allowed to use them they will have to go over during the week.
"Dust is the worst. You can't do anything about it apart form spray the road. It makes my heart miss a beat every time a lorry goes by. You feel precious towards your plants. I'm going to be cleaning every leaf. It's heart-wrenching.
"Ten days ago we had frosts. It's a tremendous change in temperature. There are a lot of challenges working in the heat.
An RHS representative said: "The Met Office say it could be the hottest May day in record. But exhibitors are well equipped to deal with it."
Veteran exhibitor Peter Seabrook said: "Heat extremes are always difficult and for people who have not exhibited before are not prepared for it."
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