Irises, peonies and wisteria are among those exhibitors are concerned about showing early or wilting in the heat after three hot, sunny days.
Every year designers and landscapers battle with the elements to bring the show gardens, which can cost as much as £500,000, to the level of perfection in time for them to be judged, filmed, photographed and enjoyed by visitors.
Too much rain slows down the build and can wash away plants if it is hard enough. Strong wind is no good either. Sun is great for landscapers who build the gardens but designers fret that their blooms will open too early and go over too quickly.
"Everyone’s covering their plants until they get judged," said Charlotte Rowe, a Chelsea first-timer. "It’s a hotter site than we thought, we thought we would be north-west facing." Her No Man’s Land: ABF The Soldiers’ Charity Garden is unusual at the show because it is a landscape rather than a garden. The design, which commemorates the outbreak of World War One, is reflective of the landscape of the Somme.
At the Cloudy Bay garden co-designers Gavin McWilliam and Andrew Wilson were keeping their irises wrapped in the kitchen paper they came from the nursery in.
"This year is an advance year so a lot of the irises are going over although things like Silvia are not a problem," Wilson said yesterday. "In a real garden that’s not a problem because you can enjoy them next year. We’ll bring the gazebo over them tomorrow but there’s not much you can do about the air temperature."
The partners have brought in Rosa Munstead Wood and Burgundy Ice Rose at the last minute to add wine-colours as the similar-coloured peonies are already in flower.
Alan Titchmarsh’s 50 years in gardening has inspired From the Moors to the Sea, designed with Kate Gould, which features a section of Yorkshire moorland with wild turf and flowers, native hedging and pine and birch trees.
"The rain at the beginning of the week was really quite nasty, the worry now is things are going to bake. The woodland and moorlands in particular," he said yesterday.
While some teams are going for camping gazebos or light materials tied to canes, Bowles and Wyer landscaper Asa Common constructed a sturdy looking canopy out of wood and canvas to protect the wisteria at Matthew Childs’ The Brewin Dolphin garden.
"If it were to fall over it could damage the flowers so I made sure it wouldn’t," he said.
The Rich brothers, of Rich Landscapes, were two designers who were nonplussed about the sun.
Harry Rich said the best thing for plants in the Bord na Mona-sponsored garden was to get them in the ground as early as possible and let nature take its course.
"Things come out but we want them to come out, we’re very happy for it to be natural," he said.
One garden designer who does not have to worry about the sun is Catherine McDonald, whose Massachusetts Garden with co-designer Susannah Hunter is sponsored by Massachusetts Office of Travel & Tourism.
The designers wanted the garden, designed to evoke the landscape of the Cape Cod coastline, to be as close to the natural flora of that area as possible.
McDonald said it was not possible to get hold of many in the UK and where necessary they have used naturalised versions or closely related species. They have small American beach grasses mixed in with similar types found here.
"Dave Root (Kelways director) said it was the hardest plant list he has ever had to source, harder than the Australian gardens here in previous years," said McDonald. "I think we’ve got some plants not seen at Chelsea before, like the Arctostaphylos uva-ursi (Bearberry) which is good for erosion control in the sand dunes and the Ilex glabra (Inkberry) and Myrica pensylvanica (Bayberry)."
The partners wanted Rosa rugosa but, as an invasive species, the natural form is banned at the show. So they have used cultivars such as the Rosa rugosa rubra (Saltspray) instead.