Cold nights this spring in contrast to warmer weather of recent years means some planned plant launches at Chelsea may not take place and the show is likely to be more green.
RHS shows director Stephen Bennett said: "It has been a very severe late spring for growers. But I like late spring because it means very fresh-looking spring shows. I predict Chelsea will be a fresh green-looking show, less blousy and colourful than usual."
Hillier Nurseries plant show manager Ricky Dorlay, who is seeking a record 65th gold medal for the company, said: "This spring, most of our outdoor-grown plant material is two to three weeks behind.
"It's not just the temperatures that have been low this spring. Light density levels have been very poor too, which is the bigger worry, and next week's weather forecast is not looking very encouraging." He added that later flowering Rhododendron are "really struggling" and Cornus are "way behind".
Hillier managing director Andy McIndoe said the planned launch of Philadelphus maculatus 'Sweet Clare' at Chelsea may not happen. "It's normally quite difficult to get into flower anyway, so my guess is it won't make it in time for the show," he predicted.
Hardy's Cottage Garden Plants owner Rosy Hardy said: "The main problem is that we are not getting root growth, so although show plants are looking good and putting on top growth, as soon as it gets hot they will dry out and wilt."
Blackmore and Langdon is the only company that has been at every show since its inception in 1912. Owner Rosemary Langdon said: "The very cold winter and lack of light mean we have had to use lots more oil. It's probably cost us quite a few thousands because we have to keep the glasshouse hot from September to May, particularly for the Begonias."
Tony Smith is designing the Easigrass Urban Plantaholics Kitchen. "The issue we have is a Verbena called 'Silver Anne', which we brought into our glasshouse in January and it did nothing because there wasn't enough light," he said. "We didn't want to force it because they get soft and floppy, but hopefully it is starting to take off now."
Companies exhibiting at the Harrogate Flower Show, which began yesterday, have had to deal with even tougher choices. Cath's Garden Plants director Bob Sanderson said: "We have had to change what we are going to use. We would normally show things like Dicentra spectabilis, Tiarellas and Heucheras but they are not ready. Instead, we will put them in another show."
Garden writer Peter Seabrook added: "It promises to be a very difficult show at Chelsea for growers. If the wind changes, temperatures will suddenly rocket and plants will be soft."
RHS advisory services head Guy Barter said: "Things are a lot later - there has been a significant delay in flowering. At the moment, people exhibiting at the Chelsea Flower Show are in a bit of a dilemma. If they force plants too fast they may be over the top for Chelsea and if they don't they could be significantly late. It is a lot more challenging this year."