According to the BBC Weather Centre, 11-12 May was the coldest May night since 1996, with temperatures dropping to -6 degsC in the Scottish Highlands, -4 degsC in Wales and 0 degsC in London.
Somerset grower Kelways owner David Root, who is growing for 12 designers including Thomas Hoblyn, Philippa Pearson, Sue Hayward and Peter Dowle as well as his own stand, said there will be far fewer irises at the show than normal. The iris and peony specialist has been exhibiting at Chelsea for a century.
Root added: "We know we will not have so many irises this year because they have not grown well enough. The cold weather has upset them and we've not had any flower spikes. But our tree peonies will be the best ever. It's swings and roundabouts.
"Tender English perennials are flowering very late and more exotic plants like Euphorbia, olives and Echiums are flowering very early. Chelsea is always a challenge and I've been doing it for 19 years. I thought I'd faced all of them and then you get a new one."
Shropshire-based Claire Austin Hardy Plants owner Claire Austin added: "The weather has been quite adverse this year. We have had to put plants back in the glasshouse. Every year Chelsea is a drama. It is very difficult with irises. The older varieties can be particularly difficult."