Estimates suggest that more than half of the trees at Chelsea will be imported from mainland Europe.
Hillier managing director Andy McIndoe said: "I can't see why they should go abroad when there are big trees available in this country and they look fantastic. I don't think it's the price, it's certainly not the range and one of the big costs of trees is the transport, so it's not the availability or delivery times."
He added: "I shouldn't think it enters any of the designers' minds that they might bring in pests. They're looking for form and shape and that's what they're buying. They shouldn't have to worry if they're buying from responsible nurseries but it has become an issue because of ash dieback.
"I always get a bit frustrated when people creating a show garden say they have to go to Germany. What's the point of putting that in front of the nation on TV?"
Majestic Trees managing director Steve McCurdy said the only way to stop problems coming into the UK is "to make it really difficult to import stock. There have been more pests and diseases come into the UK in the past 10 years than in the previous 100 years so we have to tighten up on it."
Designer Jo Thompson's awareness-raising Chelsea garden "Stop the Spread", sponsored by Defra, is designed to stop the introduction of plant-health problems. Thompson said: "We should think very carefully where we buy trees from. All my trees in my garden are sourced from Britain. It will raise the issue on the main avenue at Chelsea."
She added: "Overseas nurseries have been set up to provide what garden designers want and like any other trade we take advantage of that ease. You might pay a slightly higher price to go to Hillier but the choice and quality is there and you get just about a guaranteed product. If you bring anything in there is an obvious risk of importing something we don't have already."
Food & Environment Research Agency head of plant health public engagement David Slawson said: "There are risks bringing big trees into the country, we know from other outbreaks. It's far harder for an inspector to inspect a large tree as opposed to a small tree."
However, Hortus Loci co-owner Mark Straver, who sources from overseas, said: "It's easy to blame importers but nurseries have been importing for many years - and it's easy to blame European nurseries but we liaise with Defra all the time and have everything checked by Defra."