The RHS said it would not publish its policy until it sends out garden allocation letters for RHS Chelsea Flower Show 2018 on 12 October.
RHS council member Jon Wheatley and shows selection committee member Andy Sturgeon said on 12 September at a plant health symposium held at Majestic Trees that they are aware a ban may come in and that one has been discussed at RHS internal meetings.
One of the biggest show garden plants' suppliers Hortus Loci sales director Robin Wallis said: "I've heard it's imminent. The RHS are going to make an announcement."
The RHS may prohibit the importation of plants direct from overseas onto the Chelsea and other RHS show sites and may insist plants be put into quarantine.
A garden like 2017 RHS Chelsea Flower Show best in show winner James Basson's would be difficult to do again. Wallis said, because it was full of imported Mediterranean plants: "It's going to affect anyone wanting to do a Mediterranean garden at Chelsea. The reason they do that is to inject more colour, which is very difficult. You can't force them with light and heat because plants end up etoliated and floppy."
He said he was particularly worried about lavender and rosemary because of the volumes sold through big retail sheds, which would be imported in early season and would be quickly transported, sold and spread around the country. He said an "erosion of knowledge to see warning signs" could mean infected plants sit on nurseries or in retailers without the symptoms being spotted.
Wallis said any ban would not hit UK suppliers, but would damage the regions of Italy and Spain where the high-risk lavender and rosemary plants are produced: "They're staring down the barrel of a gun."
Wallis said the options were to confess to Fera and allow it monitor, or "if you don't know what you're looking at and it sits on your nursery for months, they are going to throw the book at you", which will mean a 10km (or 77,000 acre) exclusion zone.
He said the Government generally took more notice when food plants were involved in plant health scares.
"The worrying aspect is the speed of supply in the last 10 years. Stock can be with you the next day. Who is checking it at each station? Everyone has got to get used to the slowing down of the supply chain to make sure things are checked and look at having quarantine areas."
Wallis said some Dutch nurseries were producing tens of millions of lavenders for instance in controlled conditions with their own propagation so supply would not "dry up completely but there will be a wobble".
He said nurseries which had mature olives and other big high-risk plants in the UK could be able to charge a premium "because I don't think there will be anymore".
Wallis said an issue was the no-one knows how long Xylella can stay dormant in a plant.
Some suggestions are that prolonged use of glyphosate on olives may kill soil organisms and that have encouraged Xylella to develop.
He said there was "the one everyone's scared of" - X.multiplex - as well as the less threatening X.fastidiosa.
Crocus co-founder Peter Clay, whose company supplied and built James Basson's best in show-winning M&G garden for 2017, said new damaging pathogens like Xylella deserve a "huge amount of respect and concern" and "we only look at suppliers who minimise the risk".
Crocus is working on a Mediterranean-themed show garden for M&G for Chelsea 2018, plus an unjudged garden feature.
He said Crocus has stopped importing lavender, rosemary and herbs from Italian suppliers and stopped importing olives "some time ago".
Clay said the difficulties with the issue was that the xylella situation is moving fast and that the UK growing industry is not ready to step in to meet demand.
He said the RHS has sent out a list of host plants and "we're not using anyhting from those lists". Clay said olives, rosemary, oleander and polygalla has never been or were no longer used much in show gardens and that Basson's 2017 garden did not include them. Clay said he wants to use something different every year and does not want to use plants on the list but "it's jolly difficult" - should we ban everything from Italy, or just that part where Xylella is, and what about Spain, or all southern Europe?"
Clay said any RHS ban could "push Chelsea Flower Show gardens back to quintessential English gardens with old-fashioned herbaceous borders, which is what the public wants".
He said it was "inevitable" that Xylella would reach the UK, perhaps via a tourist coming back with a pot of rosemary from their villa and said how Italian growers were spending large amounts on testing but traders without paperwork were selling on the roadside near their nurseries. He said because there was no proper quarantine system in the UK, reporting the disease was reliant on nurseries spotting it and telling Defra.
Another big RHS show garden supplier, Kelways, did not respond to requests for comment, but has been tweeting that about travelling to Italy to tag plants for the 2018 show season.
The RHS said it would be "communicating the requirements for Xylella high risk hosts in the floral and garden allocation letters for all shows. The RHS will also publish its new Plant Health Policy later in the autumn.