Anecdotal evidence is even suggesting garden centre staff are telling customers that they cannot plant in autumn, as they decrease stocks for the winter.
George said: "There is a general move away from plants outside the very lucrative springtime market, which is anti-autumn. It is worrying how garden centres are polarising. Many centres are only buying things in the spring now. There is a trend that people are shunning way from autumn, though it is one of the best times to plant, especially for herbaceous."
He added that Christine Walkden's new BBC series Gardens from Above said you can have year-round colour if you visit your garden centre one day every month all year. But many garden centres cannot provide that because they lack the range. "You will be lucky to be able to buy something in flower every month," he said.
For 2015, growers are "quietly confident as usual but it's going to be incredibly tight and weather-dependent", said George. "The market is changing quite dramatically. Buying power is in quite a small number of hands. Several really big players are getting bigger and bigger."
Latest Garden Centre Association figures suggest plant sales were up 14.5 per cent this October compared with October 2013, but George said that is probably because of higher footfall and does not reflect large numbers of plants sold.
George added that the latest NBIS meeting showed autumn is a "mixed" month, with success in mail order, trees and at destinations such as National Trust shops.
George's nursery Prenplants supplies 31 National Trust centres. Last month the trust announced that Hillier is supplying 10 properties this year - Nymans, Polesden Lacey, Sheffield Park, Blickling, Wimpole, Anglesey Abbey, Dunham Massey, Stourhead, Mottisfont and Clumber Park - and there are plans to expand its plant ranges at 16 further gardens.