While the onset of autumn is controlled by temperature and day length, the unusually early colour being seen on trees such as maples, hazels and laburnum is a response to the dry soils left over from the spring, the society said.
The society has seen yellowing, and some red and brown tints, on leaves at its gardens at Wisley, Surrey, particularly on leaves in the middle of trees which they can afford to lose without stopping much photosynthesis.
The brown foliage on chestnuts, however, is caused by damage from the leaf miner moth – which like other insects will have benefited from the warm weather, leading to greater numbers.
Fruits are also ripening several weeks early on apple and pear trees, while autumn raspberries and wild fruits such as hawthorn are ahead of schedule.
Guy Barter, chief horticultural adviser at the society, said: “We are certainly beginning to see plants beginning to show colour because of the unusual weather we have had.”
He said a reasonably wet winter was followed by an extremely dry spring, and the rainfall since then had not been enough to counter extremely dry soils.
He said that as a result: “Trees and shrubs are under a lot of water stress.
“It’s not fatal because they are well adapted but it makes them get rid of their leaves.”
He said it was not a problem for the trees, which had a very good growing season because of the early warm weather and would be in good condition for next year.
However he said gardeners should give plants such as rhododendrons and camellia a “good soak” because they formed their flower buds at this time of year and would be vulnerable to the dry soils.
This season’s weather, with its mid-summer rain, has also led to unseasonal blooms on winter flowering plants, including some hellebores, viburnum, mahonia and magnolias.
But Mr Barter said the plants were just “chancing their arm by producing more seeds” and it would not affect their normal flowering season of November to April.