The Autumn Planting for Year Round Colour campaign will promote planting in autumn when the soil is still warm to allow plants to get established in time for spring.
Gardeners can also save money, for example by buying bare-root plants for planting in October, which often cost much less than those sold in containers.
The RHS commissioned survey showed that only 6 per cent of people purchase their plants during the autumn and 80 per cent were not aware that the focus on spring planting is fairly recent.
RHS director general Sue Biggs said: "Smart gardeners can save money and have more success if they get certain trees, shrubs, climbers and herbaceous plants, like peonies, planted now and roses, soft fruit and fruit trees planted in November.
"When gardeners are choosing bulbs in September, hopefully this campaign will encourage them to stop by the tree and shrub areas as well. With the drought this spring and then a washout summer, more than ever we need to embrace autumn gardening to get our outside spaces looking beautiful next year."
Autumn Planting for Year Round Colour takes place during September, October and November.
At the four RHS Gardens - Wisley, Rosemoor, Hyde Hall and Harlow Carr - there will be talks by gardeners and experts for ideas and advice, free autumn planting cards to take home and basic soil testing advice.
There will also be events including the Wisley Flower Show, Malvern Autumn Show, RHS London Shades of Autumn Show.
On 2 October, RHS Garden Wisley is joining forces with the HTA to plant hundreds of violas and pansies.
The HTA launched its autumn planting campaign, Plan it, Plant it this Autumn on 1 September.