Around 60 RHS staff, HTA members and volunteers will plant more than 5000 pansies and violas in containers, hanging baskets and in a large open area.
The planting is part of the campaigns of both organisations to encourage autumn planting.
RHS Wisley curator Colin Crosbie said: "As gardeners know, pansies and violas are superb at providing winter colour in gardens. Planting them at this time means that there will be colour in the garden from now until spring when the plants, having settled in should be spectacular."
HTA Seasonal Plants Group chair Charmay Ball added: "Violas and pansies are great plants to dispel any winter blues. We are really pleased to be working with the RHS on this project as part of our ‘Plan it, Plant it this Autumn' campaign. We hope that this display will get gardeners thinking about using these versatile plants even more."