Westland Horticulture and its brands Unwins Seeds and Birds & Bees will sponsor the festival line-up, which now includes television celebrities, chefs and live music.
Acts include: TV choir master Gareth Malone, Harry Potter bird trainer Lloyd Buck, cookery author Genevieve Taylor, Dallas Campbell from Bang Goes The Theory and The Gadget Show, Matt Tebbutt from Channel 4’s Drop Down Menu and Food Unwrapped and one half of the Fabulous Baker Brothers.
A stand, educational gardening talks and giveaways of flower and vegetable seeds, gardening produce and bird food are all planned for festival goers.
Humble is Birds & Bees brand ambassador.
Westland marketing director said: "We are delighted to be supporting Kate in her first ever festival venture."
Kate Humble said: "We are delighted and proud to have such talented people working with us to help make the Big Day Out an irresistible mix of food, music, craft and good old fashioned fun, all in the beautiful surroundings of our Wye Valley farm."