Overview: A look at some of the world's most threatened plants, as Alan Titchmarsh and Joe Swift host another visit to RHS Chelsea Flower Show. They are joined by gardening presenter Christine Walden, who learns more about an exhibit in the Great Pavilion which is helping to guarantee the future of the planet. There is also a chance to catch up with the RHS show manager as the cameras follow her hectic schedule in the build-up to opening.