De Thame, currently building her first Chelsea show garden backed by LK Bennett and London-based garden centre Clifton Nurseries, said: "Either you have an idea about how things should look as regards proportion and shape or you don't.
"I do feel it would have been nice to understand the technical aspects of the build side of it. But, aesthetically, I do not particularly rate it as the most important thing. It is something that can be coaxed out but really it is innate."
Thomas said: "She's saying there is no reason for choosing someone trained as a garden designer ahead of someone not trained in garden design. It's a ludicrous comment. A professional working in any creative industry benefits from having a good eye but if having a good eye was the sole criteria of being a garden designer there would not be the vast number of garden design courses on offer."
Thomas added: "Although I have a good eye that doesn't make me a good furniture designer. It's a ludicrous idea that you can design a garden without specialist knowledge and understanding of the material required."
De Thame said: "It's very important for people who design gardens that they have a strong feeling about how they should look. And I have a strong idea of how I want it to look."
She added: "I haven't been formally trained in garden design, though I've been designing gardens unwittingly for ages through Gardeners' World. I have a knowledge gap having not trained formally, though I did do two years' study (in practical horticulture and plants at the English Gardening School at Chelsea Physic Garden).
"Clifton helped with technical stuff such as step heights but I am aware it would have been great to know things such as CAD design."