Gavin says the garden will be his tallest and most unusual. Last year his Sky Garden dangled from a crane over the site.
He said: "I'm thrilled at the prospect of reaching for the skies with Westland."
The Westland Magical Garden is the company's second with Gavin.
Westland joint MD Ed Conroy said:
"Diamuid's vision for his garden far exceeds anything we've seen or done before and is exciting for everyone at Westland."
Westland will run garden centre ticket giveaways. See Horticulture Week 10 February for more.