Dobbies Garden World should open by late spring 2011, creating 120 jobs.
The 48,200sq ft garden centre will house more than 10,000 plants, trees and shrubs.
Property director Stuart Wright said: "We’re delighted, in these difficult times, to be commencing construction and being a step closer to opening our first Dobbies Garden World in Cumbria.
"We are investing £8m in the project, which will create 120 jobs.
"That will have a significant effect on the local economy for many years to come."
More than half of the bedding plants will be grown at Westwood Nursery next door.