More than 60 new staff members have been recruited for the new garden centre, which has been constructed over the last year on 15 acres of land previously housing commercial greenhouses alongside the original Roundstone Garden Centre.
The extensive centre features a 460-seat restaurant and the company says it is optimistic more staff will be recruited soon.
Haskins garden centre group chairman Warren Haskins said: "Based on the success of our garden centres in Dorset and Hampshire, we are confident that our Roundstone centre will become a favourite destination for anyone with a love of horticulture, whether it's caring for a window box, garden or a grow your own allotment.
"We are delighted that our experienced team from the original centre will be able to offer the fantastic, helpful friendly advice so appreciated by gardeners over the years."
The new centre has a 500-space car park with full environmentally friendly lighting and its own bus stop. The indoor and outdoor plant areas are twice the size of the old garden centre and include extensive indoor departments including houseplants, gifts, books, pets, clothing and furniture.
The centre was officially opened by the Rt Hon Nick Herbert, MP for Arundel and South Downs.