He joins from Glendale Horticulture, where he was general manager.
Managing director Christine Howard said: "We are immensely pleased to have Marcel on board and are eager to tap into his commercial and operational knowledge across the retail and amenity sectors."
Howard Nurseries grows a wide range of herbaceous plants in the field as well as in containers for supply to other nurseries, garden centres, garden designers and landscapers.
Willems said: "I am looking forward to working with Christine and her team with great anticipation. The company has been at the forefront of this industry for almost 50 years and I am keen to build on these foundations and surge forward in the supply of superior plants for retail and amenity planting across the UK."