Crossley took the mature tree, shrub and hedging producer from a small subsidiary of Exbury Trees to a successful company in its own right.
He said: "My role in the business has continued to evolve and with these recent changes now complete — and after 28 happy years at Wykeham — I feel now is the time to explore and pursue other wider horticultural interests, special projects and consultancy opportunities. I am looking at work on other nurseries and with garden designers and maybe more work in education."
He stepped down as general manager four years ago but continued to work in a part-time role in the sales team.
The business has changed from producing mature, field-grown specimen trees to the production of container-grown specimen trees, instant hedging and shrubs. It is a leading specialist in the north of England with a major retail side.
Crossley took on Adam Smith to help with the firm's diversification process. He took over the role as general manager four years ago and will continue to expand the company's range.
Smith said: "David was here 28 years and he built the company up from scratch really. I still pick his brains now and then because there is so much knowledge in there."
He added: "Over the past five years we have finished the final trees and what we are now majoring in is container-grown trees, instant hedging and instant shrubs. We have seen the demand for screening driven up by garden designers and the well-to-do."