He joined the nursery, which supplies B&Q and Tesco, in February 2003 after working at Florimex UK and Farplants Sales.
"I decided to look for another challenge," said Tugman, who left the firm in the middle of June.
"It was the right time to move on and I hope to stay in horticulture because I've been in it now for nearly 15 years so I think I know it reasonably well."
Tugman added that this year had been a challenge for the industry: "It has probably been one of the most difficult seasons."
B&Q sent letters to some of its suppliers earlier this year rescheduling stock orders due to poor weather, with some nurseries reporting plant orders cut by up to 50 per cent.
Tugman said Roundstone had not been hit by B&Q's decision and that despite poor weather, sales had been reasonably good.