The West Sussex centre has been hit since work started in March. Completion of the £900,000 scheme has been delayed until the end of November.
Haskins chief executive Julian Winfield said the Copthorne branch was experiencing a 3.7% loss while the company's three other garden centres were averaging a 6.7% growth in turnover.
He added: "Throughout the year the roadworks have been a major problem. It has made a big difference to trade and there has been delay after delay."
Utility companies could not find pipes and now the road needs resurfacing.
Winfield said: "The run up to Christmas was when we hoped to make up for lost sales with our extensive display of seasonal decorations, products and, of course, Christmas trees. We can only hope that eventually the improved flow of traffic created by the new roundabout will make it easier for people to reach us."
He added: "We are disappointed that the authorities do not see fit to formally name the roundabout the Haskins roundabout — as it is already known by local people.
"Sponsorship is out of the question as we feel that we have already paid a high price already during its construction."
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