The impact on the cost base is "pretty drastic" thanks to the pound weakening after the referendum vote to leave Europe, Utting said.
But Utting flagged a second, less quantifiable issue, that of the impact on workforce, particularly in warehousing.
For manufactured goods, which could be 70 per cent imports, prices could rise up to 20 per cent, possibly in a very short time frame, unless wholesalers have hedged currency, Utting warned.
Gardman has hedged currency but Utting said all big garden centre suppliers who import stock will have to raise prices in new catalogues for the Glee show in September.
"The cost of gardening goods going into garden centres is going to go up drastically over the next two-six months, with a lot depending on people's hedging policy," he said.
Utting said garden centres as a premium shopping experience shopped by an older and better off demographic could cope with higher prices but he said "it has to be a bit of a worry and you have to believe what is going on at the moment is going to shake consumer confidence a bit."Garden Industry Manufacturers Association director Vicky Nuttall said: "Following the announcement that the UK has chosen to leave the EU there is no doubt that we are now moving into a period of uncertainty both financially and politically.
"However, at GIMA it is very much business as usual. Until we learn more about the exit strategy that will be put in place to separate the UK from the rest of the EU, we can only speculate as to both the short and long-term implications for our members of this momentous decision."GIMA will be watching the situation carefully as events unfold. We remain dedicated to working on behalf of its members, and committed to doing all we can to ensure that our members cope successfully with any short or longer impact."
The Garden Centre Association would not comment. The HTA was unavailable to comment.
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