"One customer cancelled a large order for Taxus two weeks after the ban started, and we had 200 plants on our hands. In the end we stopped adding up the costs after £200,000 of cancelled orders."
McKenna told the plant procurement seminar: "It surprises me that horticulture was hit so hard while window cleaners and car washers were working flat out.
"The government and water companies treated us very badly and we did a poor job of defending our need for water."
At the height of dry spells Wyevale East spent £300 to £400 a day "just to keep plants alive - once you add it up it's scary but without water we are dead".
Palmstead Nurseries sales and marketing manager Nick Coslett agreed with McKenna that horticulture was picked on because it was a "visible user of water" unlike power or steel companies, which used more water.
"Water companies have strict contracts to supply certain amounts of water to those industries and we are seen as a poor relation," said Coslett.
"But the utilities have to be seen to be doing something even tough they take our money and don't reinvest in our reservoirs and other infrastructure."