HTA business development director Tim Briercliffe said the ban on hosepipes and sprinklers was "outrageous, what with all the work that has been done between then (the last bans in 2006) and now".
Briercliffe said this year's ban would not have as bad an impact on trade as in 2006, when the ban came in before Easter. But he added: "Our issue is that bans are too blunt an instrument. We've worked hard to come up with a machine that means gardeners won't be hit so drastically. Hosepipe bans are an easy hit."
He said United Utilities was acting against the spirit of a code of practice agreed between the HTA and water companies.
The HTA said that rather than a blanket ban, a phased approach including evening or alternate day watering, or drip-irrigation rules could have been used. Briercliffe said "nothing had changed" since 2006 in the water companies' attitudes to gardeners, adding: "More people are taking up gardening. We don't want to put them off. This will have a big impact. We need to make sure gardeners are not unfairly treated."
Jacob Tompkins, director of Waterwise, an NGO concerned with water waste, said this year's Water Management Bill had yet to be translated into legislation, so United Utilities was acting legitimately.He said it was following 1991 water act guidelines and that the drought had come so quickly the company had no choice.
But he added: "We sympathise with the HTA. There needs to be more dialogue between the water industry and the horticulture trade before hosepipe bans are called. But it has simply not rained since the floods last November. United Utilities has a statutory duty to provide water."
Tompkins has offered to act as an intermediary between the two groups: "It needs another round table and more discussion. We are not the water industry and we don't sell anything.
"The current situation is not ideal. There is no staged approach and not enough dialogue."
He suggested that the water industry should look to the US and Australia at strategies to cut water use in gardening without banning hosepipe use.
United Utilities explained that the ban would help "safeguard essential supplies".
Anybody caught flouting the ban could face a fine of up to £1,000.
Water levels in many reservoirs and lakes have plummeted to less than half their capacity because of the region's driest start to the year since 1929.
A Defra representative said: "There is an industry code of practice included in the Floods and Water Management Bill 2010 - but this part of the act is not due to come into force until October this year. At present, there is a voluntary code of practice which states that water companies must provide notice of a hosepipe ban, which United Utilities in the northwest has done."
A United Utilities representative said: "To make the hosepipe ban as effective as possible we needed to communicate clear information to our customers. By banning all hosepipe devices as well as basic hosepipes for watering private gardens and washing private cars, it helps avoid any confusion.
"Gardeners can still use a watering can to water their plants and any customers who are registered disabled or blue-badge holders are exempt from the hosepipe ban."