From this June until 2015, 500,000 customers in Hampshire, parts of West Sussex and Kent will have new meters installed and will pay 6% more than the standard rate for their water from June to September and 2% less than the standard rate the rest of the year.
Water companies including Severn Trent, Veolia Water East and South East Water already have seasonal tariffs for business customers. Seven companies are trialling consumer seasonal schemes.
HTA director-general David Gwyther said: "We are extremely concerned that this will force Britain's 20 million gardeners to pay far more to water their gardens or force them to neglect valuable plants."
The HTA claimed that Southern Water was ignoring the Flood and Water Management Bill. But a Southern Water representative said: "These tariffs are set to discourage non-essential use in the summer months when water comes under the greatest stress."
During the second reading of the bill in the House of Lords on 24 February, environment minister Lord Davies said: "We will work with the water companies to ensure that their revised drought plans set out clearly their broad priorities for conserving water and the types of concessions or phasings they propose to introduce."