Garden Organic programmes director Dr Margi Lennartsson and publishing editor Pauline Pears, along with 3,823 Which? Connect members, surveyed 14 products that neither body would name.
They found that more than half of those surveyed were confused about what the terms "natural" and "organic" meant in a gardening product context. A quarter of those surveyed also believed the words to be interchangeable.
The bodies said claims such as "100 per cent chemical free", "100 per cent natural active ingredient", "environmentally friendly", "organic choice" and "suitable for organic farming" could be misleading, adding: "Most concerning was the phrase 'environmentally friendly', which under the Government's Green Claims Code (currently being revised) is a term that should be avoided."
Lennartsson added: "What we don't want is gardeners misled into thinking that these products are acceptable for everyday use. This is why we want to help develop a code so that consumers can make informed choices."
A Garden Organic representative said: "The products are anonymous - the claims made on the product labels and the ingredients used are the focus of the report." It questioned the use of fish oil, pelargonic acid, feather meal and rock dust as organic ingredients.
HTA business development director Tim Briercliffe said: "The HTA is committed to strong environmental credentials for the gardening industry. We would point out that the Chemicals Regulations Directorate is the Government body that is responsible for the safe use of all pesticides and herbicides and to protect human health and the environment."
He added: "It conducts a thorough and scientific review of all such products before they are authorised for use, including the labelling of those products."
Which? suggested that consumers contact Trading Standards with concerns about misleading claims on packaging.
- For further details, see Organic Gardening Guidelines at www. gardenorganic.org.uk