The city's street tree stock is being managed and maintained by contractor Amey over a 25-year period as part of the controversial Streets Ahead programme.
But campaigners at Sheffield Tree Action Groups (STAG) say that the contractor has already failed to maintain street trees it has planted, many of which were to replace mature trees felled as part of Streets Ahead.
Its representative Rebecca Hammond said that while STAG has itself encouraged the public to water new trees, "this must not replace proper aftercare from Amey", adding: "We have seen Amey's watering schedules, and compared them with the recommendations from experts. Amey simply don't water often enough."
STAG co-chair Paul Brooke said: "As the contractor responsible for looking after the trees that we as council tax payers have paid for, Amey should be able to water them properly."
Amey's project director of operational services Melissa Wise said: "As part of our programme, we water trees as much as we can, but if the weather gets warmer and drier, especially over the summer months, it would be great if local people helped out by giving them extra water whenever possible."
She added: "Many other local authorities across the country are following similar initiatives to ensure young street trees get the best start in life."